Pony-Start Rotary Phase Converter

Copyright 2006-2009 by Jim Wilson
Click images to enlarge.

This is a 20HP Rotary Phase Converter I built to power my vertical machining center. CNC equipment can be sensitive to unbalanced three-phase power, so this one is "tuned," that is, it has phase balancing and power factor capacitors.

20HP Rotary Phase Converter Circuit Diagram
This diagram links to a high resolution PDF version (18kB).

The starting in-rush current for a 20 horsepower three-phase motor running on single phase power is monstrous. This design avoids that -- and the huge capacitors that a self-starting RPC would need -- by applying power to the the idler motor only after it is brought up to speed mechanically by a relatively small single horsepower "pony motor."

In the diagram above, the wild leg (also called the "manufactured leg" or "generated phase") is labeled as B. I have found the wild leg is commonly labeled C in other phase converter designs. Bear this in mind if you compare my design with others you may see.

The symbol "Vab" refers to the RMS voltage between terminal A and terminal B. Likewise, "Vac" is the voltage between legs A and C, and "Vbc" represents the B to C voltage. Note that Vac is simply the incoming line voltage, whereas Vab and Vbc involve the wild leg.


The Start/Run switch S1 toggles power between the pony and idler motors. Initially, it is placed in the Start position. Then the momentary Power On switch S2 is pressed. This latches the main power contactor K1 and supplies power through S1 to the pony motor contactor K2, and through K2 to the pony motor itself.

The pony motor is mechanically linked to the idler motor by a v-belt. After the pony motor has brought the idler up to nominal speed (this takes one or two seconds), the Start/Run switch S1 is placed in the Run position. This simultaneously opens the pony contactor K2 and closes the idler contactor K3, thus removing power from the pony motor and supplying power to the idler motor. To shut down, the normally-closed momentary Stop switch S3 is pressed, which unlatches the main power contactor K1, removing power from the system.

The pony motor start-run capacitor is disconnected whenever the idler is running. Otherwise, instead of merely coasting along, the pony would act as an electric brake and draw unnecessary power from the idler.


The capacitor values on the diagram are what my converter required in order to be optimally tuned. These values depend on the particular characteristics of my 20 HP idler motor. Your converter will almost certainly need different values. That's the reason for the adjustment notes.

Connecting multiple capacitors in parallel makes an equivalent capacitor whose value is the sum of the individual capacitors' values. I.e.,

Ctotal = C1 + C2 + ... + Cn       (See [1] for capacitors connected in series.)

In the diagram, Cp, Cs and Cpf are each represented as a single device, but in the converter, each of them actually comprises multiple physical capacitors. For example, Cs was really two 30uF capacitors connected in parallel.

To accomplish step 1, add capacitors between the A and B legs (i.e., increase Cp), as much as needed to raise the Vab voltage to 1.05 times the line voltage, Vac.

Then, step 2, add capacitance between B and C until Vbc matches Vab. Note that Vab usually changes also as Cs is adjusted. That's fine. Just tweak Cs until Vbc = Vab. You don't need to repeat step 1 or re-adjust Cp.

In step 3, a small capacitance between A and C will reduce the line current (monitor this with a clamp-on ammeter on the L1 or L2 wire). As more and more capacitance is added to Cpf, the line current will continue to decrease until it reaches a minimum value and then it will begin to go back up. Use the capacitance that gives the lowest line current.

The first two steps balance the phase voltages. The third step corrects the power factor.

This tuning method is designed to be performed with the idler running [2] but with no electrical load. (The mechanical load of the pony motor is ok.) When the idler is electrically loaded, the wild leg voltages sag a bit. For this reason, Vab and Vbc are tuned slightly above the line voltage with the idler unloaded.


None of the parts were obtained new. The pony motor is from an old pool pump in which the original 30uF start-run capacitor had failed. I replaced the bad part with a 60uF cap to increase the starting torque. That rendered the pony capable of spinning up the 20HP idler, which has a rather significant inertial mass. I was in a hurry to get this thing built, so the idler motor came from a local surplus outlet for $10/HP: $200. In the past, I've seen similar motors at the salvage yard, which sells them at 15 cents a pound, or under $30 for one this size. The few motors I've picked up at salvage required new bearings at most, or perhaps another $15. Such is the cost of impatience.

Likewise, a carefully-filled box of run capacitors was purchased at a surplus store. The cost was $127 for an excellent variety that allows accurate sizing of the tuning caps. In the end, the converter used perhaps half the box. The switches and contactors and were snagged from a junked industrial control found at the salvage yard long ago. I actually removed the parts there, and doubt they cost more than a dollar or two total, as the salvage yard sells by weight.


I'm especially grateful to Fitch Williams for his invaluable guidance and generosity when I was building my first rotary phase converter years ago. Many discussions on the rec.crafts.metalworking USENET group were also enlightening.


  1. ^Connecting capacitors in series reduces the total capacitance. This is less likely to be useful in our present application, but in the interest of completeness, the formula for series capacitance is:

    Ctotal = (1 / (1/C1 + 1/C2 + ... + 1/Cn))

    That is, the total capacitance is the reciprocal of the sum of the reciprocals of the individual capacitors in series.
  2. ^For safety, disconnect power when altering the circuit (i.e., adding or removing capacitors, etc.) and then power up again to take measurements.

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