From: John Larkin
Subject: Re: Is it possible to run a shaded pole AC motor at half speed?
Date: Wed, 11 Sep 2002 13:56:33 -0700
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On Wed, 11 Sep 2002 16:28:23 +0000 (UTC), email@example.com (Wayne
>My bathroom fan is too loud, and I can't change it because I rent. So
>I was wondering if there is some way to provide the option of running
>it at half speed. The fan motor is a shaded pole AC motor, and the
>specifications on the fan say "not for use with solid state fan
>controls". I assume that rules out the readily available fan
>I understand from reading a January, 2002 thread that it is hard to do
>speed control on single-phase motors, so perhaps I am asking the
>impossible. The following possibilities did come to mind, though:
>1) Change the frequency of the AC supply to 30Hz. Is there any simple
> way to do this other than an AC to DC to AC circuit? Is a shaded
> pole motor designed only for a particular frequency, say as far as
> the location of the shadings, or will it work at half frequency?
Yes, but too much work.
>2) Change the voltage of the AC supply. I believe that a very low end
> motor like this runs with a lot of slip. Is it possible that by
> lowering the voltage I could increase the slip to as much as 50%?
> Would that be bad for the motor?
Should be OK.
>3) Add another tap to the winding in the stator. This might be tricky
> to do, but if I could tap the motor winding in the middle, then
> would running with half the winding halve the magnetic field
> strength and increase the slip? Would that be bad for the motor?
DON'T try that. Toast city.
>Any comments would be welcome, thanks.
>Best wishes, Wayne
>P.S. How do three-speed ceiling fans do their speed control?
If you reduce the applied voltage, it will certainly slow down and get
quieter. I sometimes run 240 volt muffin fans at 120 to get a gentle
air flow and near dead silence.
Try a series resistor; wild guess 50-200 ohms, wirewound power
resistor. It will get hot! A series capacitor would work and not get
hot; maybe 10 (??) microfarads, 200 volt mylar or other nonpolarized
A stepdown transformer or autotransformer would work too. I bet 3/4 of
full line voltage would be a lot quieter. A 24-volt bell-type
transformer could be used as a buck-type stepdown autotransformer.