This page: www.bacomatic.org/~dw/3phase/3phase.htm
Main page: http://www.bacomatic.org/~dw/index.htm
Last Updated: 16 Jul 2003
File: 3PHASE.TXT
Compiled by: Dave Williams
Version: 07/05/1998
Copyright: original messages copyright by their authors as indicated
--note-------------------------------------------------------------------
Note: The messages collected below have been edited by me, sometimes
dramatically, trimming away what I wasn't interested in, as this
data is compiled for my own use. The messages may no longer
reflect the actual intended meaning of the original poster. The
original texts can be found on DejaNews or the r.c.m. archives.
--net lore---------------------------------------------------------------
[email protected] (Jim Kirkpatrick)
rec.crafts.metalworking 2 Apr 93
- I'd like to run a few 3-phase motors on single-phase power, and have several
diagrams for doing so. However, the key article I'm looking for was in (I
think) Jan/Feb 1991 Home Shop Machinist. If anybody has that article, or
another reference, I'd like to know:
- 1. What size capacitor to use for starting. This probably depends on
whether the circuit is 110V or 220V, as well as horsepower.
- 2. What to do to determine direction. On a 3-phase motor, I think
you normally just swap two of the legs. On a 3-phase motor running
kluge-mode on 1-phase power, is it just a matter of which phase
you hook the starting capacitor to?
[email protected] (Pete Lancashire)
rec.crafts.metalworking 24 Jun 93
- >Conserning the discussion of 3-phase converters: Some time back I posted
>jubilantly that I had just scarfed up on an excellent deal on a 20" long
>bed jointer. It's still a good deal, even though I can't use the 3-phase
>motor--but I guess not *as* good a deal. :-) Anyway, so I'm told one can
>buy a "roto-phase" at the electric motor place for much less than a new
>replacement single-phase motor that will allow a 3-phase to run on single-
>phase power...only that *my* motor happens to be a 7.5 horse power, which
>is "different" than the lower horse powers and won't work with a roto-
>phase.
- I don't know why but true rotary converters are expensive, unless you are
lucky. I lucked out a got my 7.5 hp rotary converter at a garage sale of all
places for $50. But one thing I don't like about mine is that when it is
unloaded it is very noisy.
- But I also have built 'rotating transformers' aka 3phase motor, as well.
Once you have done this, getting 10hp and under 3phase motors is very easy,
usually for cost of scrap.
- Here are the places to look ...
* Surplus stores, see below.
* Scrappers. I've have a very good relationship with the local scrapper
where I live. He gets at least one 3ph motor a month that usually has
nothing wrong with it.
* Water and sewage agencies. I have first dibs on the pump motor in our
little town's backup well pump house when they upgrate it. Basically if I
help get it out, I can have it.
* OLD factories and mills, when a factory or mill modernizes, the motors are
usually scrapped. But for use a a rotary transformer, old motors are
usually better, more iron == better transformer. Either get to know the
person(s) who have the scrap deal, or talk to the foreman directly.
* Ask electricians that do industrial installations, as in above.
- Here are the two of my best transforer/motor 'deals'...
The first is a GE verical pump motor, 40 hp for $100. The motor was pulled
out of a sewage pumping station where the drive end of the shaft was screwed
up. What's nice about it is being vertical it takes very little room and is
totally sealed, also very over built since it is designed to run continously
at 40 hp. MASSIVE over sized bearings. Runs forever after you turn of the
power. Because of the mass of the rotor, I had the shaft ground down and
used a 1 hp as a starter.
- It was a great conversation piece as well, with the 1 foot stand the thing
was over 5 feet tall, and took forever to stop spinning. If I remember it
weighed over 400 lbs. but only took about 2 sq feet of floor space.
- Although the shop I put it in only needed a max of 10hp, the extra iron
really helps the transformer action. The waveform for the 3rd leg looks
great. I wish now I still had it.
- The second motor was a Lincoln 50hp. It was BRAND NEW, and cost me all of
$200. The surplus store also had a 75hp for $300.
- SO look around, although these motors are very heavy, they make very very
good transformers from all the iron. And it seem after a motor gets bigger
then 20hp, they get cheaper.
- BUT BEWARE: DO NOT GO BY THE ID PLATE !! A lot of large (>= 10hp) motors on
the market have been rewound and when rewound, it is common practice to only
wire them for the voltage supplied. And in a lot of cases this is 440.
- >Too bad. That would be a great motor, a little over-powered for the usual.
>A new single-phase 5 horse--which will do the job quite nicely--runs about
>$400.00. So I'm still trying to find someone to buy the still great but
- Be careful of 5hp motors. There are a lot of 'cheap' low duty single phase
motors designed for air compressors. They usually need a lot of cooling air.
I've bought Lessons 'continous duty' motors for around $350 to $375.
- Other things to look out for use in a machine is the quality of the bearings
and don't forget about the mounting.
- >unusable by me 7.5 HP 3-phase motor. But I don't expect to have much
>success. As explained by a motor rewind guy from whom I solicited info,
>"Most 3-phase motors are used by Industry, and Industry usually wants a
>*new* motor, not a used one."
- I don't know. Most place around hear don't seem to mind buying used motors,
Where I am now, and when I lived near Philly, I was always able to find 3ph
motors up to 10hp for very little or for free.
[email protected] (Jay Stryker)
rec.crafts.metalworking 21 Jan 1994
- >> If anyone has info on using a large 3-phase motor as a phase
>>converter, ie 1-phase power to 3-phase power, I have a couple of
>>questions. First, how much power is consumed at idle load condition,
>>relative to the size of the motor being run?
- I have seen ordinary three phase motors used in this way...you can wrap a
rope around the shaft, give a strong pull, and then switch on 1 phase power.
Once running, it will draw a few amps, but as long as there is no load on
the shaft, it "idles". When you place three phase motors on the output
(recall that you have to use a three phase contactor) the current will rise.
How much...hmmm...maybe twice or more? Anyone know? I have a commercial
single to three phase converter....I have measured the three phase outputs
for balance but never measured the input current change. I run mine from a
70 Amp circuit into the workshop; when I start the converter the house light
level drops about the same as an appliance starting. When I start the 7 hp
turret lathe motor it dims the house lights for about one second as it comes
up to speed.
- >>Second, can a dual-voltage 3-phase motor be configured as a kind of "
>>rotating transformer" to step up 220V to 440V ?
- This sounds feasible if the windings are electrically separate (eg. the 220
is not a mid-tap on the 440). Heating, arc-over and insulation limits may
be involved here. If you are using a "bigger" three phase, say 7 horse,
motor, for this, and the driven motors are 2 or 3 horse, the heating may not
be a problem. Three phase motors are rugged beasties, but you want balanced
power in all three feeds to prevent overheating/meltdown. Cautions: high
voltages, high currents, high shaft torques, possible open or short
circuits, possible insulation fires, possible contactor problems,
etc.....this can be an expensive laboratory experiment....!!!! Check with
your local expert(tm)!
[email protected] (John De Armond)
rec.crafts.metalworking 23 Jan 94
- >I have seen ordinary three phase motors used in this way...you can wrap a
>rope around the shaft, give a strong pull, and then switch on 1 phase power.
- Yes, this works well. and if you connect a capacitor in series with another
winding hooked in parallel with the first, the motor will start itself and
will draw much less idling current. For 3P motors in the 5 hp range, I'd
start out with about 20 uF at the appropriate voltage. If you're interested
in best efficiency, tune the capacitor by varying it to get the least idling
current.
- >How much...hmmm...maybe twice or more? Anyone know? I have a commercial
>single to three phase converter....I have measured the three phase outputs
>for balance but never measured the input current change.
- Amazing how power factor works, eh :-)
- >>>Second, can a dual-voltage 3-phase motor be configured as a kind of "
>>>rotating transformer" to step up 220V to 440V ?
- >This sounds feasible if the windings are electrically separate (eg. the
>220 is not a mid-tap on the 440). Heating, arc-over and insulation limits
>may be involved here.
- This works fine as long as the FLA limitation of the converter is respected.
For dual voltage motors, simply connect the windings in series for the
higher voltage and then apply the incoming power to between one end and the
junction of the series windings. Take the output power from the ends of
each phase.
- Another little known fact is that an induction motor driven by another prime
mover (engine, etc) is a fine generator. It generates lots of leading VARs
so it counteracts the low powerfactor of the motors and the "converter."
Simply connect the 3P converter motor to the prime mover, bring the rotor
speed up to nameplate (1800, 3600, etc) and apply excitation as before.
Then crank the throttle open on the prime mover and apply the 3P load.
Within the current rating of the converter, the rotor will remain locked
roughly in sync with the line and no govenor is needed. Watch the phase
current to keep it within the FLA rating of the converter. Particularly
watch the phase connected to the line; "excess" power is fed back out the
power line.
[email protected] (Mark Williams)
rec.crafts.metalworking 24 Jan 1994
- Yes, 3 phase motors can serve as rotary phase converters, wiht reduced power
from the target motor.
- You basically run the 3 phase motor off 220, on two of the phases, and the
spinning motor will generate a third phase. This is a hack, since two of
the phases are the 220 volt line (180 instead of 120 degrees out of phase)
and the third phase will be of lower voltage (160 v) and between the other
phases.
- You also need a means to start the 3 phase motor/generator, either
mechanical or electrical. Either another motor and cluch or a motor
starting capacitor to the third leg will spin it up, then should be
disconnected.
- Woodworkers sometimes use this scheme to run bargain 3-phase saws, jointers,
etc. See writeups in "Fine Woodworking on Making and Modifying Machines",
available from Taunton Press.
[email protected] (Ron Ginger)
rec.crafts.metalworking 25 JAN 94
- >>> If anyone has info on using a large 3-phase motor as a phase convertor
- I measured the input current to my 2hp phase convertor, at idle and its ony
about 1 amp. All its drawing is the windage and bearing loss, since its
doing no work.
- My convertor control is about as simple as possible- a two pole switch, a
single pole momentary push button, and a motor starting capacitor- total
cost under $20. I flip the switch with my finger, while holding the push
button with my thumb, as soon as I hear the motor up to speed I let go of
the button. Much siimpler than relays.
[email protected] (Scot Heath)
rec.crafts.metalworking 27 Jan 1994
- : You basically run the 3 phase motor off 220, on two of the phases, and
: the spinning motor will generate a third phase. This is a hack, since
: two of the phases are the 220 volt line (180 instead of 120 degrees out
: of phase)
- This is not correct. If you were to look at the voltage vrs. time plot of
the 240v available in a home or the 240V available between 2 phases on a
240v 3 phase circuit, you would see the same thing, a 339v peak, 60Hz sine
wave. The two leads of the motor connected to the 230v line see this also.
The voltage from either of the applied leads to the third lead on the motor
will be, under ideal conditions, 240v and 120 degrees out of phase with the
applied voltage. This is due to the geometry of the motor, which may be
modeled as a transformer with a rotating secondary.
- I have seen this mis-information in many posts regarding 3-phase converters.
I think it is a result of trying to incorporate the center tap "neutral"
line of household single phase systems into the problem. This tap has no
bearing on the applied 240V.
[email protected] (John De Armond)
rec.crafts.metalworking 04 Feb 94
- >Thanks, but I need a way to check that is not guess work.
>I think amperage is more likely to tell me what is going on.
- Nope, sorry. The amps approach FLA quite a bit before the motor is fully
loaded. What changes near FL is power factor. The only way to tell without
guessing is to put a wattmeter inline and measure the power consumption.
Ain't no other way. Lacking a wattmeter, measuring the winding temperature
is the next best bet.
[email protected] (Paul Amaranth)
rec.crafts.metalworking 1 Jun 1994
- > 3 phase motors are much cheaper than single phase motors. However,
>unless you happen to live *very* near a commercial zone wired for
>3-phase, getting 3-phase to your home could easily cost more than the
>milling machine. I finally gave up and swapped a single phase motor on
>my old Gorton.
- It's also trivial to reverse a 3 phase motor, you don't have to worry about
switching out the starting capacitor, they have nice torque characteristics,
etc.
- Also, having only 1 moving part, there's nothing to break. They are nearly
indestructable as long as you stay within the ratings. Very important for
industrial machinery, perhaps not so important for home shop use.
- As for 3 phase power you can build a phase converter for anywhere from $25 -
$100, depending on how much you can scrounge. I'm still gathering parts,
but after I finish the converter I need for my mill, I'll probably swap my
lathe over to 3 phase as well.
[email protected] (Jeffrey R. Millar)
rec.crafts.metalworking 1 Jun 1994
- I have an old South Bend lathe with a 3 phase motor. I generate my own
three phase from a junkyard 5 hp 3 phase motor running off 220 single phase.
I hook the 220 across two phase, turn it on, and flip the shaft with my foot
to get it turning. Smoke comes out if I don't get the shaft turning right
away, three phase motors on single phase don't self start. Once it's
spinning the unconnected third phase becomes a generator output. The three
phases from the motor connect to the lathe motor and away we go. I get the
smooth reversing benefits of a three phase motor. It's neat to reverse the
lathe and watch the motor/generator on the floor hum and jump a bit from the
torque.
[email protected] (gary preckshot)
rec.crafts.metalworking 3 Jun 1994
- > "Plug"?
- With the motor running full forward, flip the switch to reverse. The motor
reverses immediately with a kind of screeching whine. Doesn't hurt it a
bit, unless you stand there plugging it continuously. Most machinists use
plugging to stop the spindle quickly. You just reverse momentarily and shut
it off as the spindle passes through 0 rpm. Very convenient.
[email protected] (Jon Winlund)
rec.crafts.metalworking 3 Jun 1994
- After my original long-winded post, I decided to see if I could find
additional info on "plugging". I found the term in "Understanding and
Servicing Fractional Horsepower Motors" by Kennard C. Graham. There are
switches avaliable that momentarily reverse the motor to slow it down
quickly. I also found out from a friend (I havent tried this) that switching
a diode into a split phase or capacitor motor circuit will stop it right
quick.
[email protected] (Randy Lee)
rec.crafts.metalworking 30 Jun 94
- I have to replace my static phase converter(the power loss with the phase o
matic was too high even befor it died) on My Bridgeport milling machine and
I I am looking at the Mitsubishi Freqrol A-024 or the Freqrol U-100 for both
Phase conversion and speed control in one package the big feature in the A-
024 over U-100 in a 1.5K size seems to be that the A-024 is UL approved and
has something called Ultra low acoustic noise operation... both units can be
computer controled which I can see as useful for facing operations.
- Do any of you have any input on these units? I have been quoted $483 on the
U100 and $559 on the A--024 (list is 805 and 950) is that a good fugure for
this sort of unit? any others I should consider? perhaps of US build?.
- the features list for the A-024 has among other things...
Starting torque of 200%(6hz) regardless of the maker of the
motor(Mitsubishi moters are quoted at 280%)
Ultra low acoustic noise Operation capability
Highspeed tripless function and restart after momentary power interruption
function as standard
Regenerative Brake resistor as well as DC dynamic Braking
output Freq range... standard default .5hz to 120hz
- My Bridgeport is an older unit and it's motor is a 3 phase 230 volt pancake
style just out of rebuild (when the phase a matic went south it took the
motor with it!) are their any problems driving an old disign of moter like
this with modern combined phase/speed controlers?
[email protected] (Randy Lee)
rec.crafts.metalworking 26 Jul 94
- I went through this process with my (much older WWII vintage Bridgeport
Round ram Jhead) and started with a Phase o matic static converter... this
did not work out for me and after much research I came up with the
Mitsubishi (908-302-2787) Freqrol - A024-1.75K Gen purpose Inverter... this
has computer control so your quill speed would be under CNC as well...
- for your unit you will need the 3.7K Size I think as when used as phase
converters they are derated by about 1/2 (My 1 HP needs the unit that would
normaly run the 2HP from a 3 phase supply)
- I went with them as they give 200% (magnetic flux vector control) starting
tourqe(min most motors have up to 280%) at 6hz (and quite flat up to 50hz
)and have an ultra low noise mode that makes them a joy in the home shop...
my price was about $600 instaled Make my old belt drive J head a modern full
speed control head which might not beimportant to you but the low noise
level compaird to the phase o matic is a true joy!
[email protected] (Dan Caster)
rec.crafts.metalworking 27 Jan 1995
- The few converters that I have built to get three phase from one phase used
a single phase motor to drive the three phase "idler" motor. One pulley was
a variable pulley so that the " idler" motor could be driven at snychronous
speed. It seems to work pretty well. I personally call the three phase
motor a generator, not an idler. I also threw in a bunch of capacitors
across the line to correct the power factor. This was necessary only
because the breaker size was marginal.
