As I said most single phase motors are not amenable to speed control. V/f is the best method, but
shaded poles and phasing capacitors are tuned to a particular frequency so don't handle changes in f
well.
If you get it wrong the motor might stall and the current through the windings can rise lots and cook
the motor. Some motors can handle such stalls, others can't - that's why the datasheet is important it
will show the current/torque/slip-rate profile.