Speed Control for Induction Motors 1

[controlnovice]

Found this in an Electrical Engineering reference document.

All of Section 18 in the attached document seems poorly written. Second paragraph in Section 18 is very confusing to me. To change speed in an induction motor, I thought a VFD was required (PWM, etc) to change in the stator.

Any help with this explanation would be appreciated.

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This is normally the space where people post something insightful.

 

[dpc]

Various methods have been used over the years to provide some measure of speed control in induction motors. Wound-rotor motors were probably the most common. There are also eddy-current drives and other mechanically-based methods. And some other electro-magnetic approaches.

Your reference looks old. Older than me. VFDs are currently the best alternative 99% of the time for variable speed induction motors.

If you have a more specific question, let us know.

 

[Skogsgurra]

I don't think that it is poorly written. It just doesn't mention VFDs - perhaps it was written a few decades ago?

There are other ways of doing speed control than with a VFD. Stator voltage is a control variable that is commonly used for smaller fans, usually in the FHP range. But I have a couple of speed controllable motors with 1.5 and 2.2 kW. The trick is to have a high enough rotor resistance so that slip increases as voltage is taken down. The losses are kept within reasonable limits due to the fact that a fan needs less torque at lower speeds.

The other method, that gets mentioned in the text, is to change rotor resistance or changing the voltage seen by the slip rings. There are different methods and the cascade control method is mentioned. Both needs a WRIM.

The cascade can either use another machine or a solid state device. Usually a rectifier and a thyristor controller.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Changing the supply frequency is also impractical in most instances.

This statement is no longer accurate. VFDs have come into common use since this was written.

Bill
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"Why not the best?"
Jimmy Carter