Hello all, another question I wanted to know why do people put idler
motors running on 3 phase power created by a rotary phase? Does this
help balance out the load on other machines? Yes I still pondering my
power issue in my shop when I turn on my milling machine and the
neighbors lights dim out abit. I have talked to an electrician and he
said 100 amp service is plenty and he thought a 25 kva transformer was
Without an idler motor, you have no 3rd phase generated. You can run a 3-phase
load motor (like your mill's) on single phase power, but it will only develop
about 1/3rd the horsepower and certainly won't run as smooth.
Idler motor = 3rd leg
Static phase converters don't have an idler, they are just starting mechanisms
which allow you to get your motor going but it won't be as smooth and you won't
get the power it's rated for.
Do I understand that you state a 3 HP 3 phase motor running from a single
phase source will develop only 1 HP at it's shaft?
Do you have any data on what would happen if a 3 HP, 3 phase motor running
from a single phase source is loaded to 2.8 HP??
Your understanding is correct. I have no data, but my intuition is it would
stall; i.e. stop turning and draw an immense amount of current.
Of course, I'm just citing conventional wisdom most of which I learned from
reading this newsgroup consistently since about 1993. I was amazed to find that
I can start and run a 7.5 hp load motor with a convertor with a 7.5 hp idler. So
maybe I'm wrong. Love to be. For years I had a Phase-A-Matic (commercial flavor
of a static phase converter) installation/owner's manual sheet. I believe it
said that you can't ever get nameplate horsepower out of a 3 phase motor using
single phase power via a static phase convertor.
I am certainly not a Phase Converter Expert. But, a couple years ago I
was inspired by Fitch's work with converters and I built a small dyno for
accumulating data on 3 phase motors driven with single phase. Don Foreman
was polite and patient enough to help me figure out how to buld a dyno and
get good data from it. The dyno that Don showed me how to construct worked
well. Excel did the rest of the work.
As with so many things like this, The more I learned, the more I became
aware of how little I really knew.
In the process of accumulating data on 3 phase motors driven by single
phase power, I learned that a 3 phase motor can deliver 98 percent of its
full name plate rated HP, but at a lower than name plate specified RPM. I
didnt even try to get data on heat and vibration. I was certainly
surprised to see that the 3 phase motor could deliver so much power even
though it was driven by single phase. I would like to se some data on the
3 phase motors that *cant* deliver nearly their name plate rated power when
driven on single phase.
I remember those threads and I am a great believer in actually trying things
to find out if and how they work. Did you get any data to determine how much
power a 3 phase motor could develop on 3 phase power if you did not consider
RPM drop and heating? Somehow in the back of my mind I seem to remember that
several times nameplate horsepower can be developed for a short time.
No I didnt follow up on learning about the heat limit for the 3 phase
motor as a limiting factor for Max HP while running on single phase. It
would have been fairly easy to do, but it would require alot of
time/interest because it meant running the motor at full load for a long
time. The 3 HP 3 phase motor that I was using when I ran the HP vs "load
power" sure didnt get warm while I was loading it to 3 HP several times
while evaluating the effects of the Idler. There is so much thermal
inertia in the motors that it might distribute the heat from the excited
windings to the heat dissipating area around the unfed windings. So, to
make a valid comparison, I probably have to install heat sensors within the
windings. there is just too little interest in the data. I discontinued
my investigation when I realized that there was so little interest in actual
I can send you the Excel data and plots I made while testing the 3 HP 3
phase motor with and without the 5 HP idler. The material is not well
organized but it is pretty accurate. It sure was repeatable. That was
accurate enough for me.
The 3 HP 3 phase motor RPM dropped to 1730 when it was loaded to 2.9 HP.
The same motor dropped to 1740 RPM when loaded to 3 HP when the 5 HP idler
One thing I found very interesting was that any/all the tests I made (and
there were many) showed that, if the 3 phase motor is loaded to about 1/2
its name plate rating, it didnt matter (for HP /RPM) if there was *any*
idler in the system. Any given 3 phase motor I tested would drop to the
same RPM when it was loaded to about 1/2 its nameplate rated HP, no matter
which idler was used, from zero idler to 15 HP idler. So, if the
home/hobby user had a 3 phase motor with a nameplate rating of about twice
whatever could be anticipated as the normal / max load HP, there is little
need for an idler *or* tuning.
I'd really like a copy of that Excel data. Just to look at, if you
don't mind. Please send it to
I've noted here a number of times, that I use a home-made auto-start
5HP RPC and distribute its output via single phase 220 "laundry"
sockets and plugs located in various places around my shop. My
machinery includes a 2.5HP geared-head lathe (which is NEVER asked to
"struggle"), and I have a second 5HP motor on wheels I can bring out
and plug into one of the sockets when I want to run my 5HP lathe. This
second motor has a starter contactor with the coil across the
"generated leg", so that this "idler" (your term---I like the term
"additional output") will not attempt to start unless the prime RPC is
On Fri, 24 Nov 2006 06:26:02 GMT, "Jerry Martes"
Thanks for the info, Jerry. I have a home-built 5HP tuned converter and have
only used it so far to run a 5HP Kearney and Trecker mill and a 3HP Thompson
surfaace grinder. It starts the mill quite rapidly. The mill has a clutch
which reduces the starting load quite a bit. I also expect to use the
converter for powering some electronic equipment.
