# Speed control on a ceiling fan induction motor

I've managed, through the help of others here on Electronics StackExchange to get a ceiling fan motor running, at least at one speed, and in the right direction.

I want to enable 3 speed settings on the motor. How can I enable speed control of the motor?

I came across the following circuit diagram, is this method advisable? Won't this cause a lot of humming in the motor?

http://electronics-diy.com/1000w-ac-motor-speed-controller.php

• The circuit you posted controls the duty cycle and is therefore only applicable to DC motors. – Rainer P. Feb 7 '16 at 18:11
• It does say "AC Motor Speed Controller" in the page title @RainerP. – jippie Feb 7 '16 at 18:13
• Usually speed control on motors in this application works by changing stator windings, are there any unconnected wires left? Did the motor support multiple speed settings before you made it work? – jippie Feb 7 '16 at 18:16
• @jippie Yes, but it is actually about DC motors driven by an alternating current. In the description it says: The circuit incorporates a self-stabilizing technique that maintains the speed of the motor even when it is loaded. An AC motor does not change speed at all. Even under load. It always runs at a speed of f/n where f is the AC frequency and n is the number of poles. – Rainer P. Feb 7 '16 at 18:20
• That kind of 'speed controller' is really just an incandescent light dimmer. It works well enough on 'universal' AC motors like you would find in a drill, but not nearly so well on induction motors. It will definitely make the motor hum for any speed not close to max. – brhans Feb 7 '16 at 18:23

simulate this circuit – Schematic created using CircuitLab

Figure 1. 3-speed fan control.

It appears that the standard speed control uses additional capacitors in series with the windings. By switching in one or two capacitors three speeds can be obtained.

Table 1. Speed selection.

| SW1 | SW2 | Speed |
+-----+-----+-------+
| Off | Off | Off   |
| On  | On  | High  |
| On  | Off | Med   |
| Off | On  | Low   |
+-----+-----+-------+


Table 1 shows that switching in all the capacitance gives highest speed. What's not so obvious is that it also gives highest starting torque. Most ceiling fans use a pull-cord switch which cycles through the sequence above. The switch deliberately goes from off to max so that the fan will start reliably. The user can then slow down the fan by one or two further pulls.

The capacitor values will depend on your supply voltage and the rating of the fan motor. They need to be mains voltage AC rated.

• are you sure that circuit is correct? – Jasen Feb 7 '16 at 19:22
• I think @transistor's circuit is correct, I might just want to find a way to calculate or double check the capacitor values. Maybe this will help: youtube.com/watch?v=0RbbwY9vvs0 – josef.van.niekerk Feb 7 '16 at 21:28

I came across the following circuit diagram, is this method advisable? Won't this cause a lot of humming in the motor?

A light dimmer type of controller will work, but it will likely cause a noticeable humming or buzzing sound and may shorten the life of the motor. Reducing the voltage of an induction (asynchronous) motor will only reduce the speed slightly and then cause the motor to stall if the load requires a substantial amount of torque to operate below full speed. The torque required to operate a fan or centrifugal pump drops off drastically at reduced speed. That allows more speed reduction as illustrated below. The red curve shows the torque vs. speed requirement for a fan load. The blue curves show the motor torque vs. speed capability for various voltages. Stable operating points are points where the load requirement curve crosses the motor capability curve an near a right angle. The bottom operating point in the illustration might not be stable.

Reducing voltage is speed control by weakening the motor and allowing the load to slow it down. Changing the capacitor value or connecting to a different point on a winding produces a similar effect.

Note that the air flow of a fan is proportional to the square of speed. A speed of 90% gives 81% air flow, 80% speed give 64% air flow and 70% speed gives 49% air flow. Power is speed X flow 72% power at 90% speed and 81% flow.

• the fan-speed controllers in my house are little multi-tap autotransformers (or maybe even merely inductors; I haven't checked). Rotating the control knob switches between different taps and therefore different voltages supplied to the fan motor, and therefore different speeds. The operating principle (power reduction leading to speed reduction) is as per Charles' answer/graph here, but the mechanism for reducing power is not triac phase control. – William Brodie-Tyrrell Feb 8 '16 at 5:00