# What's the theory behind speed control of a multi-tap multi-pole induction motor?

Ceiling fans are an application of multiple-pole capacitor-run/permanent split capacitor induction motors, and they appear to control the speed through a partially powering and shunting multiple-tap coil. How does the partial powering and shunting of such a motor control the speed?

My original question was migrated from the electrical engineering site to the diy site, but the question I was attempting to ask was about the theory of multi-tap motor coils and their use, with a ceiling fan as a practical, motivating example.

How is the speed control in this circuit I found on the internet supposed to work?

In the diagram shown below, with the switch table in the diagram, it appears that the fast speed setting energizes to only the reversing coil, and the low speed setting energizes only the shorter(/less poles?) coil but does not energize the reversing coil--how can these configurations possibly control direction? For medium speed, apparently the high #turns tap is energized, but the low #turns tap is shorted. If you energized the high #turns tap and shorted out the low #turns, would the speed be even lower?

I did not count the poles in my motor, since it appears to require a bearing puller to disassemble it. The ratio of resistance between the shorter and longer portions of the tapped coil is 30ohm:90ohms and the ratio of the resistance of the capacitor-fed coil to the tapped coil is 60:120.

I'm not entirely sure this circuit and switching diagram is correct, since it doesn't seem to control direction at the high and low speeds. How should the system work?

This diagram (http://imgur.com/qhHe5x4) is crudely modified to match my ceiling fan: A DPDT reversing switch, power comes into '1' and powers the secondary coil with 'L'. The wire colors match my motor.

Why does powering or shorting the different taps of this multi-tap motor affect the speed of the motor? Is it like a DC motor, in that there is some kind of braking action if you short across a coil? If you wanted high speed, would you power the full 120 ohm coil, the smaller (29ohm) section, or the 96 ohm section? Is the tap on this sort of multi-pole motor energizing some fraction of the total number of poles or is the tap putting more or less turns on the same number of poles? With one end of the triple tap coil tied to Neutral through the capacitor, how do you produce three speeds (High, Medium, and Low) using the two other two taps?

One thing that confuses me is that since a PSC motor should run at RPM=slip*120*Freq/Poles, the three speeds appear to manifest even when unloaded (the fan blades are removed) Does that indicate some sort of variable-pole behavior? (Apparently, slip is significantly increased by reducing the voltage, either by using resistors, capacitors, or inductors.)

I apologize for the number of questions here, but this tapped multipole motor seems a lot more complicated than the simple PSC induction motors explained elsewhere.

• electrical-contractor.net/forums/… - see scott35's answer on that thread, including diagrams. It appears that the speed control is achieved by auto-transformer action, which isn't readily apparent from the (very hard to read) diagram in your original post - the diagram in that forum thread is much better. I will answer properly once I get home. – Li-aung Yip Oct 25 '13 at 7:36
• Note by "autotransformer action" I mean that the motor has a built-in multi-tap auto transformer which is used to apply variable voltage to the motor. Think "variac". – Li-aung Yip Oct 25 '13 at 7:55
• Thanks. In Scott35's diagram, it appears that the capacitor-coil path is attached to the center tap of the auto-transformer, so that the same voltage is applied to both coils. Under no load, I would guess that the speed would be rpm=120*F/poles under operating voltages, but maybe under load, the motor would slip more, giving a reduced speed. Aside from the switch connections and logic, the found diagram in my post does seem to match my ceiling fan motor: 5 wires in two independent coils come from the housing, the capacitor is in series with the tapped coil, and does three speeds with one tap – Dave X Oct 25 '13 at 18:05
• I gave an answer a while back that is applicable here. It helps explain how changing voltage changes speed. – Eric Oct 25 '13 at 18:46
• Thanks Brad. What makes a motor a high-slip motor? Relatively high resistance coils? Squirrel-cage non-wound rotors? – Dave X Oct 25 '13 at 21:25