What is the basic principle for varying speed of ac motors? In domestic appliances, for example, air conditioners, the small ac motor speed can be controlled by the switches. I know from the formula n=120*frequency/poles that you only can change the power frequency and the motor poles number to change the motor speed. But I do not think the air conditioner motor speed is changed by the above method. And different books have different answers for it. What is the principle for that down to earth?

 

Small, single phase induction motors (fractional horse power) can be speed controlled simply by reducing the voltage (only really suitable for fan motors, or motors with a constant physical load). However, this is a wasteful method, as the voltage must be reduced through a resistor, which dissipates the voltage as heat. A variac can be used (a variable voltage transformer), but these are expensive.

The next stage up from the previous methods for controlling small, single phase motors is to use an electronic 'chopper'. This method is popular in controlling the speed of condenser fans in AC units and power tools. This method uses a triac (a form of electronic switch) to cut-off the voltage part way through each half of the mains sine wave cycle. The ‘firing’ of the triac is usually synchronised to start as the sine wave crosses through zero volts, to help reduce electrical ‘noise’ The effect of 'chopping up' the sine wave is to reduce the 'power' applied to the motor windings, thus reducing the speed. Not all small motors can be controlled by this method, as the resultant 'dirty' and distorted sine wave can produce horrendous harmonics that will heat up the motor windings. Check with the motor manufacturer first!

None of these methods are suitable for controlling three-phase motors, no matter how small. The only proper way to control the speed of three phase motors is to use a ‘drive’ (an electronic ‘inverter’). This unit rectifies incoming AC mains to DC, This DC voltage is smoothed, and applied to some form of fast-switching electronic device (such as transistors or GTO devices). Using sophisticated PWM techniques at a very high switching frequency, the sine wave is recreated on the output. However, by varying the PWM shape and frenquency, it is now possible to alter the frequency of the recreated sine wave, and hence the speed of the motor. This is a simplified explanation of a ‘drive’ – other factors such as the voltage and max current can be controlled too; the ijternal logic can even tailor the output of the drive to suit the motor application; pumps, fans, conveyers – all that require different torque and start-up requirements from a motor. Drives are so cheap and easy to set-up and use, there is really no excuse not to use them now. Other benefits of using drives are less wear and tear on mechanical plant and even reduced running costs if a motor doesn’t have to run ‘flat out’ (often the case with fan and pump applications).

Hope this helps!

Duncan Grant, Technical manager,
Ecotech (UK) Ltd,
www.ecotechuk.com

 

Typicaly in industry variable speed drives are used to change the frequency, and thus speed.
In your case, the switches are used to short motor poles together thus changing the speed. This is usually done in single phase motors only because switching multiphase motors would involve way to many contacts wires and wires to/from the motor to be economical.

 

I agree with you, that is, by changing the number of poles to alter the motor speed. But it seems Mr. Duncan Grant's comments are reasonable too. Would you please explain that for me again. Thank you for your help!

 

Thank you very much for so detailed explaination for my questions. I got a lot for that. But I am not sure if there is another approach to change single motor speed except changing poles and power frequency. Could you please spare more time to explain it for me? Thank you again!