Rotary Phase Converter

I have a 220 v 3 phase, 3/4 horsepower motor and want to run it from a 220 v single phase source. I have a 220 v 3 phase, 3 horsepower motor that I would like to use as a rotary phase converter. I have a 25, 35, 40 and 40 uf capacitors. I need help on how to wire these.

Thank you.


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For $ 156 you can get a vfd it has speed control, breaking ,foward ,reverse . Its alot better than rotary phase

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You probably don't need the capacitors.

Connect your two 240 volt incoming line to two of the three leads in the large motor, via a fused knife switch.

Connect your load (3/4 hp) motor to all three of the wires of the idler motor - ie, motors in parallel - via whatever switch you are going to use to run the load motor.

Spin the large motor up with a small single phase (pony) motor, getting going at least as fast as the nameplate speed, and then throw the knife switch to apply the excitation.

Uncouple the pony motor at this time and turn it off.

Measure the line-to-line voltages on your new three phase supply, they will probably be within ten percent or better for load motor that small in comparision to the idler motor hp rating.



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jim rozen

You know the 2 wires you connect to get 220 single phase power? Call those line 1 and line 2 (L1 and L2). Connect those to any two of the 3 legs of the 3hp 3-phase motor through a 30A single phase disconnect (about $10) using Buss slow-blow fuses in the disconnect box. Connect your load motor (the 3/4hp one) to L1, L2 and L3 via another cheap disconnect box. Put a small (10x10x6") electrical box somewhere close and run L1, L2 and L3 to that box. Put an oil-filled motor run capacitor rated for 220VAC between L1 and L3, and another between L2 and L3. These will be sized big enough to quiet the buzzing down and small enough so it doesn't start back up again. If you want to start it using a rope then you're done, otherwise you need to add some electrolytic motor start capacitors between L1 and L3 in some kind of circuit that will take the capacitors out after the motor has started. This can be done using a heavy duty pushbutton switch, or by using a potential relay from Grainger to automatically take out the start caps.

There are real good schematics available online, but I don't have the link handy. Google will tell you a lot.

Grant Erw> I have a 220 v 3 phase, 3/4 horsepower motor and want to run it from a 220 v

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Grant Erwin

I have a 220 v 3 phase, 3 horsepower motor that I would like to use as a rotary phase converter. I have a 25, 35, 40 and 40 uf capacitors. I need help

For a 3 HP "self starting" RPC, you'll need about 33 uF from the A-phase to the B-phase.

That's about 100 uF, which is achievable using the 25, 35 and 40 uF caps.

(They should be rated at least 370 volts).

For a better result, I would start by placing the 60 percent of the total available capacitance from the A-phase to the B-phase, and the remainder from the C-phase to the B-phase, thereby achieving a more balanced RPC.

If the RPC then won't self-start, then you're going to have to add a potential relay and an electrolytic cap of about 360 uF from the A-phase to the B-phase.

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Peter H.

Here is a link to the Frugal Machinist's (Roy) Rotary Phase Converter page. This man's website is one of the most informative and interesting I have seen. I used his plans, had a little problem with the relay, so I use a momentary switch in it's place.... The unit I built using his plans works flawlessly. Pat Landy

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Another question about RPCs....

Im currently running much of my shop on a 5hp commercial (but old) rotary converter, but have been running out of power once in a while. I have a big Clausing 1500 (7hp) that will start ok in direct drive, but tends to get boggy in any of the gear speeds and when I do a fast reverse, it pops the 30 amp breakers feeding the RPC.

The commercial RPC sounds like a jet turbine on windup when it runs..I have it hidden behind a lathe, but it still makes my teeth itch and actually puts me in a bad mood if I run it for a long period of time.

A guy has offered to give me a very nice, older and very well balanced

20hp motor. "pick it up or we will dump it in the trash".

What sort of cap values is this gonna need?

Should I spin it up with a pony motor? Im up to my butt in belts and pulleys, so this shouldnt be a

What sort of current draw will it have in idle? Will my electric meter be spinning like a dervish?

Should I simply hunt around for a 10hp motor instead?

Thanks all. Gorton mill runs on its own 5hp VFD.


That rifle hanging on the wall of the working-class flat or labourer's cottage is the symbol of democracy. It is our job to see that it stays there. - George Orwell

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For a balanced converter, the rough rule of thumb is 32 uF per hp. So you'll need about 640 uF total. How you distribute that between L1-L3 and L2-L3 will need a bit of experimentation to achieve best balance.

Darn straight. Starting surge on a self-starting 20 HP converter is going to be well in excess of 200 amps. I have a 20 hp converter, and I use a 1/2 hp 1 ph pony to start it. When I slam in the switch, the lights don't even flicker. But trying to self-start it always resulted in a popped breaker.

No. Windage losses, bearing drag, and I^2R losses in the windings should be under 500 watts no load. If it isn't well balanced and power factor corrected, reactive currents can get pretty high (up to around

90 amps), but reactive amps won't cause your kWh meter to spin (though they do require you to size the breaker to accomodate them). If it is well tuned, reactive currents should be much less. I run my (tuned) 20 hp converter on a 50A breaker.

Naw, bigger is better (within reason). You'll have better wild leg regulation when the rotary is much larger than the load motors. The *minimum* size is for the rotary to be 1.5 times the hp of the motors it will power. But going several times larger than that will result in better regulation and a cooler running rotary. My biggest load motor is the 7.5 hp motor in my Clausing Colchester. The rotary hardly notices when I start it.


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Gary Coffman

JUst noticed I never put the link in.......

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