[email protected] (thomas walter x5955)
shop-talk 08 Apr 1996
- My 20 year old Sears compressor is rated at 4 hp, with a large 220v (15amp)
motor on it. It does manage to put out enough to run an air sander, or paint
a car... but the 20 gallon tank runs down and takes a bit of time to
refill... which is a pain when sanding and I want to get the job done.
- Another thought: (haven't done it), but is to use a surplus A/C compressor
with the 12V clutch. Sketchy details were given in "Projects in Metal"
about a year back. Idea is to let a 220V 4hp motor come up to speed, then
the A/C clutch is controlled by pressure switches. Obvious the motor would
be running full time, but since I would like to have a "3 phase idle" motor
for my lathe & milling machine, really not too bad of an idea (just let it
idle when using the machining tools... or power the compressor as needed).
[I think Met-Lab series of 'contruction projects' also outlines the same
thing. Plans cost $4]
[email protected] (Paul Amaranth)
rec.crafts.metalworking 5 Apr 1995
- >I have a few questions about rotary phase converters. I just finished
>wiring a magnetic starter and some outlets for mine, and the electrical
>connections surprised me. There were only three connections inside the
>box on the phase converter - is this typical? I had expected five
>connections (2 in and 3 out). Instead there are 2 in and 1 out. I
>thought a phase converter was a motor generator, but all it does is
>generate the third phase.
- >So, how does this third phase relate to the 220V input? There is no
>way that the three phases are balanced... (how could they be, since the
>input phases are already 180 degrees apart). How many volts will there
>be between the two input phases and the generated phase? How will this
>vary with load (i.e., is it a bad idea to have the phase converter
>running without any load, or with a very small load in relation to its
>size)?
- There are three kinds of phase converters that I'm aware of: static, rotary
and motor/generators. OK, there's a fourth that I'll get to.
- The static kind use a bunch of capacitors to generate the other two phases.
These probably have the worst peformance overall and typically run the motor
at around 80% of rated capacity. They're also relatively cheap and adequate
if you only have one motor or tool you want to run. You can think of these
as the guts of a rotary converter using your target motor as an idler.
These aren't hard to build.
- Rotary converters consist of a 3 phase motor that is used as sort of an
auto-generator to generate the other two phases. Single and three phase
motors will run on single phase current, but neither will start on it. A
single phase motor has a startup winding which uses a starting capacitor to
generate a second phase to bring it up to syncronous speed. You can do the
same thing to start a 3 phase motor. Once it's running, the magnetic fields
from the rotating armature will generate the other two phases in the
unenergized windings. The amplitude of these phases will be lower than the
line side, but you can balance them out with judicious use of run
capacitors. These things tend to be ideally balanced for only a specific
load, but since 3 phase machinery easily tolerates +/-10% (or worse) line
variations, you can run various loads without problems.
- It's not too hard to build one of these from an old 3 phase motor and
miscellaneous parts. If you're not picky, you can leave off all of the
fancy stuff like contactors and balance capacitors and use a rope to start
the motor. It'll still work. You do lose something in efficiency with a
rotary converter - they tend to be around 70-80% efficient so the motor
(called an idler) needs to be around 1.5 times your biggest load. Without
the run capacitors, the generated phases may be down 10-20% from your line
voltage. It helps to have a clamp-on amp meter to balance things out.
- The one I built consumes a kilowatt just idling (4A at 220V), but the garage
needs the heat anyway. I probably spent $150 on it, but I had to buy most
of the parts new, except for the motor. HSM had an article on rotary
converters some time ago that covered them in a little more detail. Drop me
e-mail if you want more info.
- BTW, the input phases are not 180 apart, your input power is single phase
220 since you're not referencing it to the neutral. With 2 wires you get 1
phase, with 3, you can have 2 or 3.
- Motor/generators are another way, if you have the money. Get a big single
phase motor running a 3 phase alternator. You still have some losses, but
it's industrial strength. This is probably as close as you can get to
having 3 phase power installed without getting the power company involved.
Pricey. Also, when was the last time you saw a 10hp single phase motor?
Still, if you want to run a MIG or TIG rig, this is probably the way to go.
- Another way that's getting more popular is to use a solid state inverter.
You convert the single phase AC to DC and then synthesize the 3 phase
output. These use neat semiconductors like IGBTs or MOSFETs as control
elements. Not only do you get high conversion efficiency, but you can
fiddle with the frequency and other parameters to control motor speed and
useful things like soft starting and braking. I received some literature
the other day on one of these. It had around ten or so stored
startup/shutdown programs that you could run as well as speed control.
These still tend to be a little expensive, though. If you have the money,
these are pretty attractive.
[email protected] (David Erickson)
rec.crafts.metalworking 4 Apr 1995
- Another thing that surprised me: I opened the box on top of the phase
converter that holds the electrical components that generate the third
phase. All that was inside the box were two large capacitor banks.
Actually, I don't know if those banks were used in generating the third
phase or in starting the phase converter. Any information that could shed
some light on this would be appreciated.
[email protected] (Barry Workman)
rec.crafts.metalworking 05-27-95
- >between the converter and the idle motor. This switch is turned ON for
>starting, then OFF after the idler motor is running. Now, the question:
>How does a commercial rotary phase converter handle this condition? Is
>there something that does this automaticlly?
- It boggles the mind, doesn't? I too have started looking into
phase converters for my newly acquired lathe. I called GWM because
I liked their ad in HSM. They not only sent me a catalog of their
products, but included discussions of the pros & cons of the different
types of phase converters, how they worked, how to wire them, and diagrams
that show you where the 3rd phase comes from in each instance. I've seen
different phase converters advertised for less but a call to them gains
you some real insights. Their number is 800 437 4273. They're in MO
and in state you'll have to use the pay-for-it number, (as given on the
info) 380-7247. I assume that the AC is 816 but I dunno.
[email protected] (Barry Workman)
rec.crafts.metalworking 26 May 1995
- >Forgot, had one more question. Does anyone know what the difference is
>between a standard duty and heavy duty static phase converter for the same
>HP rating?
- Standard duty means you will not be running the machine/motor at max load
most of the time (as in welding units big compressors) GWM sez about 75%
load as a maximum.
[email protected] (Barry Workman)
rec.crafts.metalworking 26 May 1995
- >Why are people trying to convert 3-phase to 1-phase via a converter?
>Would it not be just as cost effective to replace the motor with one of
>the proper frame and hp rating? Or adapt one?
- The task is to convert single phase to three phase. Most larger electric
motors run on 3 phase circutry. Most homes only have single phase. Usually
a phase converter is less expensive than trying to find a new motor of like
HP that runs on single phase. My lathe has a 5 HP 3ph motor. The reversing
switch switches two of the phase lines to reverse the direction of the
motor.
[email protected] (Peter Brooks)
rec.crafts.metalworking 26 May 1995
- My understanding of 3-phase motors (I'm an EE, but more into
microelectronics than power stuff) is that the power is constant over time,
unlike single phase, where the power will hit a null 120 times a second
(where 60Hz power applies, anyway). Thus, 3 phase motors tend to be very
popular in industry, so it's a lot easier to find used equipment with 3
phase motors.
[email protected] (Barry Workman)
rec.crafts.metalworking 30 May 1995
- >A small company in Richmond VA is building phase converters and selling
>them for about half the going rate. Example - 3HP = $425 Their phone
>number is 1-800-219-8173
- Are these static phase converters (then the price is expensive)
Modified static phase converters (maybe reasonable) or
Dynamic capacitor phase converters (reasonable price) ?
[email protected] (David Erickson)
rec.crafts.metalworking 7 Jul 1995
- Following up on the phase-converter thread from about a month ago, I
experimented last weekend with adding start capacitors to three phase
motors. I ran 220V single phase to two of the three motor connections and
connected a start capacitor between one of 220V legs to the third motor
connection. The results I got were similar to what had been reported:
- small (less than 1hp) motors started up fine with a 50mfd cap; larger motors
required about 50mfd per hp to start.
- BTW, I purchased the capacitors from the Surplus Center, the address for
which was posted here a couple of days ago.
- if the capacitor is left in the circuit after the motor starts, the motor
growls, indicating current imbalance, but if it is removed, the motor runs
quietly, albeit with reduced power when compared to powering the motor with
three phase electricity.
- So one solution is to use a push button to supply the capacitance and start
the motor. However this has drawbacks. If the button is not pressed
immediately when power is supplied, the direction of rotation is not
predictable. Also, if the motor stalls, it will not restart automatically
when rpm falls.
- What I need is some kind of relay to detect startup or stall conditions and
engage the capacitors. Startup conditions are easy: use a timer relay. How
do you detect stall conditions? What would be an easy, fairly cheap device
to add that would reliably restart the motor when needed?
[email protected] (Ron Ginger)
rec.crafts.metalworking 7 Jul 1995
- >What I need is some kind of relay to detect startup or stall conditions
>and engage the capacitors. Startup conditions are easy: use a timer relay.
>How do you detect stall conditions? What would be an easy, fairly cheap
>device to add that would reliably restart the motor when needed?
- I have had exactly this running for more than 5 years, and I only recall
once it starting in reverse. I dont ever recall stalling the motor.
- I mounted the toggle switch right next to the push button, I put my thumb on
the PB first, then flick the switch with my finger, and after a few seconds
release the PB. Its now about a reflex action to reach up and start it.
- I dont think its worth the extra effort of adding a relay.
[email protected] (John M. Stuart )
rec.crafts.metalworking 18 Nov 1995
- >I would like to build a portable generator for 120 and 220 volts. Where can
>I get the generator itself and/or can I build one?
- There are several paths that you can take here. You can buy a real
generator from McMaster Carr or other industrial supply place. They sell
the generator units, and you supply the engine, water turbine, or other
mechanical energy source to turn the shaft.
- You can also make a generator from any motor. For example, a 10 HP three
phase motor can be driven at a speed slightly above it's synchronous speed
to generate three phase power. The 10 HP motor would be good for about 3-5
KW out. The voltage generated would match with the winding voltages, and
the frequency of the alternating current would be proportional to the
rotational speed. So if you had a 3600 RPM 220 VAC motor, you should run
the motor at about 3650 RPMs as a generator, and you will get about 210 VAC
out of it at 60 to 61 Hz. The same trick done on a brush type motor will
give DC voltage.
- Since the motors are not optimized for use as generators, you may have to
start it under a very light load such as an incandesent lamp, and apply the
full load after it is generating power. In addition, all of the resistances
in the motor windings now are working in reverse, and cause it to produce a
lower output voltage under load. If the electrical load that is placed on
the motor/generator is too heavy, the magnetic fields within the motor will
collapse, and the power will stop. The voltage regulation will be a
function of a particular motor.
- If you find your motor surplus, you may get other voltages such as 240/480
VAC. These are usable, however you may need to use an autotransformer if
you need 120 VAC. You can also use Boost/Buck transformers to correct for
low output voltages.
- Starting the generation of electricity is the trick with a motor. Usually,
there is enough residual magnetism to start the process when the motor is
rotated with a light electrical load. This has always worked when I have
tried it. If for some reason, the generation of power did not start, I
think that it could be excited by a short pulse from a battery or capacitor
in the windings to start the process.
[email protected] (Gordon W J Sorensen)
rec.crafts.metalworking 30 Nov 1995
- : I recently bought a 3 phase South Bend metal lathe, and am slowly
: cleaning it up for use. So far, I have not been able to power it
: up, since I don't have three phase. I would prefer to avoid mucking
: around with a converter (although I know how to do it).
- : I also do not want to have to change over the entire electrical
: system on the lathe - if there is a way to wire in the new motor
: so that I can use the same switch, I'd be very glad to hear about it.
- Three phase contactors have 3 sets of contacts, You need to use two of the
3 for your single phase 220 motor. Let the 3rd one dangle in the breeze so
to speak. You may need to change the heaters in the contactor to correspond
with the different loads that the single phase motor will draw. Also the
normal reversing switch for 3 phase motors will not work for a single phase
motor.
- I guess it will be difficult to avoid doing some rewiring, like a new plug
for instance. Also how extensive is the wiring on the lathe anyway? If it
has push buttons for starting and stopping you can reuse them. The main
contactor should be reuseable if it is sized large enough. You need to find
out what are the voltage requirement for the coils in the contactor are
(24,110, 220, 208, 277, 440 ? ) You may need different coils or a
transformer to step the line voltage down to the correct value.
- Depending on what you need to change, the single phase to 3 phase converter
might not be an all bad way to go.
[email protected] (Dave Lane)
rec.crafts.metalworking 30 Nov 95
- I, too, recently acquired a South Bend 10K, and power it from single-phase
220v using a Phase-A-Matic static converter. I was skeptical at first, but
the job of fitting a new motor seemed overwhelming. On the advise of the
dealer I bought from, I got the converter. It is an elegant solution, and
so far I can't see any down side. Of course, the motor runs at only 2/3 the
horsepower it would develop on 3-phase power, but new belts will slip before
the motor slows anyway. This is a <$100 solution, and is a no-brainer to
hook up.
[email protected] (Paolo Zini)
rec.crafts.metalworking 1 Dec 1995
- I do not think that rewiring the motor is a nice solution: you will have
power loss, difficult start, and problem on reverse.
- I have solved that problem using one spare 3-phase motor has phase
converter, You need a surplus (working...) 3 phase motor larger than the
motor of your lathe and wire it as explained in the following web site.
http://tbr.state.tn.us/~wgray/howto/ph-conv/ph-conv.html
- I did it and my 3 phase lathe works great, full power, reverse etc. etc. The
only comment I can add: disregard the start capacitor and relay, are
expensive (really expensive..) and a piece of rope do the job. Using that
solution you do not need rewiring the lathe's motor and you have 3 phase
power for oter tools.
[email protected] (Roger Mitchell)
rec.crafts.metalworking 4 Dec 1995
- For those who wish to connect a 3 phase drum switch to reverse a single
phase motor, the following show the connections for both.
- Typical drum switch connections for 3 phase operation
Drum Switch
Forward
Phase A _______________ ___ ______________________
Line Load
Phase B _______________ ___ ______________________
Phase C _______________ ___ ______________________
Drum Switch
Reverse
Phase A _______________ ______________________
\ /
\/
/\
Phase B _______________/ \______________________
Phase C _______________ __ ______________________
- Single phase split phase motors have 2 windings: start and run. The start
winding usually has a capacitor in series with it to provide the phase shift
to start the motor, as well as a centrifugal switch to cut out the power to
the start winding after the motor is up to speed.
Typical 3 phase drum switch being used for single phase motor.
Drum Switch
Forward
_________________________________________________________________________
|_______________ ___ ______________________ |
|
Start Windings Run Windings
Line _______________ ___ ______________________ |
| |
| |
________|_______________ ___ ___________________________________________|_
Drum Switch
Reverse
_________________________________________________________________________
|_______________ _______________________ |
\ / |
\/ Start Windings Run Windings
/\
Line _______________/ \_______________________ |
| |
| |
________|_______________ ___ ___________________________________________|_
- In Bill's case his lathe has two 3 phase contactors, one wired to produce
the foward combination and one wired for the reverse combination. In that
case, what needs to be done is to find the one terminal set that is wired
the same on both contactors versus the other 2 sets which reverse polarity
between contactors and connect them up the same as what the above digram
shows for the drum switch. Also, you will need to make sure that the
connections that supply the power to the contactor coils is connected across
the 220 volt input terminals that are being used.
[email protected] (Antonio Palama')
rec.crafts.metalworking 07 Apr 1995
- I have a few questions about rotary phase converters. I just finished
wiring a magnetic starter and some outlets for mine, and the electrical
connections surprised me. There were only three connections inside the box
on the phase converter - is this typical? I had expected five connections
(2 in and 3 out). Instead there are 2 in and 1 out. I thought a phase
converter was a motor generator, but all it does is generate the third
phase.
- The phase converter you describe is, most probably, of the Ferraris-Arno
type; in August 1994 I mailed an article to this newsgroup with the
references to the theory of the method and a practical discussion of how to
implement it using an ordinary 3-phase induction motor. I do not have a copy
of that posting handy but, should someone be interested, I could repost it
next week.
- So, how does this third phase relate to the 220V input? There is no way
that the three phases are balanced... (how could they be, since the input
phases are already 180 degrees apart). How many volts will there be between
the two input phases and the generated phase? How will this vary with load
(i.e., is it a bad idea to have the phase converter running without any
load, or with a very small load in relation to its size)?
