Drives for Web Handling

Permanent Magnet (PM) motors stators are very similar to synchronous or induction motor stators. The rotor is made of a shaft with a sandwich of steel layers (laminations) or alternately with a solid casting. The rotor is permanently magnetized, usually with rare earth magnets.

The PM produces flux and torque without the cost of magnetizing current.

The stator needs 3 phase poles with copper windings. If there are 2 poles, the motor will turn at exactly 3600 RPM (3000 RPM) at 60H (50 Hz). A 4 pole motor will rotate at exactly 1800 RPM. A 6 pole motor will rotate at exactly 1200 RPM.

PM motors produce torque only when they are turning at exactly synchronous speed. Slipping a pole due to torque overload is a serious problem which must trip the motor. The trick is in starting a PM motor since it has no torque until it gets to synch speed. Special PM variable speed drives are used with PM motors. Many servo systems use PM motors and drives. PM motors are expected to become more common, unless a shortage of rare earth magnets make them prohibitively expensive.

Warning – PM motors are always on, even when the drive is turned off. Rotating the shaft by hand or by pulling on the web will generate current at the stator and motor leads back to the drive. Lockout/Tagout should include locking the shaft.