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.


4 Answers 4


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.
  • \$\begingroup\$ After I looking up what an auto transformer is, I now understand what is going on. A typical transformer has a two windings. An autotransformer is a single winding. With an autotransformer the “height used”/impedance of the shared input and output winding determines the transformer ratio. The autotransformer in the motor works equivalent to a conventional transformer connected to the H speed line input. — why is fan speed proportial to its mechanical efficiency?? I wouldn’t think that a lower fan speed would behave less mechanically efficiency or significantly so.— Will try a transformer \$\endgroup\$ Commented May 31, 2019 at 2:32
  • \$\begingroup\$ The amount of air moved is roughly proportional to the mechanical power of the fan. That alone already means at lower speeds it's lower. But as a fan has a quadratic torque over speed characteristic, the amount of air depends on the speed to the third power. → If you plan to run a motor at low speeds, you have to oversize the cooling fan or have an external fan or a gearbox just for the fan. \$\endgroup\$
    – Janka
    Commented May 31, 2019 at 12:09
  • \$\begingroup\$ But I don’t want to keep cfm constant. I am lowered the motor speed, to lower fan speed, to move less air, but ultimately make less noise. I am okay with reduced circulation. \$\endgroup\$ Commented Jun 1, 2019 at 1:51
  • \$\begingroup\$ The question is if the motor is okay with less circulation. It heats up if the slip goes up. \$\endgroup\$
    – Janka
    Commented Jun 1, 2019 at 2:10
  • \$\begingroup\$ Can you explain the auto transformer concept a bit more, I'm trying to understand what is happening inside the motor when power is applied to the different taps. Each tap has a different resistance, however when we apply say 120V to any of the taps, is the voltage being stepped down/up like it would be in an autotransformer or is it 120, also what is the load in this case, is it the capacitor winding or is it the non electrically connected rotor? Is the difference in speed simply due to a lower currents because of higher resistances, resulting in weaker magnetic field, thus less torque? \$\endgroup\$ Commented Aug 17, 2022 at 20:24

I don't believe that you have described the connection correctly. I don't think that the poles are connected differently. The effect of the low and medium connections is pretty much the same as reducing the voltage externally with a dimmer type control or or series resistance. However the series resistance method would have additional losses in the resistors and the dimmer control would also have additional losses.

The lowest speed that can be achieved is determined by the motor and load characteristics. The motor needs to be closely matched to the load. It is likely that attempts to reduce the speed below the factory set low speed would not be very effective.

The motor efficiency at lower slip is lower, but this sort of speed control is only used for fans and centrifugal pumps that naturally use much less power at lower speeds. Fan driving power is proportional to speed cubed. Driving a fan at half speed requires one eighth of the power required at full speed. The result is that reduced motor efficiency does not result in more total power used.


IMO the capacitor it there in order to generate a 90° phase shift (while it would also reduce the voltage, which is compensated by a different design of the auxiliary winding).

Shouldn't it be possible to disconnect the capacitor and have the phase shift and the voltage reduction done by a modified 3 phase VFC, using just two of the outputs?

The VFC could maintain the phase shift while adjusting the frequency for reduced speed. The output voltages would have to be reduced with reducing speed because of the lower inductie resistance of the windings. Magnetic losses should reduce with lower frequency and lower generated power, so the ohmic losses should dominate the heat generation.

As the ohmic losses are proportional to the winding current, the output voltage of the modified VFC could be adjusted to keep the effective current at the level it is with standard frequency and voltage use. That way the motor shouldn't overheat and still have a fairly high torque. It would still be much lower torque than at nominal speed, but not too far less than the optimum at the desired speed (as magnetic losses might be reduced, higher ohmic losses might be allowable). Also, if the motor runs at lower speed, cooling might be a bit less effective, too.

  • \$\begingroup\$ Welcome to EE.SE. We tend to think of the capacitor giving a 90° phase shift but when in series with the winding inductance it will be very much less than that but enough to give a direction to the spin. I can think of a problem with your three-phase VFC idea: Most don't have a neutral and using all three phases means the voltage would be wrong. \$\endgroup\$
    – Transistor
    Commented Apr 4, 2021 at 11:43
  • \$\begingroup\$ Aren't they able to control the amplitudes as well? A PSC motor has 3 terminals, so the 3 outputs of a VFC could be used to supply the voltages and phases actually needed. It just has to provide different ones than those needed for a standard three-phase motor. Are there ones with completely free programmable outputs? \$\endgroup\$
    – GerdF
    Commented Apr 5, 2021 at 12:26
  • \$\begingroup\$ VFDs control amplitude using PWM so the voltage applied to the winding will be \$ \sqrt 2\$ times the mains voltage. For a VFD with single phase input it might be OK. For 3-phase the peak voltage will be much higher and may stress the winding insulation to breakdown. \$\endgroup\$
    – Transistor
    Commented Apr 5, 2021 at 12:32

Although the drawing is basically correct, a PSC motor will slow down and stop if loaded beyond the torque peak.

Speed control, at same AC source frequency, can be accomplished by reducing drive to operate at higher slip and/or jumping from a two pole to four pole configuration. Slip is very limited in achievable speed range, where jumping two to four poles cuts speed in half.

The capacitor, at optimum value, provides a 90 degree current phase lead to auxiliary (start) winding. This achieves maximum torque, greater peak mechanical power, better power factor, and less slip. A side effect of using a series capacitor to synthesize the second 90 deg offset source phase is there will also be an impedance transformation which causes a higher voltage on auxiliary/start winding.

Most all PSC motor have slightly greater number of winding turns with slightly lower gauge wire for auxiliary winding to account for the slightly higher auxiliary applied voltage. Swapping run and start winding will make a PSC motor run backwards but it is not advisable due to the different number of winding turns and the same value run capacitor will no longer provide the optimum 90 deg phase shift.

90 degrees winding phase current shift does not mean 90 degrees voltage shift due to difference in winding inductance and series resistance between run and auxiliary windings.


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