Single-Phase Induction Motor Control with Electronic Fan Regulator

It depends on the type of single phase motor you are using.

As you know, the Induction motor (any type) has a speed torque characteristics. A Motor must run at the slip at which the load intersects with OC curve.

The reduction in the torue may either reduce the speed or stall the motor (as in your case)

The fans used for the home fan have the characteristics that suits this (ie it has a high starting torque and matches the load characteristics)- If I remember these are shaded pole indution motors where the torque requirement rises with the speed and motor also has similar slip-torque characteristics.

Thanks for the reply you shared with me.

Did it mean that variable voltage control can not be applied to such motor for speed controlling?

Then what trick would work in my case?

My single phase motor is capacitor start type.

Regards!!!

variable frequency drive

all single phase come in several types, many of which cannot be easily speed controlled.
Only PSC and Shaded Pole motors can be speed controlled by external means.

PSC motors can be controlled with variable voltage VFDs. S

haded pole motors can be controlled also by variable voltage alone .

Other types have centrifugal switches and capacitors that will interfere with or damage speed control devices.
check a few links

http://www.designnews.com/article/9323-Three_Ways_to_Control_a_Single_Phase_Induction_Motor.php

http://www.scialert.net/pdfs/jas/2005/249-252.pdf?sess=jJghHkjfd76K8JKHgh76JG7FHGDredhgJgh7GkjH7Gkjg57KJhT&userid=jhfgJKH78Jgh7GkjH7Gkjg57KJhT68JKHgh76JG7Ff

also may search "Single phase induction motor speed control" in google

http://www.google.co.in/search?hl=en&q=single%2Bphase%2BInduction%2BMotor%2BSpeed%2BControl&meta=

If your motor has a centrifugal starter switch you have to forget the speed control. Otherwise you can control it with a variable frequency drive (maybe accompanied with variable voltage control)

Although a VSD will work, an economical solution would be a resistor-type speed control.

This is the old fashioned style, where the voltage is dropped across one or more medium duty resistors, and the applied motor voltage is reduced as a function of the speed setting. The first setting on this type is always "high" such that full voltage (no resistor) is applied as the motor starts. It should be mentioned that, this type does not use a true resistor, but a network, as it is an ac application. Although this could be considered an electronic device, it differs in what I believe you have tried- a pulse modulation or switch-mode type.

What are you driving with the motor? It may be more economical to vary the load instead of the driver speed. If you are driving a fan, variable inlet vanes will accomplish fan variability with very little electrical loss.

You cannot apply any sort of speed control to a capacitor start motor. it will not work. The starting capacitor is controlled by a speed switch, and if you lower the speed, the switch will re-engage, putting the start capacitor back into the circuit. At the very least, that will over-excite the motor and eventually damage it.

If you use a VFD, the capacitor charging current will look like a short circuit to the transistors in the VFD, and the rapid rise in current wilol eventually damage them. At the same time, the high harmonics of the VFD will overheat the capacitor, causing it to fail quickly. It usually ends up as a race to see which one dies first.

It's pretty obvious that you cannot speed control a cap start induction motor with the starter winding on. However, depending on the application, if starting torque isn't a killer (i.e. used for a wood shop disk sander, table saw, etc...), the starter switch can easily be "tuned" to kick out eariler. For that case, the motor speed should be controllable within the primary winding range. In fact, that switch can be modified so the kick in rpm (when turned off) is lower than the kick out rpm. Then as long as you start the motor at full rpm, you will get all the starting torque and can dial back the speed. The controller would need a limiter that would not allow it to drop below the starter kick in speed. In fact, at the lower speed, if the motor is loaded to drop into the starter winding, you would want a cutout switch so nothing would get damaged. With a closed loop encoder, you should be able to vary the current/frequency to hold the speed within the motor torque limits. Do the leg work and get back to me. mgc@everestkc.net. Thanks.