PSM

A permanent-magnet synchronous motor uses fixed magnets on the rotor instead of electromagnets, and its rotation is locked to the supply frequency (50 Hz or 60 Hz depending on region). Unlike induction motors, which slip slightly under load, a PSM rotates at exactly one speed determined by the incoming AC frequency and pole count. This synchronous operation makes PSMs useful where speed precision matters, though they cost more to manufacture and control than comparable induction machines.

The rotor contains permanent magnets, typically neodymium or samarium cobalt, arranged to create discrete magnetic poles. The stator is a wound coil that produces a rotating magnetic field when energized by three-phase AC current. The rotor's magnetic field aligns itself with the stator field and follows it; if the load tries to slow the motor, the rotor cannot lag without stalling completely. This all-or-nothing behavior differs sharply from an induction motor, which can tolerate continuous slip under heavy load.

Practical use and limits

PSMs demand a constant frequency supply and cannot be started by simply connecting them to AC power, the way an induction motor can. They require a variable-frequency drive (VFD) to spin up gradually; the drive must ramp its output frequency slowly so the rotor's magnetic field accelerates and remains synchronized. Once running at speed, a PSM draws less current than an equivalent induction motor and runs more efficiently, particularly at part load. For this reason PSMs dominate in machine tool spindles, extruder screws, and compressor applications where energy consumption and speed stability justify the extra cost.

The main failure mode is loss of synchronism: if transient load shock exceeds the motor's pull-out torque, the rotor falls out of step with the stator field and the motor stalls. Recovery requires a controller to detect the fault and restart the drive. Demagnetization of the permanent magnets due to excessive temperature is also a concern; neodymium loses strength above 80°C and must be derated in hot environments. Some PSM designs include damper windings (squirrel-cage copper bars) in the pole faces to improve starting torque and provide a degree of asynchronous operation during transients.

In automotive contexts, PSMs appear in hybrid and electric powertrains rather than traditional engine-driven auxiliaries. EV drivetrains often use interior permanent-magnet (IPM) motors, a PSM variant where the magnets are buried inside the rotor rather than mounted on its surface. This arrangement raises the reluctance torque and allows better speed range extension via field weakening, which is why IPM motors suit EV acceleration profiles better than surface-mounted designs.