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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.

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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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  • \$\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\$ – whiskeysip69 May 31 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 May 31 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\$ – whiskeysip69 Jun 1 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 Jun 1 at 2:10
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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.

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