- I'm going to run some experiments to find out, but I'd welcome hearing from
any EE's who could explain some of the theory here.
- Another thing that surprised me: I opened the box on top of the phase
converter that holds the electrical components that generate the third
phase. All that was inside the box were two large capacitor banks.
Actually, I don't know if those banks were used in generating the third
phase or in starting the phase converter. Any information that could shed
some light on this would be appreciated.
- A simplified theory of the method is the following: One can write the single
phase AC voltage as the sum of two 3-phase systems of the same voltage and
rotating in opposite directions. An asyncronous induction motor running near
syncronous speed (e.g. 2950 RPM for a 2-pole 50 Hz motor) has a very high
impedence for the 3-phase system rotating in the same direction of the motor
and a very low impedence for the other one. So one of the two 3-phase
systems is short circuited by the motor while the other one is left
undisturbed.
- The role of the capacitor is both that of starting the phase converter and
that of improving the balancing of the output 3-phase system. In theory one
should use a specific capacitance for every load on the converter but, in
practice, a reasonable compromise can be worked out.
- This method is inehrently risky for the load motor since the 3-phase system
produced by the converter is unbalanced and this can overheat the load
motor. Nevertheless I had no problem using this type of converter in my
workshop.
[email protected] (Roger Mitchell)
rec.crafts.metalworking 7 Apr 1995
- All a rotary converter consist of is a 3 phase motor with a capacitor and
centrifugal switch similar to what a single phase motor has to start it.
Once it is running it will generate the 3 phase quite effectively. On a 220
volt unit, you will find 220 volts between each leg. Virtually all the units
I have seen are delta connected so there is no "neutral leg". Conventional 3
phase power is typically Y connected and has 120 volts from each leg to
neutral and 208 between each leg.
- Our Trolley car here in Fort Collins is operated on 675 volts DC which is
derived by rectifing 480 volt 3 phase from a floating delta connection
which has no neutral line. The power originates from a transformer setup in
which the 13,800 volt primaries are Y connected to a common neutral and the
secondaries are delta connected. When in operation the floating delta has a
DC potential of slightly over 300 volts imposed on it.
[email protected] (Mark Kinsler )
rec.crafts.metalworking 7 Apr 1995
->It is basically a 3-phase motor being run on 1-phase power. The capacitors
>are used merely for starting the motor. Once it is up to speed, the caps
>are cut out. The 1-phase power keeps the motor running, which then
>generates the other two phases because you have coils spinning in
>magnetic fields.
- When I was a distribution engineer for Northeast Utilities my partner and I
got a call to see a guy in an old residential neighborhood in Norwalk,
Connecticut. He wanted to know how much it would cost to extend three-phase
power to his house. We couldn't imagine why, so we asked. He took us down
to the basement and there is a nice machine shop. Big turret lathe, a
couple of milling machines, and I think a punch press of some sort.
- All of the machines had three-phase motors. Well, we asked, what are you
using now to power the machines? So he strode over to a big gray motor,
punched a few switches, wrapped a greasy length of cotton clothesline around
the motor pulley, and gave a yank. Motor spun, shop came to life.
- It seems that he'd been running this small commercial machine shop out of
his basement with his wife for many years. He also worked a regular job,
and apparently she was most of the skill and brains of the shop. At least
he said so. It wasn't a hobby: a basket by the punch press was filled with
parts made for a local aircraft manufacturer. Everything seemed to work
fine with the arrangement he had, so I asked him why he wanted three-phase
power piped in at this stage in his career. He replied that his converter
seemed to be sort of a kludge and he wanted to know how much it would cost
to go legit.
- The answer, after my partner (the experienced one; I knew nothing) figured
out the cost of a couple more transformers and extensions from the other two
phases of the feeder, was a lot. I don't remember how much, but we gave
free estimates. I advised him to leave well enough alone, and I think he
did. They seemed to be a very happy couple.
[email protected] (James Kirkpatrick)
rec.crafts.metalworking 6 Apr 95
- [questions about rotary phase converters]
- Certainly, one way to do this is using a separate motor and generator, and
this will probably give more "accurate" 3-phase power. However, there's no
reason for the two to be completely separate, and in fact it works quite
well to have both motor and generator in the same case, wound on the same
armature and field coil. In fact the coils are the the same piece of wire,
and the whole thing is really just a motor. This type is sometimes called a
spinner.
- It is basically a 3-phase motor being run on 1-phase power. The capacitors
are used merely for starting the motor. Once it is up to speed, the caps
are cut out. The 1-phase power keeps the motor running, which then
generates the other two phases because you have coils spinning in magnetic
fields.
- Let's call the three wires A, B, and C. You pump in power on A and B, and
copy that to the output as well. The second phase appears between B and C,
and the third phase between C and A.
- The phases won't all be exactly at the proper angles and voltages, but
fairly close.
- Due to inefficiencies you generally use a motor about twice as big as the
largest load, e.g. make a spinner out of a 10-hp motor if you want to run a
5-hp 3-phase load.
- Disclaimer: I've not actually done any of this, but I've collected several
articles over the years in preparation.
[email protected] (Jonathan M. Elson)
rec.crafts.metalworking 25 Apr 1995
- I just got info from a company that has a LOW PRICED 1-phase to 3-phase
converter. This is not a starting capacitor or phase shifter, but a true 3-
phase inverter. It also provides speed control, from 3 Hz to 120 Hz. I've
been pricing these devices for years, and also designing my own. The
commercial units are $4000 - 6000! This new find is from :
Drivecon Corp
1840 Industrial Drive, Suite 220
Libertyville, IN 60048
(708) 918-1406
- Their 1/2 HP variable frequency drive, model EC400C, is $275.00 in single
quantity. It comes without a cabinet, but is a circuit board mounted on an
aluminum plate. You have to hook up your controls and speed pot at this
price. They will sell NEMA cabinets and all controls for a (substantial)
added cost. This drive will run a 1/2 HP 220 V 3-phase motor. It produces
up to .85 KVA, 2.5 A per line, with a 150% overload capacity for 15 seconds.
It is convection cooled, and designed to operate up to 40 C ambient. It is
7.4 x 6.7 x 2".
- Note that I have no connection with this company except that I am excited to
have found an affordable inverter after looking for years.
[email protected] (Paul Amaranth)
rec.crafts.metalworking 24 Apr 1995
- > My phase converter consumes around 1KW at idle, so the overhead is
> around 7.5 cents an hour for me. Conceivably, it might cost me
> 15 or 20 cents an hour to run my mill. If I did that four hours
> a day, every day (I wish), it would cost me around $18-$24 a month.
- >How did you measure the 1kW power? Is it voltage * current * cos(phi)
>or just voltage * current?
- I knew somebody was going to ask this.
- I used a clamp-on ammeter to measure around 4A on the 220 input line, so the
1KW figure is a generous upper limit (I am aware of the phase angle
correction, but I don't have a power meter handy). This was in response to
the quetion about how much it costs to run one of these things, and the
answer is not very much. Even at this rate I have a lot of machining I can
do before I get anywhere close to break-even on 3 phase installation. At
the rate that I'm currently getting into my shop, I should make that point
in around 3 centuries :-)
- One of these days I'll run around in back and look at the power company
meter and see if I can track what it really draws.
- There have been a number of comments about the phase angles not being
correct between the 3 phases. This has piqued my curiousity and I will see
about getting some 220/12 transformers so I can (safely) feed it into my
scope and see what's going on. I'll report back on that when I get some
info, but don't hold your breath.
[email protected] (James W. Kaiser)
email 24 Jun 1996
- -> phase, well, two phase, shop. (As a work around he "borrowed" a
-> third leg from his neighbor! I have no idea how they figured which
-> leg was the one he was missing.)
- > Bizarre! That's a whole new one to me, and I'd sure like to hear
> some more about it if you could get a rundown during your visit.
- There is 3-phase service in the area, but drops are too expensive for the
limited use. Apparantly, Bruce has phases A and B, and his neighbor has
phases A or B, and C. They found a way to tell which was C. I will find
out how, it may have been luck... I hope not.
- > I have only a tenative grasp of how all this alternating current
> stuff works, mostly because the theoretical stuff all turns into
> heiroglyphics by page three, and at the practical end it's
> simplified down to "plug it in." On the other hand, it
> took Charles Steinmetz years to figure it up, and when he published
> his first papers both Thomas Edison and Nikola Tesla told him it
> would never work...
- Then the discussion on the Scott Connection will be like Eygptian to you
too. It was to me, but I'm just an economist working in the electric
utility consulting field.
- -> have an article on a "Scott Connection" which uses two transformers
-> wired together with strategically placed taps to get three phases
- I also attached below an email message I got from the buddy who told me
about the scott connection. He speaks eygptian. There is an ascii diagram
of teh transformers used to do the conversion. The math is in the article.
- -> heard of, or used teh scott connection? Or, how did you do it with a
-> motor and how big a motor does one need? He only needs about 3 hp
- > I bought a Phase-O-Matic solid state convertor; it was about $220.
>Unfortunately it can't handle the start-up load of my Coats wheel
>balancing machine. The Phase-O-Matic is a pretty common item, most
>machine tool places carry it.
- > The instructions for the Phase-O-Matic talk about using a three
>phase idler motor in cases where startup load is high. The idler
>motor should be at least 1-1/2 times the power rating of the largest
>motor that would be run off of it. For example, if you had three
>2hp motors, a 3hp idler would be fine, as long as you didn't start
>them all at the same time. They can all *run* at the same time,
>they just can't start at the same time.
- > I found out later many people just use the idler motor alone. The
>trick is, they have to wind a rope around the shaft and spin it up
>like a lawnmower before applying current; something to do with how
>the windings are. Anyway, you pull the rope, flip the switch to feed
>it ordinary 220, and it should come up to speed. You hook A and B
>(or X and Y) wires to where the 220 input wires go, and the third
> wire out of the motor (C or Z) has the third leg. Still sounds
>like magic to me.
- Hmmm, magic, yes.
- > Anyway, the expensive Phase-O-Matic will at least start the idler
>motor, so I'm not totally screwed. Some people use a 1/3hp or so 115v
>motor to spin up the idler motor.
- Rube goldberg sounds just fine for Bruce who lives way up in the woods above
the russian river with the rest of the ex-flower children. He's a hell of a
woodworker though.
- > >derek 4 May 96
- > >Two phase to three phase (and 3 to 2) conversion can be done with a
> >pair of transformers one of which must have a center tapped winding.
> >If a 3 phase neutral point is needed then the other transformer must
> >also have a winding tapped at 0.866 (this is the tricky part to
> >arrange). The method of connection is called the Scott connection.
- > Derek: I don't understand the tapping as described in the writeup
>
> -------------------------$ A3
> $
> $
> $ $-------- a2
> $ A2 $
> $ $-------- a1
> -------$ A1
> |
> -----------------------$ B3
> | $
> | $ $-------- b2
> -------$ B2 $
> $ $-------- b1
> $
> -----------------------$ B1
- > The text says that a center tap is needed on one (B2 I guess) and that
> a tap at .866 is needed on the other (A2??) It later says that a
> 3-phase neutral could be provided by tapping "one third of the way
> along the winding from A1 as indicated." I don't get it. .866 is not
> one third of the way along the winding, and it is not quite 2/3 of the
> way from teh middle of the winding. What else would A2 be used for?
- I may have confused you. Transformer A1-A3 has 0.866 the secondary turns as
B1-B3. This follows from vector diagram 4.46b. The neutral point A2 is 1/3
A1-A3. The text is a little confusing but the math is clear (they talk
about a .866 tap but mean relative turns to N1), just look at the right
triangles in 4.46b. You want the neutral point N to be equi distant from
A,B and C. NB2/NA3 = sin (30) = 0.5 This gives the neutral tap at 1/3
- > I guess to convert 220V single phase to 220V 3-phase with similar
> amperage ratings on each side one would use transformers that had
> sqrt(3)/2 less windings on the single phase side than on the three
> phase side.
- Note the transformers are not identical turns ratio. Also you have to be
careful when quoting 3 phase voltage - are you talking about between the
phases or between each phase and neutral.
- > I also guess that the two 110V single legs would get connected to a1
> and b1 respectively to get 220V single phase, and a2 and b2 are
> connected to the neutral bus, or not? That would not seem to me to give
> 220V input, or should 1 leg be hooked to a1 and b1 and the other leg
> hooked to a2 and b2?
- I don't think you have this right, the two transformers must have their
primaries in parallel so that the magnetizing fluxes are in phase (this is
not the case with a series connection). The primary neutral has nothing to
do with it. For a 220 input you need a transformer with the right turns
ratio, most big transformers do have a split primary so that they can be
configured either series or parallel for 110 or 220.
Jim Kaiser
email 09 Aug 1996
- > Have you found a reasonably priced source for three-phase connectors yet?
- Three phase what?
- >I'm seriously thinking about just running the cords through a strain
>releif in a box cover plate and just wirenutting the things to the shop
>wires.
- I would probably get 3 (4 if running a separate ground) junction boxes and
wire nut one phase per box, just to make sure there was enough volume per
box. I've got a copy of "Wiring Simplified" acquired from the local
hardware store. It is almost devoid of 3-phase discussion, but does have
some interesting comments on running 3-phase motors with phase converters in
teh motors section. It also has information on the number of wires you can
put in different size boxes, but assumes your basic 3-wire single phase
household wiring. Wire gauge is also important, of course.
[email protected] (Adam Karpowicz)
rec.crafts.metalworking 30 Apr 1995
- >I thought that maybe there was some critical speed I had to get up to so
>I connected a 1/4 hp single phase motor back onto the smaller 2 hp 3
>phase motor with a v belt and spun it really fast before I threw the
>switch. Same thing, motor came to a halt, vibrated and the breaker
>tripped.
- I found some 3ph motors just won't start this way, I do not know why..
- >I next tried connecting a 50 microfarad capacitor (for motors) between
>one of the 220 single phase wires and the remaining 3rd lead on the 3
>phase motor. The motor spun slowly, and then the breaker tripped after
>about 5 seconds. I connected another 50 microfarad capcitor in parallel
>with the first capacitor, and threw the switch again. The motor spun a
>little faster, and then the breaker tripped after 10 seconds.
- >Presumably, if I put enough capcitors on, it will work, and hopefully the
>breaker will stop tripping. However, I haven't been able to get the
>3 phase motors to work as easily as other people have been reporting.
- >Any insights would be greatly appreciated.
- You need about 75-100 mF (at a rated voltage) per 1hp to start a 3ph motor.
My 3ph 3hp starts without any problems with 250mF.
[email protected] (Gale Carlisle)
rec.crafts.metalworking 1 May 1995
- >Presumably, if I put enough capcitors on, it will work, and hopefully the
>breaker will stop tripping. However, I haven't been able to get the
>3 phase motors to work as easily as other people have been reporting.
- Home Shop Machinist suggests on page 38-39-40 January-Febuary 91 that you
need at least 100mfd per idler horsepower. Fine Woodwork-ing Volume 4
Hardbound- pages 33-37 . They suggest 150mfd for a 1-hp motor to 650 mfd for
a 5-hp motor. Both of these publications will answer just about all your
questions that you may have.
[email protected] (Dave Williams)
shop-talk 10 Mar 1997
- -> Bottom line: What is the benefit of Three phase? Can you get more
-> "cluck for your buck" with three phase? i.e. is it more cost
-> effective to run a one hp motor on standard 220 or 3P 208??
- Three phase motors are cheaper, lighter, more efficient, easily reversed,
have high starting torque, and will give you fresher breath and whiter
teeth. Unfortunately three phase power can be hard or expensive to get;
AP&L only wants $15,000 to run it to my house, so I have a rotary convertor
for the milling machines and spin balancer.
- WRT the question of "what is three phase" someone posted earlier, here is a
simplistic (because that's the best I can do) explanation:
- Electricity is produced by three phase alternating current generators. The
"three phase" means you have three single phase AC generators wound on the
same armature; each one is 120 degrees from the other.
- Normal 110v wall current consists of one leg, or phase, and a neutral, or
return, wire. A 220v socket uses two legs and a return. You can split a
220v socket into two 110s. A three phase socket uses all three legs.
Damned if I know why that doesn't come out to 330v instead of 220, but
alternating current gets really seriously strange and there's not a whole
lot between "hook wire A to wire B" and heiroglyphic math.
