In message , Brian Gaff
writes
No induction motors have no brushes!
I'm assuming brushed motors here. Induction motors need to be driven with
some sort of variable AC frequency, not something trivial to build.

I think you can get away with a variable voltage speed control if the
load torque increases more than that available from an increase in
supply voltage.

Hence fan speed control where the fan load is a square law.

--
Tim Lamb