The rotor carries permanent magnets.

Permanent magnets supply the rotor’s magnetic field. Controlled stator currents interact with this field to produce torque. The rotor does not depend on induced rotor current in the same way as an induction motor.

This lesson concerns permanent-magnet synchronous motors, often shortened to PMSMs. Other permanent-magnet motor arrangements also exist.

The fields turn together.

1. First instant

Synchronous rotation with a constant field angleThe orange stator field direction is 0 degrees. The permanent-magnet rotor north axis is -30 degrees. Both move through 120 degrees each interval, keeping the same illustrative 30-degree separation.NSBoth advance 120°

2. One interval later

Synchronous rotation with a constant field angleThe orange stator field direction is 120 degrees. The permanent-magnet rotor north axis is 90 degrees. Both move through 120 degrees each interval, keeping the same illustrative 30-degree separation.NSBoth advance 120°

3. Two intervals later

Synchronous rotation with a constant field angleThe orange stator field direction is 240 degrees. The permanent-magnet rotor north axis is 210 degrees. Both move through 120 degrees each interval, keeping the same illustrative 30-degree separation.NSBoth advance 120°
  1. Orange arrow: stator-field direction. Controlled winding currents produce this rotating field.
  2. N and S: rotor magnetic poles. The magnets and rotor turn together. Unlike the induction example, this dark magnetic axis belongs to the rotor field.
  3. Same speed, a steady torque angle. Both fields advance by the same angle in equal time. Their angular separation, marked by the dashed arc, can remain steady while torque is produced; equal speed does not mean the fields must line up.
Simplified two-pole PMSM in steady motoring operation. The 30° separation makes the principle visible; it is not a recommended setting or a universal operating angle. Actual control depends on motor design and operating conditions.

Pole count still matters.

An eight-pole motor at 40 Hz

Synchronous speed is 120 × 40 ÷ 8 = 600 rpm. During synchronous operation, the rotor turns at that speed.

Electrical frequency is not revolutions per second. A motor with more pole pairs has more electrical cycles per mechanical revolution.

Torque depends on field alignment.

The drive must control current in relation to rotor magnetic position. It can obtain the required information through suitable feedback or estimation, depending on the system.

Starting and very low-speed operation place particular demands on position information. A feedback device and drive must be compatible with the motor arrangement.

Often paired with gearless machines.

Permanent-magnet motors can be designed to provide substantial torque at relatively low speed, making them useful in gearless traction machines. Permanent magnets alone do not guarantee a particular efficiency or ride quality.

A mechanically driven permanent-magnet motor can generate voltage even without an energised drive. Its operating state must be understood as part of the complete machine.

A closer look

Electrical rotor angle repeats once per pole pair during a mechanical revolution. Correct knowledge of this relationship allows the drive to coordinate stator current with the rotor field.

Check your understanding

Does a permanent-magnet synchronous motor need normal induction-motor slip to produce torque?

Reveal the explanation

No. In synchronous operation its rotor follows the rotating field at the same speed. Torque depends on the controlled magnetic interaction, not induction-motor slip.

Further reading.

Optional reading to explore this lesson in more detail.