variable speed on single phase capacitor motors

I have recently noticed that some inexpensive tools are using this
type drive. I had thought
they were using DC motors but this doesn't seem to be so. This has
to be very cheap to
do considering that it seems to be used on very inexpensive
machines.

I have been using DC PM motors with pulse width modulation control
for varispeed but
this seems to be more interesting.

For motor hook up look at analog control info at:
http://www.varidigm.com/products/prods_vsm.asp

I have a HVAC system that uses a specialy adapted 1/2 HP GE motor
but this is not the
ame thing I am describing.

Does anyone know these systems?
jesse