I realize the extent of my confusion about phase converters. I thought
that phase converters were motors/generators coupled on one shaft. It
turns out that they are some complicated combinations of capacitors.
I am curious why phase converters are not made as motor/generators.
There are different types of phase converters.
A static phase converter is just a couple caps and probably a relay
that allows a 3 phase motor to start and run off of 2 windings.
It does not develop full power (about 2/3) but is still useful.
A rotary phase converter is basically a 3 phase motor running off
of single phase power. It will regenerate the third phase well
enough to run a smaller 3 phase motor pretty well. Commercial
products use a custom wound motor that works a little better
than a standard 3 phase motor (so I'm told).
Phase converters are not made from motor generators because it
is expensive and not necessary unless you have a unique application
such as a need to generate 50 Hz from 60 Hz.
Although I dont mean to correct you about the 2/3 power statement, most 3
phase motors will develop its full name plate HP even when fed single phase.
They will slow to a lower RPM than the name plate RPM rating, and get
hotter, quicker with single phase than with 3 phase.
It is interesting to me to learn that an idler of equal size of the tool
motor provides very little MAX. HP performance compared to no idler at all
when fed from single phase.
The HP from a 3 phase motor driven from single phase without an idler will
perform very nearly the same at that same tool motor driven thru an "equal
size" idler up to about 60 percent of MAX. HP name plate on the tool motor.
Maybe thats where the 2/3 HP figure comes from. Idlers and tuning do
help the tool motor measurably when the 3 phaes tool motor is heavily
I suppose they could be considered a motor/alternator, just not separate
Don't let the thought that they are complicated slow you down, they're
not all that involved. A clamp-on ammeter is good for balancing, they
can be had on ebay for around $10 the last time I looked- the basic
Radio Shack unit works well.
You don't have to use any capacitors at all if you don't want- you can
start it with a small single-phase motor or if you want to go all the
way simple you can start it with a pull-rope wrapped around the motor
shaft.. but make sure the is pulled clear of the shaft before switching
it on. The advantage of the caps is that you can make it more efficient..
What Grant said . . . a single phase motor driving a generator would, in
fact, be a motor/generator -- but -- the 3 phase generator portion of the
combination would have to be an alternator; a machine with a stationery set
of coils (stator) and a revolving magnetic field set of coils, or vice
versa. The field would either have to be excited by direct current (DC) or
be made up of permanent magnets. Large steam driven turbo-alternators are
an example. Those are synchronous machines, whereby the speed of rotation
determines the output frequency; their magnetic fields are generally excited
by DC generators of 110 volts, sometimes 220, and their may be other DC
voltages for excitation.
Contrast this to a rotary phase converter. The type RPC we are familiar
with is basically 2 three-phase motors running on single-phase current with
their 3rd legs tied together. The motors are not operated in parallel.
They may run with or without capacitor augmentation which aids in supplying
phased current to the 3rd leg of each motor. In a manner of speaking each
motor is a sort of rotating transformer which is responsible for energizing
the 3rd leg or "manufactured phase". The idler motor, viewed as a source
( not a strictly accurate analogy) has to be larger than the load motor in
order to keep the 3rd leg "generated" voltage from sagging down.
Capacitance from the single-phase line to the 3rd leg(s) lowers impedance in
those paths and may be thought of as forming a very broadly tuned series
resonance circuit. Current flow in a RPC is quite complex and does not
readily yield itself to mathematical modeling..
And these have a few capacitors and a voltage sensitive relay to
make it self-starting. They may (and good ones will) have additional
capacitors to tune the balance of the generated phase, and to balance
out imaginary current in the draw from the breaker box which can draw
more from your breakers than is convenient.
Or a need for 400 Hz 3 phase to run aircraft electronic and
A maker of that style of converter is Georator (in Manassas VA).
And yes -- they are a *lot* more expensive than an idler motor
used as a rotary converter.
I think I read that on the phase a matic web site but I could be wrong.
I always thought it was related to the fact that the the motor is
running on only 2 out of its three windings. At 2/3 rate HP, the
two working windings will be drawing full rated current. Anything
over 2/3 would imply that the 2 working windings must draw more
that full load current. Based on this logic, I accept the 2/3 rating
Wouldn't that imply that it might be operating outside of safe limits?
