some reading material on VFD's

http://www.findarticles.com/p/articl..._55817948/pg_3

describes method of using pulses to create a simulation of a sine wave
for the motor - pretty good article

Each cycle of the sine wave may be chopped into 20 or more on-off
pulses with widths simulating the voltage present during those phase
angles of the wave . . . Close to zero crossing the pulses are very
narrow becoming progressively wider as 90/180 degrees is reached, then
becoming more narrow at the next zero crossing. As frequency is
lowered all the pulses in each cycle can become narrower -
particularly with a fan load since the effort required falls off
rapidly as speed is lowered - no need for constant torque

That takes a fair amount of discrete logic or a programmable chip

If I were to attempt to design a vfd for a fan, I think I'd look to an
analog design to handle the pwm - be somewhat easier than using all
discrete logic.

http://en.wikipedia.org/wiki/Variable_Frequency_Drive Wickipedia's
dumbed down explanation.

You probably have one of those old Sears fans with the cast iron hubs
and steel blades. Built like a tank . . . My very OM was still using
the one I remember as a kid . . . . died about three years ago after
working for about 40 years with no maintenance.

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