- Solid state phase convertors use two 220v legs and "synthesize" a third leg
to drive three phase equipment. The legs are all supposed to be 120 degrees
apart; hearsay picked up from reading the periodic Phase Wars in
rec.metalworking indicates not all convertors manage to produce 120 degree
phases. The results, from various self-proclaimed experts, range from
"makes no difference" to "thermonuclear meltdown."
- The cheap solid state phase convertors lack start-up power, particularly for
equipment like my Coats spin balancer. My Brown & Sharpe horizontal mill
not only has less starting load, it even has a clutch, which is seriously
unusual on a mill. So to get start-up power you need a rotary convertor.
Underneath all the mystery, a rotary convertor is just a three phase motor
with a solid state convertor. The convertor lets the motor self-start, and
the motor (sometimes called an idler motor) does the actual phase
conversion. Lots of people dispense with the electronic gadgetry and spin
them up with a piece of rope before switching on the juice.
[email protected]
shop-talk 11 Mar 1997
- >Bottom line: What is the benefit of Three phase? Can you get more
>"cluck for your buck" with three phase? i.e. is it more cost effective
>to run a one hp motor on standard 220 or 3P 208??
- I believe you can get more power for the same size wire from 3 phase, but I
have tried to work it out mathmatically.
- Another advantage is less pulsing; single phase (and I include 220vac in as
single phase, because that's how it is used in AC motors) has 120 dips (0
volts) every second-the motor rotor is not being pulled at this time.
(because of this, many AC 110vac motors can go backwards if the starter
coils are wired backwards-the magnetic field is the same to the rotor
spinning either direction).
- 3 phase has distinct peaks between all 3 pairs of the three wires-as someone
else said, 120 degrees apart (or around 2 milliseconds, I think). 3 phase
has no point at which there is no voltage or current flow, and
smaller/higher frequency pulses in the rotor. because the rotor is always
being pulled forward, three phase motors can be smaller for the same power.
(and I think they are more efficient, but I don't know that for sure)
- It's a lot easier to see this with graphs and drawings, but that's the idea.
(and I'm doing a really poor job of explaining this-maybe someone else can
do better (if anyone cares)
[email protected] (Steve Flatt)
shop-talk 12 Mar 1997
- There's another explanation of 3-phase at:
http://landau1.phys.virginia.edu/Education/Teaching/HowThingsWork/
- Use the search facility there to look up "three phase".
Mark J Bradakis
shop-talk 14 Mar 1997
- > >>Can someone give me a simple clear explanation of 3phase electricity?
- > It's easier with a graph but bear with me.
- Well, I can even do that. Due to the nature and timing of the 3 sine waves,
the resultant power curve looks like this:
_____________________________________________________
- That's right- flat, with no peaks. The highs & lows of all 3 phases cancel
out to make a nice, constant power output. The result? With the peaks of
single/dual phase, you get pulsing at the line frequency, which can be seen
in the final work. With 3 phase, it's smooth, like DC.
[email protected] (John T. Blair)
email 15 Mar 1997
- Nice explination!
- > Damned if I know why that doesn't come out to 330v instead of
> 220, but alternating current gets really seriously strange....
- The reason for this is because of the phase difference, you just don't
numericall add the 3 voltages, you have to do it algebraicaly to account for
the phase difference. But who cares, you explination has right on.
Bob Hale
Hotrod 20 May 1997
- > [ Three phase can be a problem, but it is not impossible to make it.
> The Mitsubishi phase converters discussed on and off for the past year
> on rec.crafts.metalworking seem to work quite well. But they are
> certainly not free. --FEP ]
- I thought that Dave wanted to use the motor as a generator, to soak up the
power from an engine. If so, he'll be making his own 3-phase power. If
not, then there are a variety of ways to create pseudo 3-phase power from
single phase power; some work better than others but running a motor is
usually one of the easier things to do. For example, a capacitor can be
used to generate a 90 degree phase shift for one leg, and the motor is
presented with 0, 90, and 180 degree power - certainly not optimal, but
frequently usable.
- BTW, there are a few residential places where the power drops are two legs
from a 3-phase distribution system. If the legs are labelled A, B, and C,
then succeeding houses are connected to AB, BC, CA, etc. People who live in
such areas are cautioned that they don't have 240 volt power - the voltage
between the two hot legs is 208, while the voltage between either hot leg
and neutral is 120. If you happen to live in such an area then it is
inexpensive to get true 3-phase power installed.
- The most commonly found power is single phase. Most residential service
brings in a center tapped 240 volt single phase feed, and the voltage from
either side to ground is 120 volts.
- There are no utility 2-phase power systems. Two phase power has a 90 degree
angle between each leg, and is not as efficient as 3-phase power, so the
utilities don't use it.
- [ Bob is correct in all accounts here, but I'd rather not drift off into
phase theory and application, so lets cut the discussion here. --FEP ]
[email protected] (Jonathan M. Elson)
rec.crafts.metalworking 9 May 1995
- : Or, in a small shop, one tool at a time setup, perhaps you could just have
: say, a 5 HP single phase motor drive a 5 HP three phase motor used as a
: generator, which then drives a 3 HP # phase load. That ought to give you
: true three phase?
- No, you can't use an induction motor as a generator. There is no energy
supplied to excite the rotor field. An induction alternator can only be
used to bridge onto an existing power main, in this case, you would already
need to have 3 phase power.
[email protected] (Paul Amaranth)
rec.crafts.metalworking 10 May 1995
- >>I've heard from a few people that using a phase converter can cause
>>problems for CNC machines.
- >At least some of the commercial vendors offer 'CNC compatible' models - I
>remember seeing reference to them in the spec sheets when I was shopping
>for a convertor. I have no idea what the differences are.
- I believe the real point of CNC compatible models is regulation. It's
possible for an ordinary rotary phase converter to be down 20% or worse on
the generated phases under extreme load conditions. If your electronics
happen to be running off that phase, you could have serious consequences.
Computers can get pretty flakey when their power gets marginal.
- Ordinary 3 phase motors are much more tolerant of power variations and phase
differences. I have heard, but haven't confirmed, that newer motors are
less tolerant than older ones since they are designed to tighter specs, but
unless you have a commercial operation and are running the machines 10 hours
a day, it's unlikely you'll see any problems.
[email protected] (Mark Anderson)
rec.crafts.metalworking 12 May 1995
- : >I believe the real point of CNC compatible models is regulation. It's
: >possible for an ordinary rotary phase converter to be down 20% or worse
: >on the generated phases under extreme load conditions. If your
: >electronics happen to be running off that phase, you could have serious
: >consequences. Computers can get pretty flakey when their power gets
: >marginal.
- : It would be very foolish to hook the CNC control electronics to a
: generated phase of a convertor. It would be hooked directly to a separte
: line, very likely a single phase 120volt line, so should have nothing to
: do with the 3 phase 240 volt to run the drive motor. Almost all CNC stuff
: these days is PC based, and Ive never seen one of them with a 3 phase 240
: volt supply.
- I ran into this problem with my LeBlonde lathe. It has all sorts of cool
electrical features (implemented with relays) to detect things like power
fail. I run it off a 15HP rotary converter (the lathe is 7.5) at 220V.
When I put it all together, the lathe would start to turn, then idle to a
stop. After scratching my head for a while I rotated the phase wires 120
degrees and then the lathe would spin up. My guess is that the power fail
detect stuff was connected to the synthesized leg and this would droop more
than the real power coming in.
- (A point about my personal electrical supply, I only have a 60amp feed for
the entire house and garage which was probably installed by Thomas Edison as
a high school prank. When I spin up the lathe, my neighbors know about it.
If I had a little more current, things may have worked better.)
- Like Ron said, you probably don't want to hook the control electronics to
the generated leg. I wouldn't feel too bad about connecting up to the real
leg though.
Mike Rehmus
rec.crafts.metalworking 18 May 1995
- >Although I did not measure any efficiencies the start-up time of the load
>motor was perceptibly longer with the idler than without it and since I
>couldn't tell any difference in operation during a week trial period I
>removed the idler for lower noise in the shop. Have I given up anything
>with this approach?
- I use a 'heavy-duty' static inverter on a 1.5 hp Bridgeport. I can stall
the motor fairly easily compared to the same model of machine on real 3-
phase. Also, one cannot start the mill at much beyond 1/2 speed or the red
light comes on (on the inverter) and the relay drops the load out. A real
bother to have to always reduce the speed to prevent dropping the load on a
restart. This means that instant reversing is not really available either.
The idler motor would seem to solve some of these problems. Why don't you
give it a test and give us a report?
[email protected] (Peter Northway)
rec.crafts.metalworking 25 May 95
- >Why are people trying to convert 3-phase to 1-phase via a converter?
>Would it not be just as cost effective to replace the motor with one of
>the proper frame and hp rating? Or adapt one?
- I don't know about the US but here in New Zealand a new 1.5 HP single phase
motor runs about $NZ350 and they are like hen's teeth on the second hand
market. A 1.5 HP three phase motor can be readily aquired second hand for
about $NZ35 one tenth of the cost. Three phase is also very expensive to
have installed in a home workshop, if you can convince the power authorities
that you need it. Bear in mind that here single phase is 220V and three
phase is 440V.
Steve Wellcome
rec.crafts.metalworking 13 Jun 1996
- With a 3-phase motor, "instant" can be pretty instantaneous.
- For industry (with real 3-phase power), people quite often push reverse/off
to stop the motor, so the machine will stop quickly. After all, time is
money and all that.
- In a home shop...who cares?
[email protected] (Jonathan M. Elson)
rec.crafts.metalworking 28 Jul 1995
- There was a thread some months ago about various schemes to convert single
phase to 3 phase power. I found out about a very inexpensive inverter from
Drivecon Corporation. I finally ordered it, and have tried it out. It is
their EC400C model, list price (US) $275.00 in SINGLE QUANTITY! It
definitely works! It is rated at 1/2 HP, which may be a little too small
for some machines, but is a perfect match for my old bridgeport. I had a
Cedarburg "Phase Converter" which is no more than a starting capacitor, and
it was not able to start the motor when the belt was on the highest speed
setting. It also was easy to stall the motor on the 3 higher speed belt
positions. On the next to the highest speed position, 2100 RPM, I could
stall the motor with my bare hand on the spindle! Using the Drivecon, it
might be possible to stall the motor on the highest speed, but it would
cause a severe burn on the skin. Clearly the motor is producing much more
power on three phase than on single phase.
- The drivecon EC-400C is a bare bones version, with a circuit board mounted
on a 7" x 7" heat sink. It takes either 110 or 220 Volts single phase in,
and produces 230 V (L-L) 3 Phase output (up to 850 VA) at variable voltage
and frequency. It can go from 0 Hz up to 120 Hz, although you get less
torque above 60 Hz, because it can't increase voltage above 230.
- You set min and max speeds with pots on the board, as well as ramp time,
overload current trip and starting torque boost (up to 30% above max output
voltage). You set the desired speed with a user provided 10K Ohm pot, and
start and stop the motor with a user provided switch.
- I have noted that the motor runs warmer at idle than when running on single
phase. This is probably because this inverter produces square voltage
waves, not sine waves (or even an approximation). The current waves are not
sinusoidal, either. I will have to do some real cutting, and see whether
these wave shapes cause excessive motor heating. If the motor heating is
not a problem, I will really appreciate the improved torque!
- Drivecon can be reached at (708) 918-1406, they are in Libertyville, IL.
Andrew Mawson
rec.crafts.metalworking 28 Jul 1995
- > There was a thread some months ago about various schemes to convert single
> phase to 3 phase power.
- If it is of interest I've just completed a 7.5 KVA single to 3 phase
converter to drive my MOOG 1000 Hydrapoint NC Milling machine. The machine
draws 6.6 KVA, main load being a 5 HP hydraulic pump.
- I ruled out subtle electronics and opted for a transformer / capacitor bank
solution with a 5 HP pilot motor running unloaded. It works very well in
this application, as did a previous 10HP version I made for a compressor,
though I don't use a pilot motor on that one.
- My converter needed to go from 230V single phase to 415V 3 phase which I did
with the following :
- Transformer was originally 220V to 220V isolating rated at 7.5 KVA each
winding being centre tapped at 110V. I connected the primary and secondary
in series, and feed 230V in across 110V worth of each winding thus:-
_415 Live 1
[
[
_230 Input Live
[
[
- no connection
[
[
_230 Input Neutral
[
[
_ 415 Live 2
- (The top and bottom legs need a few turns removing to reduce L1 / L2 to 415
from 460)
- 415 Live three is connected through the capacitor bank to 415V live 1, and
is divided into 2 groups. Group 1 is the 'run' capacitor and is permenently
connected. Group 2 is the 'start capacitor' and is fed through a contactor
switched by a voltage sensitive relay that looks at the 415V Live 1 to 415V
Live 3 voltage. When this gets up to about 400V the contactor drops out,
leaving only the run capacitors in circuit.
- For my application I needed a total of 10 capacitors rated at 30 uF 440v AC.
connected as 90uF 'Run' and 210uF 'Start' (In fact I have a 3 way switch
that can switch in more or less start and run caps) Each Capacitor has a
220K resistor across it to discharge when left idle.
- The pilot motor is straight across the three lives, and hums to itself in a
corner. This gives a far better balance between the phases. If your machine
has its biggest motor running ALL the time the machine is on then you don't
need the pilot motor, but if like a lathe, the motor is on and off often,
smaller motors like the suds pump will burn out if the phases are
unbalanced.
- If you make something like this you MUST know what you are doing with
electrics. The voltages concerned can kill you VERY quickly. Good earth
connections. Over rated cables and insulation. Housed in a case that fingers
can get nowhere near !
Mike Rehmus
rec.crafts.metalworking 11 Aug 1995
- I am confused after reading a few 3-phase motor labels:
Bridgeport 1-1/2 horse motor - 4.8 amperes at 230 volts
2.4 amperes at 440 volts
2.2 amperes at 460 volts
These numbers work out to 1056 watts power consumption which, ignoring
efficiency seem to give us about 1.42 HP
- If we assume that the efficiency is around 70%, the apparent HP is more like
1. If we take account of your square root of 3 factor, Jim, we end up with
too much calculated HP.
- Also, where you might be correct in a delta-connected motor, in a wye-
connected motor, all the current has to go through a single leg, or can we
treat the current summations in that manner?
- Just to confound things more, the Lima motor on my Rockford Lathe has a 1-
1/2 HP rating and a label values of 220 volts and 15 amperes or 3300 watts.
That seems excessive unless they are considering the parasitic power
consumption of the attached 4-speed gearbox.
[email protected] (Jim Hanrahan)
rec.crafts.metalworking 30 Aug 1995
- I have followed the phase converter threads over the last couple months and
have received some helpful advice. I have just built a converter using a 5
HP three phase motor and have begun some interesting tests balancing the 3
phase currents (and voltages) using capacitors across the motor terminals. I
have put two graphs of my data (so far) where I compare putting capacitance
between only two of the three lines (Line 1 and 2 being the single phase
input, line 3 the generated third phase), and putting equal capacitance
between lines 1-3 and lines 2-3. These available by anonymous ftp
oasys.dt.navy.mil in the /pub/ directory. They are in GIF format.
phase_15.gif is using capacitance between lines 1&3 only.
phase_16.gif is using capacitance between lines 1&3 and lines 2&3.
- This shows that putting capacitance between both lines is better than only
one.
[email protected] (Joseph Hartvigsen )
rec.crafts.metalworking 11 Sep 1995
- > I am looking for information on taking a large three phase motor and
>making a generator out of it for emergency backup power.
- I have read that by connecting 3 capacitors between the three legs of the
induction motor the residual magnetism in the motor coupled with the
capacitors will allow self excitation. The electric load must not be
connected before the motor/generator is up to speed and voltage. If there
is not sufficient residual magnetism for excitation the article said to run
DC through a pair of leads for a short time. The voltage generated varies
with the size of capacitors used. Capacitors should be motor run type, not
motor start electrolytics. Start by sizing them for to the amount required
for power factor correction for the induction motor. Grainger is a handy
source for these capacitors. The article I read was from the Home Power
archives at:
Sunsite.unc.edu pub/academic/environment/alternative-energy
vol 3 pg 17 by Paul Cunningham
- I have plans to try this for a small hydro system soon. Let me know if you
run into any problems.
john haynie
rec.crafts.metalworking 30 Sep 1995
- I took a quick look in the Thomas Registry of companies
http://www.thomasregister.com
searching on key words compressor and gilbert. Nothing came up.
- I've been saving everything that I've seen in the "3-phase from 1-phase"
thread for the last year or so. I can send you the whole thing or specific
bits if you like.
