Speed control single phase induction motor.

[Eagleye]

I need to design a circuit which controls the speed of a single phase induction motor( 3 selection/switches for 3 speed level). I need to design this without using any microcontroller but it need to be efficient.
But I am not getting any idea that how should I do it. Most of the designs I have seen so far is using microcontroller which is not compatible with me.

So,It would be a great help if anyone can give me a starter, can give me any idea that what concept should I use or can you please refer links which have useful instructions or process.

Thanks,
Eagleye

 

[Harald Kapp]

Try a dimmer. Does the motor respond to different setting of the dimmer? If yes, you can:
1) set up three dimmers plus three switches to select one of the dimmers and thus the desired speed
2) replaxe the rheostat of the dimmer by three appropriatre resistors plus three switches to set the dimmer to a fixed phase angle.

No. 1 requires more investment, but uis probably safer since you don't have to open the dimmer.
No. 2 id cheaper, since it uses only a single dimmer, but you'll have to open the dimmer and manipulate the seeting control (rheostat).

Whatever you do: be extra careful when dealing with dangerous voltages and current. It is completely up to you to take all necessary safety precautions.

Harald

 

[duke37]

Yes but ....

If the load decreses as the speed comes down, such as a fan, then this could work. If the load torque is constant, the currents and heat in the motor will rise and, due to the reduced speed, the cooling will be less.

If the motor is small with shaded poles, it will be inefficient to start with and may not object to more heat.
If the motor uses a capacitor, current disribution between the two windings may be prejudiced.

 

[Harald Kapp]

I know, a dimmer is not the ideal tool for this purpose, just a quick off the shelf solution.

 

[Eagleye]

Actually the motor i need to control is a motor of a table fan and I need to do it by controlling voltage. In that case how should I do it?
Please help!

 

[duke37]

With a low powered motor driving a fan, the best soluton is to use a dimmer as Harald has said. You can get one that plugs into the wall and the fan will plug into that. Simple safe and cheap.

The dimmer varies the effective voltage by chopping the waveform with a triac.

 

[Eagleye]

Thanks to both of you for your help - Harald and duke.