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Since87 · 12-03-2002, 11:16 AM

I've been doing a little research on AC motors lately and I don't think you can count on a Variac working well either.

Many AC motors are designed to operate in sync with the AC line frequency. Oftentimes the normal RPM of an AC motor will be:

(Line Frequency / 2 ) * 60 seconds/minute = 1800 RPM (@ 60Hz)

The best way to adjust the RPM of these motors is to vary the frequency of the power applied. ( Google VFD controller )

A variac will adjust the amplitude of the drive signal but not change the frequency.

I don't know enough about AC motors to say with any accuracy what happens when the voltage amplitude is lowered. I believe the fan will probably stay at it's normal RPM until the force applied to the blades prevents the motor from staying synced to line frequency, at which point the motor may behave erratically.

12-03-2002, 11:16 AM	#2
Since87 Pro/Guru - Uber Mod Join Date: Sep 2002 Location: Indiana Posts: 834	I've been doing a little research on AC motors lately and I don't think you can count on a Variac working well either. Many AC motors are designed to operate in sync with the AC line frequency. Oftentimes the normal RPM of an AC motor will be: (Line Frequency / 2 ) * 60 seconds/minute = 1800 RPM (@ 60Hz) The best way to adjust the RPM of these motors is to vary the frequency of the power applied. ( Google VFD controller ) A variac will adjust the amplitude of the drive signal but not change the frequency. I don't know enough about AC motors to say with any accuracy what happens when the voltage amplitude is lowered. I believe the fan will probably stay at it's normal RPM until the force applied to the blades prevents the motor from staying synced to line frequency, at which point the motor may behave erratically.