What type of motor has its rotor locked in step with the rotating stator field?

The correct choice is the synchronous motor, which is characterized by its rotor being locked in step with the rotating magnetic field produced by the stator. This means that the rotor's rotational speed matches that of the magnetic field, maintaining synchronization.

In a synchronous motor, the rotor typically has permanent magnets or windings that create its own magnetic field. When this rotor is introduced into the rotating magnetic field of the stator, it responds by locking into synchronization, ensuring that both fields rotate at the same frequency. This feature allows synchronous motors to provide precise speed control, making them suitable for applications where maintaining a constant speed is critical.

In contrast, an induction motor operates differently; it relies on electromagnetic induction to create current in the rotor, which typically does not match the speed of the stator's rotating magnetic field. Instead, the rotor lags behind the stator field, allowing for slip. The universal motor can run on both AC and DC power but does not have the synchronization characteristic of synchronous motors. A DC motor converts direct current into mechanical energy and operates based on differing principles, such as electromagnetic forces, without the synchronization aspect inherent to synchronous motors.