From what limited testing I have done, I agree that an idler motor does
not apear to do much until the tool motor is at a higher slip than the idler
motor. There was a guy on here that stated his surface grinder did
not work perfectly with a rotary phase converter and worked much
better with a VFD. This appears to support this conclusion.
One thing that a RPC does do is provide instant reverse that you
cannot get with it.
Frankly I decided not to get two wrapped up in phase converters because
I prefer VFDs.
Would it be correct that all these phase converters that are not based
on motor/generators, are unable to run more than one 3 phase device at
a time. The reason for this suspicion is that they seem to do
something wholly different when a motor starts, than what they do when
So, with such a 1 phase converter, I cannot have a compressor and a mill.
Permit me to turn your statement inside out a bit, but retain the
original meaning: "All these phase converters based on motor-generators
(sic) can run more than one device at a time."
As others have suggested, motor-generator setups are not typically
found in this application. The rotary phase converer which consists
of a large three-phase motor, acting as a sort of rotary transformer,
has rotating elements and will provide nearly factory-fresh 3~
power, and can run many machines at once. In fact, the more load
machines running, the larger the apparent capacity of the system.
My idler motor setup powers a milling machine and two lathes:
Thanks. I feel quite lost. Is what you have not the same as
motor/generator (with the 3 phase motor being the generator)?
I can get a 3 phase motor basically for free, this or next week. A
very pleasant guy from whom I already got a bunch of stuff from his
liquidating factory, needs to dispose of his stuff. I posted what I
bought in the "My latest catch" thread. I am VERY interested in making
my own phase converter.
I am beginning to "get it".
You spin up your 3 phase motor by using the little 1 phase motor.
Then you apply single phase power to the 3 phase motor.
The third leg of the three phase motor starts producing an out of
phase thrd phase.
At this point, you turn off the little 1 phase motor and start using
the 3 phase motor. Is that right?
Are you the guy that lives in downers grove IL? If so, you are
close to me and I would also like to find a motor to make a phase
converter. A 5HP would be about right. A 7.5 would probalby work too.
Exactly correct. If you read the .txt file it sorta goes
through the drill on using it.
Close small switch to energize pony motor (it's a 1/8 hp
repulsion-induction motor), and wait for the 5 hp motor to
come up to speed. That takes all of one second or so.
Then I close the fused knife switch to apply single phase
240 volt excitation to two leads of the 5 hp idler motor.
It then picks up and locks to the line frequency.
Then I shut off the pony motor, and lift up the hinged platform
so the belt flips off. I deftly catch it and tuck it under
the platform so the thing doesn't rattle.
Now the three wires (two of which go to incoming 240 volt line)
from the idler motor give pretty good three phase power, which
is routed via my distribution buss, to each load machine that
I would want to run.
The converter is quiet enough so that I've been known to
accidentally leave it running when I quit working.
I live near downers grove.
Unfortunately, I called the seller and he said that he only has motors
up to 1 HP. Which is a little strange, as I saw some that were a
little big for 1 HP. He also had a 7.5 HP compressor that he sold for
$200. A very nice looking lub-dub unit.
He is selling a 2HP 3 phase compressor for $100. You can put a harbor
freight 1 phase motor there, I suppose.
With a motor-generator the generator generates all three phases. With
the typical Rotary Phase Converter only 1 of the three phases is
generated. The other two come from the two legs of the 220 single
phase that power the RPC. So when you connect your three phase motor
to the RPC two of the wires connect to the single phase wires that are
also connected to the RPC. The other wire connects to the wire from
the RPC that is not connected to the single phase line. Capacitors are
used to balance the RPC and to provide a self starting feature so the
small single phase motor can be disposed with. But a quick and dirty
one can even be built using just a three phase motor and a rope. Wind
the rope around the motor shaft and pull it to spin the motor up. When
the rope comes off turn on the single phase power and if the motor is
spinning fast enough it will speed up to normal running speed. I have
a well balanced RPC in my shop that is made from a 15 HP motor. At
idle it draws about 1500 watts. It can start a 15 HP motor but can run
60 HP when the motors are connected in steps. Currently it is used to
run two CNC lathes with 10 HP spindles, 1 CNC mill with a 5 HP
spindle, and various other manual manual machines.