single phase motor modified to mimic a 2 phase motor


Thread Starter


But I found this:
Two phase motors are suitable for speed control"
Unfortunately no more; can a single phase cap start be modified by reducing cap value to mimic a 2 phase motor? (avoiding series resonance??!!)
If so, then can a 3 ph. VSD be connected, using just these two phases so created?

- this proposed here to avoid the mount probs assoc with a generic end mounted motor vs my specific one.
Where are you proposing to connect the neutral, to the 3rd phase as you have no neutral point.
I think you would find most VFDs would trip on Out of Balance.

In theory I see no reason why a VFD could not be programmed for 2 phases, it is generating 3 from DC after all. Perhaps someone knows of one that can configured to produce odd waveforms.


Phil Corso, PE

Anonymous, there could be two problems, one related to starting-torque; the other related to start-winding losses:

1) Starting-Torque for a 1-Ph Source.
Typically, a split-phase capacitor-start motor, has two parallel windings. They are physically displaced by 90-deg in space and a little less than that in time. The capacitor insures that the there is almost a 90-degree relationship between the start-winding current, Is, and run-winding current, Ir. Actual displacement is probably closer to 80-85 degrees. Without the capacitor, the run-winding current is about 25-degrees. Starting-torque is proportional to the SIN(<Is)/SIN(<Ir) where the symbol, <, means angle. Thus, starting-torque equals SIN(<85)/SIN(<25) or about 2.4 to 1.

2) Starting-Torque of Motor Using 3-Ph Source.
The phase-displacement in time for a 3-wire, three-phase, source is 120-degrees! Then, using the same relationship, the starting-torque SIN(120)/SIN(25) or about 2 to 1.

3) Start-winding Losses.
Because the start-winding has less impedance than the start-winding, current during normal operation is greater than that of the run-winding. Thus, the centrifugal-switch to disconnect the start-winding must still be used!

The above scenario is based on the 3-ph source having an a-b-c rotation. If you don't identify the actual sequence, you run the risk of reverse rotation.

In conclusion, I recommend that you not proceed!

Regards, Phil ([email protected])
2 phase motors that I have found are modified for higher voltage opperation, and removing the mod allows them to run on a lower voltage. The mod became something simple like a suitable cap. Single phase speed control would require a dedicated drive to a dedicated motor. Small AC drives have allowances for motor selection depending on the drive, namely a shunt resister to adjust the drive to a type and size of motor. Now given that I know nothing about your motor or application & needs, if details are a little sketchy and the original motor is no more, then good luck with the mod or the application or the modification or the repair.
Anonymous, if you need only the variable-speed function, try this:

Set the VFD to the motor's rated frequency, start the motor. Wait until it has accelerated to normal speed and the start-cap is disconnected! Then adjust the VFD to any desired speed!

Of course, some precaution is necesssary!