- One nifty thing that I learned from an older book on electrical
installations and motors was not mentioned in that thread. It turns out
that you can increase your capacity to generate 3-phase by using any 3-ph
motor in your shop. In fact, you don't even have to buy a separate 3-ph
idler (see below).
- Single phase comes in to the shop and is connected to two legs of your 3-
phase idler. The first motor (your idler) needs a starting mechanism,
either a 1-ph motor and belt to get it up to speed or capacitors. Now
you're sending 3-phase to the rest of the shop. Any 3-phase motor should
start on its own. If you need more capacity for a particular motor, start
up one of your unused 3-ph machines and let it idle. Now your initial idler
and the idling machine will both generate the third leg, spreading out the
current draw in the windings over both. In fact, you don't even need a
dedicated idler since the job can be done by any 3ph motor in the shop as
long as it is large enough to start the next one, has some means of
starting, and can be run without a significant load.
- This is what it looked like in the book, obviously overcurrent protection
and switches are omitted for clarity.
- leg 1
----+-----------------------+------------------+-----------(ad inf)
1ph | leg 2 | |
----)---+-------------------)---+--------------)---+---------(ad inf)
| | leg 3 | | | |
| | +---------------)---)---+----------)---)---+--------(ad inf)
| | | | | | | | |
(3ph idler) (3ph motor) (3ph motor)
(not self-starting) (self-starting) (self-starting)
(generating 3rd leg) (generating 3rd leg) (load)
- I have to say that it all looked pretty obvious in the diagrams in the book
but I'm not an electrical engineer so don't take my word for it. Get
someone else's advice on how to burn down your shop. Do not try this
without approval of your local inspector, it may be illegal or unsafe. ;<}
[email protected] (Roger Mitchell)
rec.crafts.metalworking 4 Dec 1995
- For those who wish to connect a 3 phase drum switch to reverse a single
phase motor, the following show the connections for both.
- Typical drum switch connections for 3 phase operation
Drum Switch
Forward
Phase A _______________ ___ ______________________
Line Load
Phase B _______________ ___ ______________________
Phase C _______________ ___ ______________________
Drum Switch
Reverse
Phase A _______________ ______________________
\ /
\/
/\
Phase B _______________/ \______________________
Phase C _______________ __ ______________________
- Single phase split phase motors have 2 windings: start and run. The start
winding usually has a capacitor in series with it to provide the phase shift
to start the motor, as well as a centrifugal switch to cut out the power to
the start winding after the motor is up to speed.
Typical 3 phase drum switch being used for single phase motor.
Drum Switch
Forward
_________________________________________________________________________
|_______________ ___ ______________________ |
|
Start Windings Run Windings
Line _______________ ___ ______________________ |
| |
| |
________|_______________ ___ ___________________________________________|_
Drum Switch
Reverse
_________________________________________________________________________
|_______________ _______________________ |
\ / |
\/ Start Windings Run Windings
/\
Line _______________/ \_______________________ |
| |
| |
________|_______________ ___ ___________________________________________|_
- In Bill's case his lathe has two 3 phase contactors, one wired to produce
the foward combination and one wired for the reverse combination. In that
case, what needs to be done is to find the one terminal set that is wired
the same on both contactors versus the other 2 sets which reverse polarity
between contactors and connect them up the same as what the above digram
shows for the drum switch. Also, you will need to make sure that the
connections that supply the power to the contactor coils is connected across
the 220 volt input terminals that are being used.
[email protected] (Jonathan M. Elson)
rec.crafts.metalworking 26 Jan 1996
- : I am trying to get a hold of a schematic for a circuit to run a 3 phase
: motor on 2 phase converter. I recently saw a posting that said they
: were available at web site http://plains.uwyo.edu /"metal, when I try
: access it I recieve an error- forbiden. Can any one provide me with the
: schematic, or tell me another way to get them. Thank you.
- What do you mean by 2 phase. Do you mean 220 with a neutral? That is not
really 2 phase, just center-tapped single phase. REAL 2 phase has a 90
degree phase shift between the two phases, and it CAN be converted to REAL 3
phase with a simple center-tapped transformer, called a Scott-T. But you
probably don't have 2 phase power with the 90 degree phase shift. The so-
called phase converters are really a starting capacitor and relay. it drops
out after the motor starts, and you get very little torque from it. I used
a cedarburg converter on my 1/2 HP Bridgeport for years, and it was a real
trick to do any roughing cuts without stalling the mill. I finally got a
Drivecon 1/2 HP variable speed drive (which takes 1 phase in) and now the
motor never stalls. I used to be able to stall it by holding the spindle
with my hand! I don't want to try that trick now!
- Unfortunately, the Drivecon gets expensive for the larger models.
[email protected] (Paul Amaranth)
rec.crafts.metalworking 26 Jan 1996
- >Unfortunately, the Drivecon gets expensive for the larger models.
- Using a 3 phase idler motor generates "real" 3 phase within limits and
allows you to get close to 100% of the rated power on your tools. The metal
web article is an excellent treatise on this subject. You can probably
build one for $100-$150, maybe less depending on how fancy you want to get
and what you can scrounge. As somebody else pointed out the URL is
http://tbr.state.tn.us/~wgray/index.html
- I have absolutely no interest in trying to stall my mill (1hp Bridgeport)
using my hand :-)
- The variable speed inverter is a really neat technology which gives you
infinitely variable speed. If you can afford it, this has a lot of
advantages. I did some looking into this a while back to see if it was
reasonable to build one from scratch. I concluded that it's possible, but
going much over 1HP would get difficult (i.e. expensive). These things work
by changing your 220AC into DC and then synthesizing the correct three phase
waveforms. You can cheat a little by making a stepped square wave instead
of a true sinewave without getting into too much trouble, but the more
sophisticated devices use pulse width modulation techniques to get the
correct Voltage/Current characteristics as seen by the load as well as
varying the frequency with the motor speed. Controlling 220V at 15-20A is
not trivial (in other words $$$)
[email protected] (Jonathan M. Elson)
rec.crafts.metalworking 28 Feb 1996
- : >Can anyone recommend a good static single-to-3 phase converter?
: Some 3-phase motors can be started with a pull-rope off of 220 single
: phase, too. You lose some power and top-end performance but most
: hobbyists don't need or use that much of the machine.
- I used a cedarburg converter, which connects a starting capacitor, but then
runs the motor on single phase. It was not very good. The motor could be
stalled easily with the BARE HAND! Instead of the nominal 1/2 Hp, it must
have been closer to 1/10 Hp! I stalled it several times a day. The motor
wouldn't even start with the belt on the highest speed pulley step. I now
have a variable frequency static inverter, and I've never stalled the
machine since. A BIG difference!
[email protected] (Bob Bickers)
rec.crafts.metalworking 16 Mar 96
- >I presently own a 1.5 hp, largest motor capacity, Roto Phase brand rotary
>phase converter. This runs my Bridgeport and my surface grinder, both
>are 1hp. and I only use one at a time, this is a hobby shop and I am the
>only operator. I have a chance to purchase a better mill but it has a
>2hp. motor. My question is: will the 1.5hp roto phase run the 2hp mill
>or will I burn some thing up? I can afford the mill but not a mill and
>another phase converter, I may be able to afford a static phase converter
>but I am not sure how well they work. Thanks for any input.
- Two thoughts, check recent Metal Projects for artical on making your own
rotary rotary converter, $100 or so, or install amp meter on line and adjust
feeds and speeds to keep the numbers in a safe range. Cats are made for
skinning.
[email protected] (Ron Ginger)
rec.crafts.metalworking 2 Apr 1996
- >I will be aquiring another Mill a Brown and Sharpe #2A Universal, and I
>think it has either a 5hp or 7.5hp 3ph motor on it. My rotary phase
>converter that I have at present is only rated to start a 1.5 hp motor, I
>plan to make a phase converter out of another 7.5hp 3 ph. motor but this
>motor runs at 1800 rpm. My question is does the rpm of the generating
>motor make any difference?
- NO, the rpm is not important, as long as the motor runs at the speed it was
designed for, and on AC it must lock into the 60 cycles to run.
- >A second question is I also have another 3 ph motor with no name or hp.
>plate on it, is there a way to tell the hp of the motor? Thank you.>-
- No real way to tell just by looking, but the difference between a 1hp and a
5hp ought to be pretty obvious. But you cannot tell a small difference,
particluarly with old and new motors. Some new motors, with better magnetic
alloys for construction are MUCH smaller than old motors.
Larry Matthess
rec.crafts.metalworking 4 Apr 1996
- Has it really been established that the idler motor speed does not matter?
In Jim Hanrahan`s article `Building a Phase converter` he states that: `. .
. . the higher speed might produce slightly better phase angles, but the
lower speed is generally easier to start`.
- Not having found a source for used motors, am considering a new motor (for
about $200) and thus can choose low or high speed. Am not now sure which to
choose.
[email protected] (David Erickson)
rec.crafts.metalworking 7 Jul 1995
- Following up on the phase-converter thread from about a month ago, I
experimented last weekend with adding start capacitors to three phase
motors. I ran 220V single phase to two of the three motor connections and
connected a start capacitor between one of 220V legs to the third motor
connection. The results I got were similar to what had been reported:
- small (less than 1hp) motors started up fine with a 50mfd cap; larger motors
required about 50mfd per hp to start.
- BTW, I purchased the capacitors from the Surplus Center, the address for
which was posted here a couple of days ago.
- if the capacitor is left in the circuit after the motor starts, the motor
growls, indicating current imbalance, but if it is removed, the motor runs
quietly, albeit with reduced power when compared to powering the motor with
three phase electricity.
- So one solution is to use a push button to supply the capacitance and start
the motor. However this has drawbacks. If the button is not pressed
immediately when power is supplied, the direction of rotation is not
predictable. Also, if the motor stalls, it will not restart automatically
when rpm falls.
- What I need is some kind of relay to detect startup or stall conditions and
engage the capacitors. Startup conditions are easy: use a timer relay. How
do you detect stall conditions? What would be an easy, fairly cheap device
to add that would reliably restart the motor when needed?
[email protected] (Ron Ginger)
rec.crafts.metalworking 7 Jul 1995
- >What I need is some kind of relay to detect startup or stall conditions
>and engage the capacitors. Startup conditions are easy: use a timer relay.
>How do you detect stall conditions? What would be an easy, fairly cheap
>device to add that would reliably restart the motor when needed?
- I have had exactly this running for more than 5 years, and I only recall
once it starting in reverse. I dont ever recall stalling the motor.
- I mounted the toggle switch right next to the push button, I put my thumb on
the PB first, then flick the switch with my finger, and after a few seconds
release the PB. Its now about a reflex action to reach up and start it.
- I dont think its worth the extra effort of adding a relay.
[email protected] (Dave Williams)
[email protected] 30 Dec 1997
-> run ok, but the pump motors require more start torque. A friend gave
-> me an old motor type "rotophase" unit, but nothing is marked and I'm
-> not sure how to wire it up. It has a large box with capacitors in it
-> and a motor with the shaft cut off. Perhaps someone out there can
-> provide me with a diagram???
- I wound up spending a lot of money and time learning the 3-phase ropes too.
There's a tremendous FUD factor with the whole thing, and though you can
find plenty of "hook wire A to post B" or math with symbols I don't even
recognize, there's not a whole lot at an intermediate level.
- This is what I have come up with, some of which may be oversimplified:
Power is generated as three phase. The generators are actually three
electrically separate units sharing a common shaft, 120 degrees out of
phase. They total 220 volts. Their output is a sine wave. Since the three
legs are 120 degrees apart, their sum is always 220 volts no matter what the
level of any particular leg.
- What you usually get from the pole is "220", which is two legs. I suppose
you could call it two phase, though the electrical types get affronted if
you refer to it that way. The 220 is separated into individual 110 volt
legs for the wall sockets. This is single phase power, the ordinary stuff
you plug lamps and stereos into.
- The 110 volts is the single leg referenced to the "neutral" wire. A 220
volt circuit is two 110 legs, and 220 is the difference between the legs.
When you add the third leg to make 3 phase the difference is still 220
volts; you're just filling in the "missing" pieces of the waveform.
- 3 phase motors are common on industrial equipment because they're simpler to
make and less expensive to purchase and operate than single phase motors.
- The power up at the pole may be 3 phase if you are near a transformer
substation. If so, sometimes adjacent houses get power from different legs.
In that case you can borrow a leg from a neighbor's house and have 3 phase
power.
- For the rest of us some type of phase convertor is necessary. There are two
main types - solid state and rotary. I bought a solid state one that simply
didn't have enough startup oomph to power some of my machinery. The
instructions recommended using an "idler motor" in that case. This
basically turns the solid state convertor into a rotary convertor; the solid
state part now is just a starter for the idler.
- The simplest, cheapest phase convertor you can come up with is an old 3
phase motor rated at 1.5x or more of the largest hp rated motor you intend
to run from it. You only care about the largest, and that only for startup.
I believe the 1.5 rule of thumb is simply to keep the idler from stalling
when a motor is switched on. You can wire the idler motor to the wall, any
two legs going to both the wall socket and the machine to be driven. The
third leg of the idler motor goes only to the machine. All you need to do
is wind a piece of rope around the shaft of the idler motor, yank it like a
lawnmower to spin it up, and flip the breaker or plug in the cord to give it
220v from the wall socket.
- Since the idler is three motors sharing the same shaft, two are driven by
the 220v from the wall and the third is along for the ride, working as a
generator. That's where the "new" leg comes from. Voila! Now your machine
has 3 phase power. Many commercial rotary convertors are rated at some
percentage of the idler's rated power. I believe this is due to the
generated leg being slightly less powerful than the two joined to the wall
socket due to inefficiencies and losses inside the motor. It doesn't seem
to hurt anything. From what I've been told everything isn't perfect with
the solid state convertors either - their legs may not be 120 degrees out of
phase, for example. Whether this is meaningful is unclear since the EE
types usually draw their knives and go for the throat in that sort of
debate.
- If you don't want to spin it up each time, you can purchase a small solid
state convertor and use it as a starter. Then everything is automatic.
That's how the commercial rotary phase convertors work. And that's how a
"solid state with idler" works.
- There are lots of different ways solid state convertors can work, and lots
more on the subject overall, but I'm still not clear enough on it all to
explain much. The above will get you going.
- Motors come in fairly standard sizes - 5, 7.5, 10hp. You can sometimes find
them at flea markets, salvage stores, etc. I once found a very large pre-
NEMA (nonstandard frame) motor at a commercial motor rebuilder's shop for
dirt cheap since they had had it sitting around for years taking up shelf
space.
[email protected] (Dave Williams)
[email protected] 31 Dec 1997
-> Thanks, Dave, for the really good explanation of set up of an idler
-> motor arrangement to run three phase shop machinery.
- Thank you [bowing]. There are still a few things I'm not clear on. When I
get them taken care of I plan to write up a FAQ of some sort. Meanwhile, if
anyone else has any commentary feel free to put in your two bits' worth!
-> I would only add that the "starter" has to be wound on the idler
-> motor shaft the same way each time to make the reversing drum switch
-> on the machine work correctly.
- Whoops! Sorry about that. I've been using my $250 "solid state convertor"
to start my $65 7.5hp "idler motor" so long I forgot about the direction
thing.
-> Also, if the motor on the machine tool doesn't start immediately when power
-> is applied, switch one of the wires going to the wall with the third leg
-> going to the machine. If it's not wired as it needs to be, the machine
-> motor will not start running immediately.
- True. Note you can switch any pair of wires to reverse a 3 phase motor. I
thought that was really neat.
[email protected] (Carla Fong)
rec.crafts.metalworking 21 Jan 1995
- >I want to purchase a Hobart TIG welder (used and VERY cheap!!)but don't
>know if a 3 phase welder will work on a rotary converter like a mill or
>lathe. Will the electronics that produce HF and square wave be adversely
>affected? Any help would be greatly appreciated!
- The phase converter boxes used on motors basically turn the 3-phase motor
into a split-phase (capacitor start, capacitor run) single phase motor. It's
an inefficient setup at best.
- The 3 phase tig welder depends on having all 3 phases in place to provide
relatively good dc to the chopper (HF) unit. How well it will work will
depend a lot on how good the filtering is on the output of the rectifer
stack.
- It _may_ work, but i'd certainly want to have a return agreement on the unit
before spending any money for it. Adding additional filter capacitors may
make it work better.
