Electronic – Controlling speed of PSC induction motor (Questions about operating at high slip)

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Induction motors typically run synchronous speed since most motor types (CS/CSCR/Permanent Split Phase) have high low-end torques. A PSC motor on the other hand doesn't have much low-end torque.

FAN applications don't require much starting torque, this means you can operate at a significant slip away from synchronous speed (speed reduction) by lowering the toque created by the the coils without fear of stalling the motor.

(The synchronous speed of the motor will remain the SAME at ANY supplied voltage but the resulting operational speed can be significantly different when using lower power coil-torque configurations. At low coil power the motor will settle at a higher slip operational point on the performance curve.) Coil-torque can be controlled by varying the voltage supplied to the coils.

Here is a Motor Torque vs Fan Load at different operational voltages.

Motor Torque vs Fan Load

After much research, common (under 1HP) 3-speed PSC motors have speed control implemented by using extra windings on 2 of the 4 poles of the MAIN WINDING.

Here is the motor schematic:

{PSC motor wiring schematic

Motor Wiring (Red = Main Winding, Black = Auxilary Winding, Blue = extra speed control windings)

I have a few questions

1) It seems the voltage reduction present, feeding the main+auxilary windings, is only due to the resistance present from the two additional coils (blue). Is there anything significant about inductance present from these windings? (Would a resistor voltage divider have the same performance and efficiency loss?

2) If bypassing the extra windings, can you instead connect a transformer OR dimmer (triac) to the HIGH line input and achieve the same speed control but with greater efficiency?

3) How much slip can be achieved? If adding a transformer or a dimmer onto the line input, can speeds lower than what is built into the motor via the low speed wire be achieved?

4) What happens to motor efficiency at high slip?

I've searched for hours on google, but still couldn't find the answers.

Best Answer

You have to understand an induction motor is just a transformer with an odd rotating, shorted secondary.

  1. Those additional primary windings are magnetically coupled with the main winding and thus, the arrangement works like an autotransformer (plus the main transformer with the odd rotating, shorted secondary).
  2. You get a finer speed control at slightly less efficiency. But the main problem with a triac dimmer is the voltage and in result the currents become non-sinusoidal, which makes the motor rattle audibly. That's why ceiling fans typically don't use this method.
  3. The more slip, the more losses in the rotor. As the fan on the motor axle is less efficient at lower speeds, that's a double penalty.
  4. The electrical power going into an induction motor is pretty independent from the speed. (Of course, at lower voltage, the electrical power is also lower.) All what's not turned into mechanical power is turned into heat. So you don't want to operate an induction motor continously at more than 10% slip. Ceiling fans seem to be the one exception, because people demand it.
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