Permanent magnet (PM) motors have found a lot of application in web handling in recent years. The rotor is magnetized with permanent magnets rather than squirrel cage induction as in an induction motor or a dc field requiring an exciter as in a synchronous motor. The PM motor has the advantage that no electrical current is required to magnetize the rotor with resulting energy savings.
PM motors are very similar to synchronous motors, which are different than induction motors. Thus the Variable Speed Drive (VSD) must be matched to the requirements of the PM motor. Not all drives are capable of driving PM motors.
Induction motors magnetize the rotor when its coils (squirrel cage) cut magnetic flux lines established by the stator windings and the ac power feeding them. The induction motor relies on the rotor turning more slowly than the magnetic field (slip). Without slip, the induction motor cannot develop any torque.
Synchronous motors magnetize the rotor whenever the field is energized. Excitation should be applied only when the rotor RPM closely matches the magnetic field established by the stator power. Otherwise, the motor will trip on overcurrent or pole slip and mechanical damage may occur. That implies a separate means must be used to bring the rotor up to speed before energizing the field. Often the rotor is shorted to make it act like an induction motor for starting. Synchronous motors are not generally used in web handling.
PM motors cannot have the rotor de-energized. Thus they must be accelerated as synchronous motors. Special VSD models are required to do this. With PM motors, torque is available instantly as not flux up time is required.
PM motors also introduce new safety concerns since the rotor is always magnetized. Any small rotation of the motor will make it act as a generator with full flux. Therefore Lockout/Tagout for PM motors usually requires the rotor be locked in position in addition to the power feed being locked out.
PM motor rewinding also comes with new hazards. These magnets have a serious attraction to one another. Painful finger pinches must be avoided. The magnets will also attract any iron filings or metal tools in the area.