Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!think.com!mintaka!ogicse!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Simple question
Message-ID: <1991Apr15.180603.6851@milton.u.washington.edu>
Date: 15 Apr 91 18:06:03 GMT
Article-I.D.: milton.1991Apr15.180603.6851
References: <1991Apr11.131807.2565@msb.com>
Organization: University of Washington, Seattle
Lines: 23

In article <1991Apr11.131807.2565@msb.com> cmm1@msb.com (Chris Mauritz) writes:
>Hello!  I want to add a pot to an AC fan I added to my power amplifier.

	Sorry, won't work.  For best efficiency, many (most) AC motors
operate as hysteresis/synchronous motors, so to change the speed
requires changing the AC line frequency.  Adding resistance as
you propose will reduce the stall torque first, then (as the lubrication
and air drag begin to stall the motor) will cause the motor to overheat
and slow.
	If you want to make the fan QUIETER, consider mounting it in a longish
pipe, so the turbulence off the blade edges doesn't hit nearby objects in 
the air path.  A hunk of 'rubatex' pipe insulation is a good sound-dead
material for such an acoustic application.
	
	There ARE AC motors that CAN be slowed as you suggest, but 
those motors have brushes (so-called universal motors).  While this
type of motor isn't as maintenance-free as the usual type, you
MIGHT be able to find a fan with this sort of motor.  If you do,
the acid test is to see if it runs on DC (use low voltage, 'cuz
the wrong type of motor will just burn the fuse out if you don't
feed AC to it like it wants.)

	John Whitmore