[TheForge] Dimmer switch as motor controller

[Paul]

Rich Maynard wrote:
> www.screwfix.com dimmer part number 47327 says it does resistive/inductive loads up to 250VA. Is a little forge fan motor an inductive load? It says 48W on the label.
>
> Rich M.
>
>   
Hello Rich
The problem with the resistive/inductive scenario is the type of load. 
If your 48 watt motor is an 'universal' that is to say it has brushes, 
it will most likely run without protest for years to come. If the motor 
is a strictly ac motor, synchronous or inductive, the motor may run but 
because these motor depend on the FREQUENCY of the supply voltage to 
control the speed (see physics lesson )
http://en.wikipedia.org/wiki/AC_motor#Three-phase_AC_synchronous_motors
http://en.wikipedia.org/wiki/Electric_motor
http://www.coolmagnetman.com/magdcmot.htm
they will fail.
The upshot is if you drop the voltage of a ac motor the motor will try 
to run at the frequency of the lowered voltage anyway, and the lower 
power input will result in lower speed for a while and overheating. It's 
just wrong.
The proper way to control the speed of an AC motor is with a Variable 
Frequency Drive, I have them and I use them, but not for fan speed. Much 
to expensive for this application.
I like the idea of the automotive heater fan and blower assembly from a 
wreckers yard. A cheap source of 12 DC is an old power supply from 
computer.
Paul S.