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?  I was thinking that I could vary the voltage on an analog input to
get a desired speed value in a register but then how to I get a
corresponding output from the PIC that will vary the firing angle of a
triac or something similar?  You may be asking why I want to use a PIC for
the speed control.  I'm already planning to use a PIC to control run time
of the fan, to perform a thermostat function, and to automatically open and
close the fan's shutters.  I'd like to go a little further and use it for
speed control as well.

Any ideas, comments, or warnings will be greatly appreciated.  Thanks so
much.

Mark