- All of the above assumes a transformer/rectifier setup in the welder you are
considering. If it is a dc/dc conversion using a switching regulator, then I
don't know...
[email protected] (Mark McDade)
rec.crafts.metalworking 16 Oct 1996
- : I would agree that a second idler would help but I don't have another
: 3 phase machine and the one idler makes enough noise as is. I prefer
: my 'fix' over running two idlers all the time. If I ever run across a
: bigger idler I'll switch motors though.
- As for the noise, I ran 25' of Romex out from my shop to a heated storage
area we have in back of the house and put the idler motor out there. I put
a blue "party bulb" (120v) on one leg near the mill so I can tell when I've
left the idler on (I can hear it slightly, too).
[email protected] (Kevin AstirCS "1U" KO0B)
rec.crafts.metalworking 18 Oct 1996
- Sizing a phase converter is often a delima, as I discribe my own, but last
night I soved my own delimma, thought others might benifit.
- Last weekend I found some motors at bargain price, as mentioned in another
posting, and have built a phase converter.
- The motors I found were 5 and 7.5HP. For now, I have only a 3/4 HP lathe
(which I am still restoring) to be run, so even 5Hp was overkill.
- But my "someday dream" mill will most likely have a 5 HP motor, which (I
think) would cause grief with only a similarly sized idler.
- Partly because the 5 HP looked like a better deal (still had mfg. tags, and
had never had a pully on it's shaft) and partly because my scots blood took
over, I have built the smaller converter, to save extra caps, bigger
relays, and power waste of running a bigger idler.
- Still, I was suffering a lot of anxiety over the thought of (hopefully)
needing to repeat the exercise someday
- The solution: When/if I need more oomph, I will add a second motor, and
more run caps. Only the larger motor (or one of two peers) will need a
starting circuit, the second one can just be thrown on line, after the
master is up and running, starting like any other machine.
- I then realized this idea has a lot going for it if a shop has several
machines of greatly different power, as lathes and mills normally are.
- Because the surplus dealers I know price motors at a "per HP" rate, two or
three smaller motors are as cheap as one bigger one. In addition, the run
caps for larger converters normally require a bank of several per leg, so
these are easilly distributed among several idlers as well. (and you would
need only about the same total capacitance) With only one smaller motor to
start from standstill, the starting circuit becomes simpler and cheaper, and
the starting current requirements go WAY down.
- The only negitives I can find for this idea:
1) the need to switch the extra motors....then again , surplus contactors go
for ~$10 where I shop.
2) You might get some low frequency "beating" noise, as it would be
suprising if the motors ran at exactly the same speed. Belting them
togethor would solve that, but negate most of the advantages of the
scheme.
3) Several small motors will probably waste a little more power than one big
one...but thats only when you are using them all...assuming you use the
big machine only half the time, you'd realize a fair savings.
- If you used, say, a 5 and a 3 Hp motor, you could select 3, 5, or 8 HP of
idler as a job required. Only the 5 hp would need starting, and it could be
switched off after the 3 was running when you only needed 3 hp worth of
idler. A little hassle, I know, but motors are cheap, and if you LIKE
paying electricity bills, then you are even more of a sicko than me.
- On those monster relays for big motors: (warning...I'm a EE...just ask if
something needs clarification..)
- For a motor larger than 3 Hp, it takes a lot of capacitance to start it, and
a big mother starting relay to match. Getting that much capacitance takes
two or three of the biggest starting capacitors available. Now, if you
switch each of these capacitors through one pole of a multipole contactor,
then you can buy a much cheaper relay: A two pole, 30A contactor costs only
about 60% the price of a single pole 50A. (if you can find one!) If you do
this DO NOT wire togethor the relay side of the capacitors, as one set of
contacts would then take all the abuse...each set of contacts should switch
only 1/nth the total capacitor. The " legs" of the starting circuit are
tied togethor only at the input line, and at the middle (or "generated")
terminal of the idler
- Finally, my surplus dealer had 120V _DC_ coiled contactors coming out his
ears, but 120AC excited ones were not to be found. Four diodes (a bridge
module would work as well) and a 1 �F capacitor later I had a dirt cheap
120V AC contactor, new , in the box.. Mind the voltage ratings on the
components, and use only polypropelene, or oil filled capacitors for this.
You may get some TV interferance if you leave out the cap across the relay
coil. The cap doesn't filter out the ripple, but id does allow the diodes
to turn off at the AC zero crossing, stopping a lot of hash.
- With 120AC available there are similar tricks for using almost any available
relay. If you need help, email me...please include permission to quote you
to this NG, so I don't have to answer same question twice.
[email protected] (Jonathan M. Elson)
rec.crafts.metalworking 19 Oct 1996
- : With all the threads on 3 phase, you'd think it would all be
: covered - and maybe I've just missed this one . . .
- : Is there any reason not to throw my three phase mill head motor
: into reverse while it's running to stop it quickly? If I try it,
: the converter box relay kicks in and I'm afraid of burning it out
: so I don't actually do it, now. If I made a switch or relay setup
: that would take the converter box off the circuit after starting
: the idler motor, would there be any problem just reversing the
: mill to slow it down at that point?
- If your phase converter is big enough, and if the mains supply to the
converter is heavy enough, then you can use instant-reverse on most machine
tool spindle motors. It is a bit hard on the motor, but not too much worse
than plain starting. If your phase converter is too small, or the main
supply is too weak, then it may stall or nearly stall the phase converter
motor. The converter relay switching in indicates that the converter is no
longer running in near-synchronous operation, and the spindle motor is not
getting good, balanced 3-phase power.
[email protected] (Kevin AstirCS "1U" KO0B)
rec.crafts.metalworking 21 Oct 1996
- The clicking relay means you are using a static converter.
- These use electrolytic starting capacitors, and those are normally rated
for only 30 3 second starts/hour. Reversing-on-the-fly counts as 2
starts.
- It will live through an occasional "hot" reverse, but if you are tapping
under power, look out!
- I know of one machinist, (Milt Cumiford) running a shop out of his garage,
who blew up a static converter while power-tapping (he was using a forming
tap, and doing LOTS of holes).
- Also, remember that this is very similar situation to single phase
reversing, If the starting relay drops out before motor stops, it may run in
wrong direction.
[email protected] (Fitch R. Williams)
rec.crafts.metalworking 22 Oct 1996
- >I've got a Cedarberg Series 1 static phase converter on my
>milling machine. It works.
- >The only real drawback is the lack of power when drilling.
>I expect I'd do better with a rotary converter.
- I have a similar unit on my mill, similar experiences. I bought it because
that was the only way to power up the mill after all the $$$ I spent buying
it. The box has a 240V single phase voltage controlled starting switch, a
starting capacitor, and a bleed resistor.
- Near term I plan to add some run capacitors to all three phases on the mill.
It shouldn't cost to much and has the potential to considerably spruce up
performance for not to much money. I figure 15 to 25 uF per phase (2 HP
motor) should help it considerably. And it will still be totally silent
when ever the mill is turned off - a feature based on reading other posts
recently.
- The caps would be usable on a dedicated 2HP rotary when the time comes to
build one, or could be paralleled with additional caps to build a bigger
rotary if that was needed.
- Note, (building on Kevin's post elsewhere in the NG) that with the added
caps, and the belt off, the mill motor would become the rotary for a 1/2 hp
motor on my small lathe, or a 1 HP motor on the 12" lathe, or a surplus 2
speed motor on the drill press, etc. That modular concept of his has
virtually limitless variations once one starts thinking about the
possibilities.
- The static converter (motor starter really) is not a great way to go, but
for the last 4 years it sure has beat looking at the mill and wishing I
could turn it on. Its usable and expandable. Probably in terms of bang for
the buck, it is hard to beat as long as you don't need all the power built
into your machine.
[email protected] (Dave Shepard)
rec.crafts.metalworking 22 Oct 1996
- With regards to the large start relay needed for larger idler motors:
- I have seen lots of single pole solid state relays that are rated at 40 amp,
220v. These tend to cost around $25 in the catalogs I get. They can be
switched on 110v or 220v. This is what I had planned on using for my start
caps (got one for free). Sound reasonable? I have seen them rated up to 80
amps for about $40 I believe.
[email protected] (Kevin AstirCS "1U" KO0B)
rec.crafts.metalworking 22 Oct 1996
- >And it will still be totally silent when ever the mill is turned off - a
>feature based on reading other posts recently.
- By playing with the value of the run caps, the idler noise is greatly
reduced...it can be much less than a small fan. It actually makes a good
way to tell when the values are about right. By switching an experimental
cap in and out you can A-B tune for lowest noise....it does need a hefty
switch ,and it is hard to tell when you pass the lowest noise point. If you
do this on your mill, you will also be tuning for best finish.
- Mounting the motor with some vibe isolators to a solid base (I'm thinking
maybe a stout wooden box filled with sand?) should help also. If the rotor
is balanced fairly well, and you have quite bearings, the dominate nose
becomes the cooling fan built into the motor. Letting the motor rattle on a
concrete floor is probably worst case from a noise standpoint, but is the
setup I have usually seen.
- Fitch did mention one slick idea that hadn't occured to me: Starting a 220V
idler motor on 120VAC will greatly reduce the required starting current.
Good if you occassionally pop a breaker on starting. It does complicate the
switching however, as you need to run it at 230. I noted that this also
allows the use of hardware store 120V starting caps.
[email protected] (c.a.sherwood)
rec.crafts.metalworking 22 Oct 1996
- All this talk about 3 phase converters has made me think about an
alternative approach. I say think, because I dont need three phase power now
(yet).
- I have a large (300 lbs) three phase alternator which is rated at 15KW into
a 0.8 power factor load. I could drive this with a single phase motor and
get REAL 3 ph power. The efficiency would probably only be about 80 %
though, but the high inertia (the rotor weighs 100 lbs) in the system should
handle instant reverse.
- So if I drove it with a 5 HP motor, I could only get enough power to run a 4
hp 3 ph motor. Actually maybe even less, because I didnt include the
efficency of the second motor in that calculation. Would a 5 hp motor be
able to start that much rotor turning without burning out its start winding.
I think it would be ok, because the alternator is 1800 rpm and my 5 hp motor
is ~3600 rpm, and the 2:1 speed reduction would make it easier to start.
- Would it be possible
A) Drive the alternator with two 5hp single phase motors. This is equivalent
to asking: can I use two 5hp motors to make a 10hp motor. I tend to think
that this is not possible, because the drive motors would probably not be
in sync and would fight each other. I think the largest single ph motor
that I have seen is 7.5 hp. This might make a reasonable setup if one was
available cheaply. It should drive one large machine or several small
machines.
B) Start the alternator with a single phase motor and somehow turn it into a
phase converter. From what I know about brushless alternators, I dont
think this is possible, but I not sure. The voltage regulator senses
voltage across two legs and drives the field (rotating) to set this
voltage. The third leg follows the first two. If it was possible to
connect the line voltage to two legs and connect the voltage regulator
between the third leg and one of the line legs, it might work ok. Sounds
too risky to me though.
- I would probably be better off just getting a large 3 ph motor to build a
phase converter, but Im just thinking about what I can do with what I
already have.....
- At one time I had this alternator coupled to a 2 cylinder 30HP diesel engine
and it would start my 5hp air compressor or my central air conditioner. It
did make that engine bark pretty loud when they started though. With the
engine at 2400 RPM and the alternator at 1800 RPM, I could get about 13KW
before the engine bogged down.
"Steven O. Smith"
rec.crafts.metalworking 22 Oct 1996
- > With regards to the large start relay needed for larger idler motors:
> I have seen lots of single pole solid state relays that are rated at
> 40 amp, 220v. These tend to cost around $25 in the catalogs I get.
> They can be switched on 110v or 220v. This is what I had planned on
> using for my start caps (got one for free). Sound reasonable? I have
> seen them rated up to 80 amps for about $40 I believe.
- Sounds like a pretty good solution to me.
- Be sure to buy one of the zero voltage switching types. If you try to use
one of the random switching types, imagine how much current flows when the
capacitor at zero volts is connected to line at the peak of the line voltage
cycle. A whole lotta amps.
- It might also be worth while to figure the charging current even with zero
voltage switching. In this case, the current should be:
123k*capacitance (at 230VAC)
- So if you have 200uF start capacitance, the charging current is 25 amps.
- 220V might be a little low; your phase converter won't necessarily have
balanced leg voltages during start up. At $25 a 'pop', its probably worth
buying a higher voltage unit.
- A friend of mine says that solid state relays often show up in large copier
chassis. Their purpose is (guess what) to switch a voltage onto a large
capacitor. I wouldn't recommend tearing the one at work apart, but maybe you
can get lucky scrounging.
[email protected] (Kevin AstirCS "1U" KO0B)
rec.crafts.metalworking 22 Oct 1996
- >........Would it be possible
>A) Drive the alternator with two 5hp single phase motors. This is
>equivalent to asking: can I use two 5hp motors to make a 10hp motor.
- Yes. Two ways:
- Buy two identical motors, and two identical pullys for them , your in
business. slip/torque curve is broad enough that identical motors should
share +/- 10% which is well within a 1.15 SF. (BTW, Northern Hydraulic has 5
HP compressor motor on sale for $163) even if you find double shafted
motors, I wouldn't couple them end to end...don't know how much torque that
last shaft /keyway/ bearing are rated for
- OR, get two surplus motors and an ampmeter. (clamp type is handy, and fine
for the purpose) load up the alternator. (your wifes dryer AND stove?)Put an
adjustable pully on one motor, and tweak the pully untill an ampmeter
indicates an equal percentage of the nameplate current for each motor. No-
load current can vary widely with motor type, so you need some load to get
into a more linear part of load/current curve. If you do this at above 50%
load, you should be reasonably close at 100% load. Wouldn't hurt to check
this even if using identical motors.
- Now mark both pullys with a sharpie, take a florescent light and view the
pullys while running. (alternator unloaded) Make sure the strobe lines
appear to be turning same direction. (different speeds are to be expected
however) If you drive one motor over 3600 rpm then it becomes an induction
generator, and you might've been measuring it's output current!
- BTW, go a little under 2:1 on the reduction, say 1.8:1, so that you'll have
1800 rpm under load. No harm in being a _little_ over 60 Hz, but some
machines might draw excessive current if much under.
- If you put an idler pully in the system , it may be useful to start motors
unloaded, one at a time, then throw the alternator on line....starting 10HP
+ an alternator might trip a breaker, or at least dim the lights! If you did
this with clutches (ala "projects'" articles) then you could run only one
motor for powering small machines.
- >..............if it was
>possible to connect the line voltage to two legs and connect the voltage
>regulator between the third leg and one of the line legs, it might work ok.
>Sounds too risky to me though.
- I think so too, you also have to get it started, which is harder to do with
a sychronous machine than induction motors.
-
>.... It did make that engine bark pretty loud when they started though
- Think "Flywheel".
- With this type of setup run the biggest flywheel you can start,, and if belt
coupled, put it on the alternator, not the drive motor. This flywheel is
effectivly coupled to the machine, and will result in smoother finishes, The
flywheel will cause longer surges of lower current (into drive motors) when
starting loads..
- You might find a big 3ph motor real cheap surplus. If you use run caps, you
can probably get 75-80% rated power out running it 1Ph. Starting a 12 or so
HP motor could be fun..might be good to try out Fitch's idea of starting at
1/2 voltage.
- It will cost you more to run it, than the typical induction motor based
converter, but your machines should run really nice. If you ever decide you
just _have_ to sell that alternator real cheap, let me know!
Jeffrey Prothero
rec.crafts.metalworking 22 Oct 1996
- > All this talk about 3 phase converters has made me think about an
> alternative approach. I say think, because I dont need three phase
> power now (yet).
- > I have a large (300 lbs) three phase alternator which is rated at 15KW
> into a 0.8 power factor load. I could drive this with a single phase
> motor and get REAL 3 ph power. The efficiency would probably only be
> about 80 % though, but the high inertia (the rotor weighs 100 lbs)
> in the system should handle instant reverse.
- Another approach is to drive drive a three phase motor with a single phase
motor. This actually works well. You need to use at least one variable
pulley so you can adjust the amount of work that the single phase motor
does. I built a converter using a two hp. single phase motor and a 7.5 hp
three phase motor. I used a bunch of capacitors to correct power factor and
help start and run the three phase motor. I adjusted the pulley so that
the single phase motor was running close to its nameplate current. Using
this approach the three phase motor runs closer to its synchronous speed,
and the three phase power is closer to what the power company would supply.
- It starts well, supplies more three phase power than a converter built with
just the 7.5 hp motor would supply and doesn't draw too much current.
