A permanent magnet motor is an ac motor that uses magnets imbedded into or attached to the surface of the motor’s rotor.
Controlling the speed of ac motors is accomplished using a variable frequency drive (VFD) in most cases. While many scenarios involve using VFDs with induction motors with stator windings to generate a rotating magnetic field, they also can achieve precise speed control using speed or position feedback sensors as a reference to the VFD.
In some situations, it is possible to obtain comparably precise speed control without the need for feedback sensors. This is made possible using a permanent magnet (PM) motor and a process called the "high-frequency signal injection method."
A permanent magnet motor is an ac motor that uses magnets imbedded into or attached to the surface of the motor’s rotor. The magnets are used to generate a constant motor flux instead of requiring the stator field to generate one by linking to the rotor, as is the case with an induction motor. A fourth motor known as a line-start PM (LSPM) motor incorporates characteristics of both motors. An LSPM motor incorporates a PM motor’s magnets within the rotor and a squirrel cage motor’s rotor bars to maximize torque and efficiency.