One thing to remember, is that when you want to machine with either
the mill OR the surface grinder, crank-up your RPC, then the machine
you DON'T want to use. Gives you that additional "umph".
A 3HP grinder does some serious work, and the 5HP RPC won't quite
provide enough power.
It's my understanding that a 3 phase motor running with only a phaseamatic
box with no idler develops 1/3 less HP or stated differently is running at
2/3 of the rated HP. Since it's only running on 2 legs out of 3.
All induction electric motors act as generators when running. The
opposite is also true. Induction motors spin because of a rotating
magnetic field. In a three phase motor energized from the single phase
line the rotating magnetic field generates current in the third,
unenergized, leg. This leg is called sometimes the wild leg or the
manufactured leg. Using an idler motor bigger than the motor run from
the converter will enable the smaller motor to do things like instant
reverse, just like the motor would do running from utility supplied
three phase. When using a static converter to start a three phase
motor the capacitors in the converter shift the phase so that the
motor will start to spin. Once the motor is spinning the starting caps
are disconnected from the circuit, leaving only the running caps
connected. It is not necessary to use capacitors to start a three
phase motor from single phase. If the three phase motor is first spun
up to a minimum RPM the motor will continue to spin when energized.
Some rotary converters use a small single phase "pony" motor to spin
the large idler motor and when spinning fast enough the idler is
connected to the single phase line. Once spinning under its own power
the idler is disconnected from the pony motor. A real simple way to do
this is to use the weight of the pony motor to tension a belt that
drives the idler. Once the idler is up to speed a board is used to
raise the pony motor so that the belt flips off. If the idler is spun
up to to operating speed with a pony motor and then energized it will
draw much less current at start. This helps prevent dimming lights.
Induction motors can draw more than three times the current when
starting than when running. Capacitors are used with rotary converters
to either start the motor or balance the voltages from the converter.
A rotary converter can have either or both. For a Bridgeport mill a
simple rotary converter can be made with a 5 hp three phase motor and
a 1/4 hp single phase pony motor. The three phase output won't be
perfect but it will work fine for the 2 hp mill motor.
I've not seen any quantified data but the theory is that the idle motor
acts as a generator to stabilise the 3 phase supply. As a generator
needs to be driven, we must ask what mechanical input exists. There is
no direct power driver so it's only source of energy is it's own
inertia. Adding to this energy store with a flywheel will therefore
increase the amount of electrical energy it can produce. The exact
electrical effect will depend on the generating efficiency of your
Exact mechanical figures can be calculated from energy stored in a
flywheel. Google flywheel energy without quotes and there are lots of
One point to be careful of is to make sure your flywheel is capable of
withstanding the centrifugal forces developed. When flywheels let go,
the results are dramatic and can be fatal. I take no responsibility for
any failures I'm afraid. Again, Google for details.
Perhaps you could just tell me which "Googgled" site has data to support
the statement that suggests that a flywheel on an idler in the RPC provides
benefit of any kind.
With one RPC test, I monitored the idler's RPM while the load on the tool
motor was adjusted from zero to full load. My instruments were not able to
detect *any* RPM variation of the idler thruout that test.
My first thought was that a flywheel should help with sudden loads on
the RPC. But when people said they ran tests and found it had no
effect, I revised my thinking. Without a flywheel the idler motor can
slow down quickly. When it slows the back emf against the single phase
input power drops , and current goes up. So I would expect that a
flywheel would lessen the current spikes on the input power, but the
generated three phase would be about the same, with or without a
jerry has conducted careful tests. I have conducted some "careless" tests.
I have an Abene mill that has a 6 hp spindle motor and a 2 hp traverse motor
that moves x,y,z axis. The rapids on this machine are actuated by
"instantly" reversing the 2 hp motor. I have only single phase power. To
make the mill start, I've added a capacitor and a relay (they fit into the
junction box at the rear of the mill) - the capacitor is in the circuit only
when the voltage on the missing leg is below about 60V (with respect to
neutral), then the relay pulls in and takes it out of the circuit. This is
about what a phase-o-matic type static inverter does, except that I took
advantage of the neutral to simplify the circuit a bit.
Now, some "experiment" results - 6 hp motor starts up fine by itself. this
is as you woudl expect. 2 HP motor starts and reverses "instantly" with or
without 6 hp motor running. Conclusion - no need for RPC, static inverter
is adequate. And, a capacitor and relay, even installed in every machine is
smaller, cheaper, and more efficient than a continuously running RPC motor.
Now, a VFD is another thing entirely.....
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