[email protected] (PnP Nolan)
rec.crafts.metalworking 23 Oct 1996
- >using for my start caps (got one for free). Sound reasonable? I have
>seen them rated up to 80 amps for about $40 I believe.
- Could you use pre-owned oil filled caps? If so C&H has em by the dozen for
around $8-$10 a piece. Oil filled generally last forever I am told.
"Steven O. Smith"
rec.crafts.metalworking 23 Oct 1996
- > Could you use pre-owned oil filled caps?
- American Science and Surplus (really) in Chicago has oil filled run caps for
$3-4. I find this typical of C&H--if you can find it anywhere else, it is
half the price.
[email protected] (Kevin AstirCS "1U" KO0B)
rec.crafts.metalworking 23 Oct 1996
- I can't add to or argue with Steven Smith's replys, except that Allied lists
370VAC 60 �F run caps at $12 apeice in onesies. Perfect for a 5 HP idler.
This is getting pretty close to surplus prices on a $/�F basis, and saves
paralelling of smaller units.
[email protected] (Fitch R. Williams)
rec.crafts.metalworking 24 Oct 1996
- >An interesting new data point is that I added a 1/2 hp 3ph grinder to the
>line last weekend and it instant reverses without cutting in the static
>starter.
- You might try reversing the mill with the grinder turned on and idling on
the line. Grinders usually have significnant inertia with grinding wheels
on the shafts. If the relay in the static converter still clicks, and it
might, but the converter and grinder don't slow down to much, try a switch
to take it out of the circuit after your idler is started. I am assuming
milling that requires a lot of reversing is reasonably infrequent, so the
distraction of having the grinder idling shouldn't happen to often, and the
idling grinder just might make the difference.
Marc Davis
rec.crafts.metalworking 24 Oct 1996
- > Does that mean that if I just beef up the idler with a larger motor,
>flywheel, etc., it might stop the mill, too? Also, what if I just design a
>switch to remove the static starter from the circuit during the braking
>process?
- I have a 7.5hp converter and a 2hp mill motor. I can reverse the mill
without effect on the converters speed. The 7.5hp converter is a large
frame unit. I use a 110v motor to start the converter.
[email protected] (Steve Jasik)
rec.crafts.metalworking 24 Oct 1996
- >I have a similar unit on my mill, similar experiences. I bought it
>because that was the only way to power up the mill after all the $$$ I
>spent buying it. The box has a 240V single phase voltage controlled
>starting switch, a starting capacitor, and a bleed resistor.
- >Near term I plan to add some run capacitors to all three phases on the
>mill. It shouldn't cost to much and has the potential to considerably
>spruce up performance for not to much money. I figure 15 to 25 uF per
>phase (2 HP motor) should help it considerably. And it will still be
>totally silent when ever the mill is turned off - a feature based on
>reading other posts recently.
- I have been running Caps accross 2 legs of my phase convertor (12 uF per HP)
and tried adding some additional Caps accross the other 'legs' and it din't
appear to make a bit of difference in terms of the measured voltages.
[email protected] (Kevin AstirCS "1U" KO0B)
rec.crafts.metalworking 25 Oct 1996
- >I have been running Caps accross 2 legs of my phase convertor
>(12 uF per HP) and tried adding some additional Caps accross
>the other 'legs' and it din't appear to make a bit of
>difference in terms of the measured voltages.
- And so it shouldn't if you have heavy enough wire run to the converter, but
it can significantly reduce the idleing current to the motor .(by improving
the power factor). Caps here can do nothing to improve the "phase balance"
of a converter.
- Caps across the input line can save a little power, and reduce nusance
tripping of circuit breakers. But, if you never blow a breaker when starting
a load, and you didn't measure _any_ increase in the line voltage when you
added the caps, then there is little reason to add capacitance across the
input line.
[email protected] (Fitch R. Williams)
rec.crafts.metalworking 26 Oct 1996
- >I have been running Caps accross 2 legs of my phase convertor
>(12 uF per HP) and tried adding some additional Caps accross
>the other 'legs' and it din't appear to make a bit of
>difference in terms of the measured voltages.
- Thanks for the data point.
- I wouldn't expect to see much if any difference in terms of voltages. I
would expect to see the line current decrease noticably, especially at idle,
and the converter might get slightly quieter when idling. A typical 5 hp
converter with run capacitors on two legs can draw 7+ amps just idling. On
occasion, adding the extra caps across the third leg can reduce that to
about 2 amps. It won't reduce the electric bill proportionately although it
will reduce it some, but does reduce voltage drop due to line loss (IR),
should improve slightly the systems ability to cope with surge power
demands.
[email protected] (Kevin AstirCS "1U" KO0B)
rec.crafts.metalworking 11 Nov 1996
- > I have seen these and the old style motor convertors, I would like some
>opinions on which would be better to run a lathe and a drill press.
- You will get a better finish with rotary (motor type) converter, as the
motor will run with less vibration. The rotary type gives better reversing
action, If you don't wait for motor to stop ("plugging") There is also no
danger of burning it out with frequent starts, or plugging. You get full
power with rotory converter, but only 2/3 or so with the static type
converter.
- A motor type will eat about 100W/HP (HP rating of idler motor) all the time
it is running...You' will probably want to turn it off during those long
setups. The running idler motor makes some noise too.
- > I have been told that the "motor" type is easy to build, but no one
>can tell me all of the detailed information.
- Jim Hanrahan can,
Point your browser at:
http://tbr.state.tn.us/~wgray/howto/ph-conv/ph-conv.html
It's all there.
- By trolling the surplus yards, I built a 5 HP unit (sutable for up to 3 HP
machines) for <$100, including $35 for a new NEMA box to house the caps and
relays.
[email protected] (Jens)
rec.crafts.metalworking 13 Nov 1996
- >Its probably twice to three times bigger than you need unless you have
>10HP or larger motors on your machinery.
- >If you can trade it for a motor nearer (equal to or slightly
>larger than) the HP rating of your largest three phase machinery drive
>motor you might be happier with the final product.
- Based on my experience, the idler should be 50% to 100% larger than your
biggest motor. I currently have a 5 hp idler driving a 5 hp lathe and there
isn't enough power available for the highest speed. In my application I
should have used a minimum 7.5 hp idler.
unknown
rec.crafts.metalworking 21 Nov 96
- >> > Further, there are power
>> > companies whose policy it is to *not* supply 3phase to residences
>> >as the potential is there for the DIY electrician to install a breaker
>> >in his panelboard on the 277V leg rather than the 120V leg..
- >> Huh ????
>> ... and here I always thought that 3 phase power consists of three
>> 120V phases.
- >> (totally confused and wondering if my homemade three phase converter
>> has only 2.77 phases ....)
- >As I responded to you via e-mail this morning, one of those legs is in
>the case of 208V 3-phase, 192V, or for 240 3-phase, 277 volts...I
>believe, though I'm not entirely sure, that if you are using an idler
>motor, it should actually generate the full voltage on the third leg, I
>don't think the solid-statew converters do, though as the motors run at
>about 2/3 the rated HP, according to the blurbs I've read.
- 277 volts comes as one leg (to neutral) off a 440 volt 3 phase service. The
main use is for industrial flourecent lighting. The ballasts are available
with 120 or 277 volt inputs. By using 277v you avoid having to use a step
down transformer.
[email protected] (Peter Wiley)
rec.crafts.metalworking 22 Nov 1996
- >... and here I always thought that 3 phase power consists of three
>120V phases.
- That's 240V for *real* power :-), but you're right. I have 3 240V lines
coming in to the main board (plus neutral) and 3 meters. From this, I can
pick off 415V 3 phase or multiple 240V single phase circuits. I ran 3 phase
cable to the workshop, put in a distribution sub-board with its own circuit
breakers and pulled out the circuits I needed.
- As the main switch is rated at 90A per phase and the sub-board is 30A per
phase, I can't trip off the house power even if I do something really stupid
in the shop.
- If I ever get to drawing more than 30A over 3 phases, I won't be able to
afford the power bill anyway, so I don't worry about it. Meanwhile, I can
run a 250A MIG welder flat out while my wife is running clothes dryers etc
on separate circuits.
j klessig
rec.crafts.metalworking 21 Nov 1996
- > As I responded to you via e-mail this morning, one of those legs is in
> the case of 208V 3-phase, 192V, or for 240 3-phase, 277 volts...I
- Not really true. Three phase power that you are likely to see may come in
any of three "flavors" depending upon howit is grounded.
"Wye"
"Hot Leg Delta"
"cornor Grounded Delta"
- For a nominal 120 v system
Wye has Qty (3) 120 V to neutral phases ("legs"), that are 120 degrees apart
in phase, giving 208 volts phase to phase. Different voltages available =
120, 208
- "Hot Leg Delta"
FOr a system that supplys 120 V THe neutral is the center point of a one
phase of a 240 volt, three phase delta secondary winding. If this is the
Phase B-C winding, there are the following voltages,
RELATIVE TO NEUTRAL
B 120 V
C 120 V
A 207 V
The Phase to Phase voltages are all 240
- "Corner Ground Delta"
- One phase point of a three wire delta secondary is grounded, if this is
phase C then there is
120 V A- (C = neutral)
120 V B- (C= neutral)
120 V A-B
Mark Winlund
rec.crafts.metalworking 26 Nov 1996
- >(totally confused and wondering if my homemade three phase converter
>has only 2.77 phases ....)
- Most 3 phase power (at the user level) is 220 or 440 volts. When measuring
leg to leg, you will see these (approximate) numbers. When you measure from
leg to neutral or ground, things change. Most setups will have a "hot leg"
whose voltage to ground is much higher. It depends on how the transformer is
wired, and where the neutral is connected. When using a connection that
provides single phase 220 as well as 3 phase 220 (as in my shop) you must be
very careful to know what you are connecting to! (my connection is known as
a split delta, where the secondary windings are connected in a delta
(triangle) shape, and one of them is split in two to provide two 110 volt
sources. The neutral is connected to the center. (Also, as a matter of fact,
three transformers are not required to provide three phase... two can do it,
with the third being a "phantom phase" derived from the other two. True!)
unknown
rec.crafts.metalworking 22 Nov 1996
- >Three phase power that you are likely to see may come in any of
> three "flavors" depending upon howit is grounded.
- Aha! Thank you for clearing up this question in my mind. Your explaination
is very clear and concise. It is also instantly obvious why one would use
hotleg delta - to be able to supply 240 volt power using a three phase
distribution system that supplies 120. I had not realized that one of
phases was *centertapped*.
unknown
rec.crafts.metalworking 23 Nov 1996
- > For God's sake, *why*? It appears from articles etc that your wiring
> standards generally are a damn sight looser than ours (all wiring earthed
- I can tell you the "why", Peter, or at least one version of it: Back in the
nineteenth century Thomas Edison, of light bulb fame and the elder Siemens
(Carl or Karl, I think) from Germany were working together on such matters
as constructing a power distribution network, generating stations and such.
Siemens favored the higher 240-volt system, Edison had a bunch of stuff
patented that worked on 110 volts and wanted the world to use that as the
standard. They got into an arguement, a *hot* one, they say, and Siemens
went back to Europe and built the power grid that is used today, 240V, 50
Hz., with the express purpose that nothing that Edison had patented would
*ever* be sold in Europe...It's true, the higher voltage is better, I'm told
that electronic stuff, especially TV's perform better on it. our 110V 60Hz
system, however, is what we in North America have always had, it's all we
know so far as residential service is concerned.
- I worked for a few years as an electrician, we have a National Code Book
with Standards set by the National Fire Protection Association, a consortium
of Insurance Companies; this book has been adopted as the standard for the
industry and is included as the reference in all building codes, it is
updated every two years. for several years now (I dunno, 25 at least) 3-
wire ""earthed" or as we say "grounded" circuits have been mandatory, though
earth-leakage breakers (over here, they are ground-fault circuit
interrupters) are not except in new construction where they are required in
the bath and at the kitchen counter that contains the sink. Single-phase
power is provided to residences with 2 110V legs (actually, the voltage
ranges from 115-135 volts, depending on the demand on the power grid). Most
appliances, lighting, etc. are 110V, large moters and welders are 220V with
both legs feeding the machine. As far as the high amperage is concerned, we
just use bigger wire.....
- ...Which is more than you *ever* wanted to know about the North American
electrical situation.......
[email protected] (Reckless)
rec.crafts.metalworking 10 Dec 1996
- >I've seen loose grounds do all sorts of odd things, including
>the result (more power) you are seeing.
- >In the past, wicked and long discussions have been written about
>the 'impossibility' of some of the results of bad grounds. Most
>of writers have been ignorant and offensive, and I do not want to
>have another round of than.
- >Spend long time checking your ground conditions.
- >Ground problems I've seen include 'start elevator blow up computer',
>'turn on printer, blow up power strip', 'start washing machine, blow
>light bulb', 'plug in printer, it lights on fire', 'sun comes up,
>circuits go off', and the list is much longer than this.
- >I suspect that you are driving the 3phase with a converter, and that
>converter is on the 'other' side of a center-tapped circuit. With
>high load on the other side, the ground is moved, giving more voltage
>to the welder.
- OK, from what I understand of Cecil's setup, he has a substantial shop with
3-phase power. I don't believe his problem is a ground problem per se, but
more likely a loose phase connection between the power panel and the
transformer. Loose neutrals do this in single-phase circuits. If the
welder is single-phase (most MIG units are unless they're really big), its
connected to only 2 of the three phases, and one of those has the loose
connection. When the compressor goes online it acts as a rotary converter
and stabilizes the loose phase, but when it shuts off the voltage on that
phase drops, hence the symptoms he describes.
- My suggestion: check the grounds (for safety reasons, of course), and pay
extra special attention to all the hot leads and any neutrals. Check the
voltage at the breaker panel when welding and at the welder outlet. I
suspect that both will be low when welding. If so, the problem is outside
somewhere between the panel and the transformer(s). If the supply lines are
aluminum, they have a tendency to get loose with time. Make sure everything
is tight. The power company may need to be called if the problem is not in
your equipment/building.
- I had a similar problem at the last house I lived in. The neutral
connection was loose at the pole transformer, and after I tightened all the
connections in the breaker panel (without changing the problem), Edison was
called and they replaced a crimp on the neutral wire. Solid power once
again.
[email protected] (Jim Hanrahan)
rec.crafts.metalworking 17 Dec 1996
- >>I just recently bought a used Leblond Regal 15x54 servo shift lathe.
>>I am (unfortunately) using a static converter to get it to run on single
>>phase power. The main motor (7.5hp) will run, but when the servo shift
>>kicks in to change gears, it doesn't seem to have enough power to actually
>>shift them. So I'm stuck at one speed!! Anyone have any experience with
>>this?
- > The first thought I had was that you should check
>the nameplate data on the motor for the servo shift. Is it possible
>this is a single phase motor? If so, you may be pulling power for it
>from the "artificial" leg that your converter is making up. You may
>want to switch the connection to a different leg.
- I agree with Dave, check those things. All solenoids, magnetic contactors,
single phase motors, etc. must run off the original single phase lines, not
the "generated" third line. Also, check that all the above are wired to
operate at the voltage you are giving them. ie, you are most likely giving
them 230 Volts, make sure they are not set up for 115 or 440; transformers
may be required for certain "accessories" that can't use 230 Volts.
tapdungeon
rec.crafts.metalworking 19 Dec 1996
- > >I had the opportunity yesterday to bring home a nearly-new looking 2hp
> >3-phase 440 volt motor from the salvage yard (buying buy the pound is
> >a wonderful thing!)
- > >I've found on the net instructions about 2 to 3 phase converters and
> >was wondering If anybody has ever run a 440-volt motor off of a 220
> >converter. Do I need to scrounge for a transformer? What will work
> >with this motor?
- > I meant single-phase 220, not 2-phase, sorry for the error!
- I have to use a rotory phase converter and a transformer.. the EM feild in
the transformer doesnt alow the motor to run off a static type converter...
So I us the static converter to start a 3-phase motor,then I isolate the
static converter and put the trasfomer on line then I am able to run the
440 volt motor.
[email protected] (Reckless)
rec.crafts.metalworking 21 Dec 1996
- >It may be a "dual voltage" three phase motor. Many are. If so,
>you can rewire the field windings in parallel instead of series
>and cut the line voltage requirement in half. If it isn't you can
>just run it on 208 volt three phase. What will happen is you will
>get 1 HP instead of 2 from it at half voltage.
