Which type of motor has a magnetic field that stays fixed while the rotor turns?

The correct answer is the synchronous motor. In a synchronous motor, the rotor turns at the same speed as the rotating magnetic field produced by the stator. This is achieved by having a magnetic field that remains stationary, which is generated by either permanent magnets or field windings on the rotor.

This characteristic allows the rotor to lock in step with the magnetic field, ensuring that it maintains synchrony with it. As a result, the speed of the motor is directly related to the frequency of the electrical supply. This is distinct from other types of motors.

For example, while an induction motor also has a stator that produces a rotating magnetic field, its rotor typically follows at a speed slightly less than the synchronous speed, which generates slip and allows it to operate. The universal motor, on the other hand, operates on both AC and DC and can have a rotating field, but it does not have the fixed magnetic field characteristic attributed to synchronous motors.