# single phase motor modified to mimic a 2 phase motor

A
Thread Starter

#### Anonymous

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.

R

#### Roy Matson

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.

Regards
Roy

P

#### 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!

4) CAVEAT.
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])

P

#### peter

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.

#### PhilCorso

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!

Phil

Similar threads