A permanent-magnet synchronous motor, usually abbreviated PMSM, is the electric machine that drives the great majority of modern battery electric and plug-in hybrid vehicles. It earns this dominance because its rotor carries its own magnetic field, generated by powerful fixed magnets rather than by electricity fed in from outside. Because that field is permanently present, the motor wastes no energy creating it, which gives the PMSM exceptionally high efficiency and a very favourable power-to-weight and power-to-volume ratio. The result is a compact, light unit that delivers strong torque from a standstill, exactly the characteristics a passenger car needs.
Mechanically the motor has two main parts: a stationary stator wound with copper coils, and a rotating rotor in which the magnets are embedded. When the inverter feeds three-phase alternating current into the stator windings, it produces a rotating magnetic field. The rotor's magnets lock onto and chase this rotating field, so the rotor spins in step, or synchronously, with it, which is why the machine is called synchronous. In most automotive designs the magnets are buried inside the rotor's steel laminations rather than mounted on its surface; this interior permanent-magnet layout adds a useful component of reluctance torque and lets the rotor spin safely at high speed without the magnets flying off.
The magnets themselves are typically neodymium-iron-boron rare-earth magnets, often with added dysprosium or terbium to keep them from demagnetising when they get hot. Their strength is the source of the motor's efficiency, but it is also its chief drawback. Rare-earth elements are costly, their supply is geographically concentrated, and their mining and refining carry an environmental and geopolitical burden. This has pushed manufacturers to reduce magnet content, redesign rotors to use less heavy rare earth, or in some models pair a PMSM on one axle with a magnet-free induction motor on the other.
Precise control demands precise knowledge of where the rotor is, so the motor relies on a position sensor, typically a resolver, feeding back to the inverter, which adjusts the current waveform many thousands of times a second. The same hardware allows the motor to act as a generator during regenerative braking, turning the car's momentum back into stored charge. A practical quirk follows from the permanent field: because the magnets always induce a back-electromotive force, a PMSM cannot freewheel as cleanly as an induction motor, and at very high speeds it must be actively managed through field-weakening to avoid generating an unwanted braking drag or excess voltage.
Within the wider drivetrain the PMSM sits between the EV inverter, which supplies its carefully shaped current, and a reduction gear that matches its high rotational speed to the wheels. Its principal alternative is the induction motor, which trades a little efficiency for lower cost and freedom from rare-earth materials, and the choice between the two is one of the defining engineering decisions behind any electric car's powertrain and its overall efficiency.
- Rotor uses fixed (usually rare-earth) permanent magnets
- Highly efficient and compact — the most common EV motor
- Needs no energy to create its rotor field
- Relies on rare-earth metals and can't freewheel as cleanly