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.
- Stator fieldRotates at the commanded electrical frequency.
- Rotor fieldComes from the magnets.
- Synchronous motionRotor and field share the same rotational speed.
Pole count still matters.
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.