"mc" wrote in message
...
"Electromotive Guru" wrote in
message reenews.net...
I agree much with Default, yet there is one general rule for any
induction motor: They are not variable speed. You can select
different speeds based upon the wound poles, but to cause the motor
to slip excessively is not an operable condition. The only way to
safely adjust the speed of any induction motor (aside from
shaded-pole) is to reduce frequency and voltage amplitude on a
measured scale.

Am I right in thinking the RMS voltage should be proportional to the
frequency when this is done?

Yes, the voltage and frequency are usually reduced. To vary the speed of AC
motors a device called an inverter is used. Internally it will convert the
AC to a DC voltage and then convert it back to an AC type of voltage that
the frequency and voltage are matched to the motor load. I say AC type
because most of them do not put out a sine wave but odd looking waveforms.
Some use several steps in the output to step up to the maximum voltage and
then back down to 0 and then the same for the negative half cycle. We use
some where I work that put out a very odd waveform and normal volt and amp
meters will not work correctly on them. They use something called a
carrier wave that is operating at from around 5 to 15 kiloHz. No where the
normal 60 Hz or less you would normally think of in the US.

Here is a link to the type I am thinking of.

http://www.yaskawa.com/site/products...Drives~J7.html