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Most common type of variable speed control for 3ph induction motors?

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Ok, in the EPRM manual (Camara) p. 42-13 section 20, he seems to say that variable speed control is most often done by introducing a foreign voltage into the secondary (rotor) circuit to oppose the induced voltage in the rotor. He also says that it is impractical to change the supply frequency in most instances.

In the industry I work in, we always use variable frequency drives for speed control. They are expensive, bulky, hot, and introduce harmonics. I don't know anything about this foreign voltage deal, but we would probably use it if it was a viable alternative to VFDs. Also, in the NCEES sample exam, problem 114 has a 3ph motor with a variable speed drive made up of transistors.

What am I missing here?

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Ok, in the EPRM manual (Camara) p. 42-13 section 20, he seems to say that variable speed control is most often done by introducing a foreign voltage into the secondary (rotor) circuit to oppose the induced voltage in the rotor. He also says that it is impractical to change the supply frequency in most instances.

In the industry I work in, we always use variable frequency drives for speed control. They are expensive, bulky, hot, and introduce harmonics. I don't know anything about this foreign voltage deal, but we would probably use it if it was a viable alternative to VFDs. Also, in the NCEES sample exam, problem 114 has a 3ph motor with a variable speed drive made up of transistors.

What am I missing here?

Seems like he's talking about how wind generators are able to sync to the grid without having a constant speed. I'm on the utility side, but I've only heard of the VFD drives as common use.

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Ok, in the EPRM manual (Camara) p. 42-13 section 20, he seems to say that variable speed control is most often done by introducing a foreign voltage into the secondary (rotor) circuit to oppose the induced voltage in the rotor. He also says that it is impractical to change the supply frequency in most instances.

In the industry I work in, we always use variable frequency drives for speed control. They are expensive, bulky, hot, and introduce harmonics. I don't know anything about this foreign voltage deal, but we would probably use it if it was a viable alternative to VFDs. Also, in the NCEES sample exam, problem 114 has a 3ph motor with a variable speed drive made up of transistors.

What am I missing here?

Before VFDs were popular with the introduction of power semiconductors that could handle the high power required, a doubly fed wound rotor induction machine was used as a variable speed motor. The rotor current frequency was varied to vary the speed. I think the Theodore Wildi text has the best explanation of the concept if you can borrow a copy. I would say it would be unusual to run into many of these motors today but they they are also used as generators in wind turbines. The complexity of the VFD is a trade off as the doubly fed machine typically has slip rings to deliver the rotor current which can cause another set of challenges.

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I guess Camara is just oldschool then. I have that Wildi book, I'll take a look. Thanks for the responses.

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