Many motors can use a variable transformer for speed control.
Motors that usually can be controlled by varying the motor voltage:
Universal Motors
Series Wound Motors
Torque Motors
Shaded Pole Motors
Motors that sometimes cannot be varied by changing the motor voltage:
Split Phase Motors
Permanent Split Capacitor Motors
Repulsion Induction Motors
Motors with centrifugal switches
Motors with fan cooling can sometimes be damaged due to the decreased cooling effect at lower speeds (depending on load).
We recommend that the motor manufacturer be queried to determine whether varying the voltage can control your particular motor.
