Shaded pole motor speed control

Question

I've recently been given a 24W 230VAC 2 pole shaded pole motor(Mellor Electric AC1004) to make a fan out of. This fan needs to have a variable output speed depending on the temperature of the room. I am quite new to motors and was wondering how to make the motor speed variable. I have done some research and seen that by just varying the voltage the speed can be reduced, however I believe this is not the best way to go about this. I've also seen people use Triac based dimmers in order to accomplish the same thing as well as VFD's but they are quite expensive(VFD's) and i don't think they are suitable to this. I would like some input as to what method would be best. Thanks.

Answer 1

Voltage-only control of a Shaded Pole (SP) motor is an acceptable method and is the most common. It doesn't work on other types of AC motors, but because of some of the inherent design aspects of what make a SP motor work in the first place, the torque / speed relationship is more or less linear, or can be made to appear so. The point is, you CAN use a single phase output VFD if you have money to burn, but a "dimmer switch" works fine and will not hurt that motor.

Answer 2

You first have to understand the speed of any drive is the result of the torque of the motor and the countertorque of the mechanism you want to drive. This is called the operating point. Changing the speed means moving the operating point.

These are (roughly) the hilly torque/speed characteristic of an asynchronous machine (as a shaded pole motor is) and a pump/fan.

• If you placed an obstacle in the air channel of the fan, you made its characteristic steeper, so the speed of the whole drive would reduce about a tiny bit. I give this example to show you how speed control works.

• If you reduced the voltage, the motor characteristic shrinks along the torque axis. Again, the speed of the whole drive would reduce only a tiny bit. If you reduced the voltage too much, the motor hadn't got enough torque to drive the fan. It never reaches the part of its characteristic right of the hill.

• If you reduced the frequency, the motor characteristic shrinks along the speed axis. That's a great way to control the speed of the drive, but to avoid magnetic saturation of the iron parts, it had to be accompanied by a proportional voltage reduction for a given motor.

• If you placed a resistor in row with the rotor windings of the motor, you made it's characteristic tilt left. Now voltage reduction will reduce the speed of the drive much more. That's what's done in motors designed for that kind of control, the ones usually used for fans, vacuum cleaners and so on. This is the so called squirrel-cage rotor. If your motor has such a rotor, reducing the voltage is the best way to control it.

Answer 3

Triac-type dimmer switches are commonly used to "throttle" ceiling fans. They have the shaded pole motors, so if you have a shaded pole motor device, go right ahead to use a normal dimmer as a throttle for that device. Also, series-wound "universal" motors can be throttled the same way. So can many (though not all) brushless DC motors can be throttled. Note that brushlesss DC motors have electronics to do the commutation for otherwise a 3 phase synchronous motor.