- Actually it will be 1/2HP since at 1/2 line voltage, only 1/2 current will
be drawn (max). Since power is volts multiplied by current it will be 1/4
(in this case). Note: 208 instead of 220V will reduce output power as well.
Hopefully it is a "dual-voltage" motor.
[email protected] (Bill Neely)
rec.crafts.metalworking 22 Dec 1996
- > >It may be a "dual voltage" three phase motor. Many are. If so,
> >you can rewire the field windings in parallel instead of series
> >and cut the line voltage requirement in half. If it isn't you can
> >just run it on 208 volt three phase. What will happen is you will
> >get 1 HP instead of 2 from it at half voltage.
- > Actually it will be 1/2HP since at 1/2 line voltage, only 1/2 current
> will be drawn (max). Since power is volts multiplied by current it
> will be 1/4 (in this case). Note: 208 instead of 220V will reduce
> output power as well. Hopefully it is a "dual-voltage" motor.
- If the motor has 9 leads in the connection box it's dual voltage, 3 leads it
will be single voltage.
[email protected]
rec.crafts.metalworking 13 Jan 1997
- > It is possible to make a non-polarized electrolytic out of 2 polarized
>electrolytics. Tie them '+' to '+' or '-' to '-' , it doesn't matter
>which. You will have to double the value to get what you need, but you can
>half the voltage requirement. For example, two 100 uF, 100 volt caps will
>make a 50uf, 200 volt cap. (I personally always overshoot the voltage a
>bit. Couldn't hurt)
- NO! Such a connection works, because it operates each capacitor with a DC
bias. The _full_ voltage appears across _each_ cap on alternate crests of
the AC waveform, when the voltage on the "other" cap is zero. (as it _can't_
be negitive)
- Accounting for the voltage boost of starting service, and the RMS to peak
conversion, each of the two capacitors need a peak voltage rating of at
least 550VDC (EACH!) for 220VAC motor starting service. The "WVDC" rating
is more conservative than the peak rating, so it could be safely used if a
peak (or max) rating is not given. Using 110V caps as suggested in the
quotation would be asking for a bang. Bill's "overshooting" may have been
more necessity than caution, and the series impedance of the motor winding
may have averted catastrophy.
- Otherwise the information above is accurate. If EE types want me to
elaborate on why this doesn't agree with the "textbook" case for series-
connected caps, please contact me by e-mail, or check DejaNews for extensive
sci.electronics.* threads on this topic...I already push the limits of what
is on-topic for this group.
- Enough dire warnings...on to solving Sasha's converter starting problem....
(I'm just sick enough to find this fun!)
- The electrolytic capacitors Jim Hanrahan's article refers to are special AC-
rated motor starting capacitors. Inside them (effectivly) are two "normal"
DC electrolytics connected as discussed above.
- It is probably false economy to buy _any_ electrolytic capacitor used. (But
that doesn't stop this scottsman, any more than a russian!) They contain
water, and dry out over time, and with use. They often fail in a spectacular
way, so use caution when turning on power with a "dumpster" cap. Their
"smoke bushing" is designed to fail in a mannor that distinguishes them from
a hand-gernade, but like other safety valves, it only works _most_ of the
time. This is why the voltage rating I argue above is a matter of safety.
- You can buy new motor starting caps from Allied, Newark, or Grainger...that
I know of, I'm sure others have thier favorite source. I bought mine from
Grainger.
- Starting a 15HP motor will require 5-6 of the largest 220V starting caps
Grainger sells, (~300uF , $12.03 each) and a couple of 30A, 3 pole
contactors, with 1 cap/pole.
- But, the starting-surge current may well blow the main-breaker on a
residential service. Don't use electricity to weld, cook, heat water, or dry
clothes at the same time, and it _might_ work.
- Use DejaNews, and find Fitch's idea of starting on 115V instead of 230v.
This would require a (rather) complex voltage switching arraingment, but
less than 1/4 the cost in caps. Starting current should then be less than
1/2 the 230V case. Smaller (cheaper) contactors could be used for connecting
the starting caps as well, and that could well offset the cost of voltage
switching circuitry.
- Get a rope! (or not...): Starting my 5HP, 3450 rpm idler with a rope took
about the best pull I could give, and I can't claim to be a small guy. It
took a _much_ stronger pull than, say, my 3 1/2 HP gas lawn-mower. I think
it is not feasable for a motor 3 times bigger. If you try this, be aware
that the rope comes off the shaft impersonating a bull-whip. Eye protection
is in order, which I luckily figured out after only a whelt on my leg. Do
NOT wrap the rope around your hand! If that rope hangs on the shaft, a 15 HP
motor is PLENTY to SERIOUSLY hurt/maime/kill you....Just consider how much
damage 15 of the hay-powered type could do if they were angry at you, and
you couldn't run. Now consider that electricity is ALWAYS angry!
- How I'd do it: Using a "small", 1 ph. starting motor may be the best
(simpilest and cheapest) option. A 15 HP motor will have significant
bearing and cooling-fan drag. Given the idling losses reported by Jim
Hanrahan, and my own measurements, plus some guessing about how much is
mechanical vs. electrical loss, I think you'll need about 1 HP to start your
big idler. Gear it (with pullys) about 1.25 : 1 to achieve about 2700 rpm
on the big motor. It should spin up from there when you throw the main
switch. 1 : 1 is OK, but it may take a little more than 1 HP ...that is why
I say gear it down a little. (fan HP loading should decrease as _cube_ of
speed) As you will not run this motor continuously, you can push the
nameplate current a _little_. If you already have a 3/4 HP motor, go ahead
and try it, but I think 1/2 HP is almost certainly too small.
- BTW, that 15 HP motor will waste (as heat) well over 1KW when just idling.
(no 3 ph tool running) That is as much as a typical electric heater uses, so
you will probably want to shut it off while setting up for the next
operation....(Unless it is cold like Albuquerque today...Brrrr...) best make
it easy to re-start.
Mary & Matt Colie
rec.crafts.metalworking 13 Jan 1997
- > I read the article in Metal Web Mews about building the phase converter. I
>will be making the one with separate start and run caps. The motor is 15HP
>3600RPM, so I need a huge oil capacitor to start it. The article says to
>use electrolytic caps for starting if the oil ones cost too much. The
>electrolytics have polarity. The question is, how do I wire them. The
>positive goes to the first or third line?
- We (my late father and I) have been doing this for years. The oil capacitor
is only to keep the third vector on the right rotation during sudden load
changes (like backing a lathe to stop the sindle). Yes - the oil is
expensive, so go to a local refrigeration supply house and tell the people
you want a univeral replacement starter. This is a high reactivity non-
polar electrlytic with a self heating thermister to take it out off-line so
it doesn't burn up. Before I found out about that, we used to to put a
sheeve on the "converter" motor and kick start it before hitting the switch.
"Randolph Lee"
rec.crafts.metalworking 26 Dec 96
- > I think we are discussing two diffrent animals. Your description sounds
>like a Ac inverter frequency controller for three phase motors. The unit I
>am reffering to is the static type phase converter. These units are
>advertised to convert single phase current to operate three phase motors
>not to supply true three phase. On the other hand rotary type converters
>are advertised to supply true three phase current with each of the three
>sine waves 120 degrees apart.
- > The inverter type converters cannot handle instant reversing loads or
>motors with more than one speed. They are also only suitible for induction
>loads not resistance type loads.
- Tim I wasjust responding to your statement:
- >The only phase converters that supply true three phase
>current are rotary converters that have a motor running to provide the
>missing third phase of the current.
- There was no qualification in that quote and I was pointing out that the
Frequency converters do provide true 3 phase...
- I can reverse the spindle on my B-port very nicely by either flipping the
reverse switch on the machine or by commanding the reverse on the freqrol...
if the latter is used then the control will ramp down the CPS (at a user
selected rate and then ramp it up again in the other direction... the
dynamic braking will be used in the ramp down and rate that it ramps down is
dependant on the size of the resistor bank you have installed and the mass
of the rotating items... if I reverse by just flipping the machine switch it
seems to reverse at about the same rate as it did when I had it hooked up to
'real' three phase at the used tool dealers before I bought it... if this is
not what you ment by "The inverter type converters cannot handle instant
reversing loads" could you explain further?
- My exp. Is only with the Mitsubishi Freqrol unit Mitsubishi (908-302-2787)
Freqrol - A024-1.75K Gen purpose Inverter to be exact and it is as a user
rather than as any kind of expert.
- ref your statment:
>They are also only suitible for induction loads
>not resistance type loads.
- I did not know this and I don't quite understand it... I thought the loads
that a machine tool moter had were induction loads... and why would you need
a converter for resistance loads like heaters?
- In the spring of 94 the phaseOmatic that came with my Bridgeport ate itself
and I inquired here in the group about these Freqrol units... I had little
in the way of response and so tried one and reported on installing and
learning to use it here in r.c.m over the summer of 94... here is a copy of
a sort of 'final' report from the fall of 94:
"Steven O. Smith"
rec.crafts.metalworking 14 Jan 1997
- > This calls for a static type convertor. My question is, can I simply use
>the plans for running the rotary convertor, of the self start variety, with
>a run cap on the 3rd leg of the motor, and obtain useful power from the
>drill press?
- A static converter is nothing more than capacitors added to your existing
drill press. You also need the starting method, such as an additional start
cap.
- Static converters give you 50 to 70% of the rated motor HP.
Rotary converters give you 80 to 90% of the rated motor HP.
(depends on whose books you read)
- The reason is that the winding currents in the motor are not really
balanced. If you try to pull 90% of the HP out of a static converter, one
(or is it two) of the windings are probably running at higher current than
designed for and will overheat. A rotary converter has a much better balance
between the windings--essentially the idler motor is the one with the
imbalances. This is one of the reasons why the idler should usually be
larger than the load motor.
- I would think that a static converter would work great for a drill press. I
would expect belts to slip before you maxed out the power available from the
motor.
"Steven O. Smith"
rec.crafts.metalworking 15 Jan 1997
- > I went back and checked my books (it has been a long time) and my voltage
>ratings are correct for my example, I may not have explained it properly.
>When capacitors are in series, their voltage ratings add. Two 100volt caps
>in series makes a rating of 200 volts.
- With unpolarized capacitors, you are correct. The voltage ratings add. The
problem is that an electrolytic is polarized. It will stand hundreds of
volts one way, but reverse the polarity and the cap will blow up. With back
to back caps, one is reversed.
[email protected] (Bill Browne)
rec.crafts.metalworking 15 Jan 1997
- >> >DO NOT use a single, polarized electrolytic! It will blow up. You can
>> >make an unpolarized electrolytic out of two of them in series, with
>> >polarities opposite, "+--+" or "-++-". I haven't done this, and don't
>> >know if there are other tricks to it (such as diodes or resistors
>> >needed).
- >> It is possible to make a non-polarized electrolytic out of 2 polarized
>> electrolytics. Tie them '+' to '+' or '-' to '-' , it doesn't matter
>> which. You will have to double the value to get what you need, but you
>> can half the voltage requirement. For example, two 100 uF, 100 volt caps
>> will make a 50uf, 200 volt cap. (I personally always overshoot the
>> voltage a bit. Couldn't hurt)
- >I question your comment on voltage requirement. It seems to me that if
>the cap won't stand reverse voltage, you can't count on that standoff
>capability. I think you need both caps to be full voltage.
- >I like to use 330VAC caps in phase converters. Since you are generating
>the third leg rather than having it supplied by the utility, the voltage
>is uncertain. As you add more capacitance, the generated voltage goes
>up. I suspect that during starting the voltage on the third leg can be
>significantly greater than line. On the other hand, the converters I've
>built with 220V caps work ok too.
- I went back and checked my books (it has been a long time) and my voltage
ratings are correct for my example, I may not have explained it properly.
When capacitors are in series, their voltage ratings add. Two 100volt caps
in series makes a rating of 200 volts.
- This only covers the rating of the capacitor. It does not take into account
peaks that may exist in AC circuits, or peaks generated by inductors.
120volt line is actually 170 volts peak, or 340 volts peak to peak (+ to -
swing). Also, inductors in a circuit can cause some pretty high peaks.
Since I never did understand how a phase converter worked, I couldn't begin
to imagine what sorts of strange voltages are flying around in there.
rec.crafts.metalworking
[email protected] 15 Jan 1997
- > Also, American Science & Surplus has run caps, but many of them are only
> rated at 280VAC, not the 370VAC or higher recommended. I can get the
> values I need with their 370 VAC caps, but will have to add them
> together, while if I could use the 280VAC, one would do. I suspect that
> there must be peak voltage spikes (porbably at startup) that exceed this
> value, but would like the groups knowledge.
- It sounds like you intend to use the so-called "self starting" configuration.
- Spikes are not the problem. Under no-load conditions, this configuration
puts elevated voltage (continuously) on the start/run cap, (The reason is
the subject of some debate, but the effect is easy to reproduce, and
measure) and that is why at least 370VAC rated caps are suggested.
- Be aware that this set-up also starts rather sluggishly..it may not start
with the DP set-up for high spindle speed. A larger cap will improve
starting performance, but the voltage rise then becomes even greater....so
you have to compromise.
- This voltage, on an idler, drops when a three phase load is connected in
paralell with the converter. I do not, off hand, know what the effect of
adding a mechanical load to the shaft would be. My insticts say it will
cause additional voltage rise...
- If you can locate a surplus 20A contactor, ( A relay with special beefy
contacts) then using a switched start-cap will be cheaper than buying a 1
ph. 3/4 HP motor. But a new contactor will likely run you as much as a
surplus 3/4HP motor. If you can find a reasonable price on a 1 ph. motor
that will fit the machine, then that will probably be the most satisfactory
solution, in the end.
- Most larger cities have shops that specialize in motor re-winding, and
though it will cost more than a surplus store, they usually give a 90-day
warranty or some such, so there is less risk...also less time wasted (if you
consider it so) digging through junk at a surplus place. In this size,
probably only the nicer TEFC motors are worth re-building...so that may
cause an even greater price spread.
- If you want a new motor, Grainger offers one-stop shopping for most any type
you'd be likely to need, but they only want to sell to businesses.
[email protected] (Bill Browne)
rec.crafts.metalworking 15 Jan 1997
- >NO! Such a connection works, because it operates each capacitor with a DC
>bias. The _full_ voltage appears across _each_ cap on alternate crests of
>the AC waveform, when the voltage on the "other" cap is zero. (as it
>_can't_ be negitive)
- >Accounting for the voltage boost of starting service, and the RMS to peak
>conversion, each of the two capacitors need a peak voltage rating of at
>least 550VDC (EACH!) for 220VAC motor starting service. The "WVDC" rating
>is more conservative than the peak rating, so it could be safely used if a
>peak (or max) rating is not given. Using 110V caps as suggested in the
>quotation would be asking for a bang. Bill's "overshooting" may have been
>more necessity than caution, and the series impedance of the motor winding
>may have averted catastrophy.
- The numbers I gave were for example purposes only. If someone is going to
dig into such equipment, they should know the values they need. Perhaps my
20 year old electronics theory doesn't belong in this NG. My experience
with industrial motors is limited, so I'll leave future posts to more
experienced folks.
[email protected] (Paul Amaranth)
rec.crafts.metalworking 25 Jan 1995
- >I agree that the converter box picks up the third phase only to "start" the
>motor. Even if you put an "idler" motor into the loop, the third phase
>might be OK for another motor, but I'd be skeptical about using it for a
>welder, and it would take a very large "idler" if you use the formula of
>needing a motor 50% larger than any you intend to use under load. I don't
>know what the amperage of your proposed welder is, but it might be high
>compared with a motor. You might give Phase-A-Matic a call in Palmdale,
>CA. They make the converter boxes and might have some idea.
- Remember that the welder amperage is the arc current, not the primary draw
from the mains. My 170A Mig tops out around 25A at 220V single phase.
There should be a tag somewhere on the thing specifying the power
requirements (so many amps at 3 phase at whatever voltage). Use that to
figure your needs. Even pros mess up on this; I was talking with a couple
of local shop guys and they were amazed that someone (not me) was going to
put a 200 A welder in their garage. They thought he was going to melt his
house wiring.
- I would think that a properly balanced idler would work OK. Balance is load
dependant though, so it might be a little tricky. It's hard to say what
running in unbalanced 3 phase would do without looking at a circuit diagram.
- I don't think I would use a static converter.
- A 3 phase inverter would probably work fine; they convert your 220V AC to DC
and then synthesize the 3 phase current. They do tend to be a little pricey
though.
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