Mark Wrote...

> I'm trying to make a speed control for a whole house fan I purchased.  The
> manufacturer warns in the instructions not to attempt to use a solid state
> speed control device with the fan.  This brings up a couple questons.
>
> 1.  Firing a triac at various angles to regulate power to a device is
> pretty straight forward.  Does the method used to do this vary depending
> whether you're dealing with a resistive load versus a reactive load like an
> induction motor?  Just how does motor speed control differ from a basic
> lamp dimmer?
>
> 2.  What is the method to use to have a PIC control the speed of such a fan
> motor?
>
> Any ideas, comments, or warnings will be greatly appreciated.  Thanks so
> much.
>

A shaded pole motor (most fans) is a synchronous
motor, it turns at a speed close to a function of its poles
and the mains frequency, with some slippage.
The proper way to vary its speed is to vary the mains
frequency. This is also true of 3 phase and other
induction motors of various types.
Needles to say varying the power frequency is a bit of a
challenge and not a task for youre average beginner.

Simply altering the phase angle (triac etc type switching)
will at best simply cause more and more slippage with a
huge drop in efficiency but more likely lead to burnt
motors as the turning of the motor normally limits the
current flowing through it (cutting flux and all that) and a
slowing or stopping of the motor will cause destructive
currents to flow.
The only AC motors that respond to triac (and other
switching) speed control are universal motors like those
used in handrills and small appliances etc. They are a
kind of DC motor with a commutator (the commutator is
the give away to what kind of motor it is) and their speed
is related to the current flowing through the windings.

There was discussion on the PICLIST a while ago on
simple , rather rough, speed control of a shaded pole
motor by skipping cycles.
This might be worth an experiment (I have NOT done this)
and should avoid (no guarantees!!!!!) the old motor burn
out problems.

What that means is the PIC detects zero-crossings and
will fire a triac at the beginning of that cycle and the
conducting triac will remain on (by virtue of its inherent
operation) until the next zero crossing.
As there are two zero crossings per cycle.. It would be
best to have a minimum of 2 so a full AC wave is fired.
Now I would have a 'duty cycle' of waves fired to waves
not... say a pattern of 10 where 10% would be 9 cycles
fired to 1 off, 20% 8:2, 30% 7:3 etc.

BUT do not let the motor stall or go very slow as you will
be in a situation where the motor is constantly in
"starting' condition which might cause overheating.

This technique (cycle skipping) is called 'burst firing' and
will work with resistive loads like heaters and other
thermaly slow devices (a light bulb will flicker at lower
settings).
The really BIG advantage of burst fire is its really low EM
emissions, often NO or little suppression is needed on
the mains feed.

WARNING.. these voltages are very dangerous, all PIC
interfaces should be isolated and explosive component
damage can result from incorrect component selection or
application.




_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

www.psych.auckland.ac.nz

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