Permanent magnet motor

A permanent-magnet (PM) motor is a synchronous machine whose rotor field comes from rare-earth magnets rather than a wound, excited field, giving high torque density and efficiency in a compact, lighter frame. On ships PM motors drive azimuth and podded thrusters and rim-driven units, where their compactness suits the pod, and serve as shaft generators and in hybrid propulsion. They run from an inverter that controls frequency and torque, since they have no starting cage. The trade-offs are magnet cost, demagnetization risk at high temperature, and the need to isolate the drive because the spinning rotor always generates voltage.