Permanent Magnet Synchronous Motor (PMSM)

An AC motor with permanent magnets on its rotor, spinning in strict sync with the supply frequency.

A permanent magnet synchronous motor has permanent magnets (usually neodymium-iron-boron) mounted on its rotor; the stator is fed three-phase AC current that creates a rotating magnetic field, and the rotor follows that field at exactly the same frequency, which is why it's called “synchronous.” Its structure is nearly identical to a brushless DC motor's; the main differences are the shape of the back-EMF waveform (sinusoidal versus trapezoidal) and how it's driven — a PMSM uses sinusoidal current together with field-oriented control (FOC), giving smoother torque and less noise. In robotics circles, the two terms are often used loosely and interchangeably. A PMSM is efficient and has high power density, but it needs a position sensor, such as an encoder, to tell the driver where the rotor is. Electric-vehicle drive motors, industrial servo motors, and most robot joint motors fall into this category.

ExampleThe frameless torque motor at the heart of a humanoid's joint module is, in most cases, essentially a permanent magnet synchronous motor driven with FOC.

Also called

PMSM, Permanent Magnet AC Servo Motor

Related

Brushless DC Motor · Field-Oriented Control · Rare-Earth Permanent Magnet (NdFeB) · Servo Motor · Frameless Torque Motor · Joint Actuator Module

Sources

Synchronous motor - Wikipedia