NEIEP Mechanics Practice Exam

What happens to the rotor of a motor when it experiences slip?

1. It runs at synchronous speed

2. It exceeds synchronous speed

3. It slows down compared to the magnetic field

4. It stops completely

The correct answer is: It slows down compared to the magnetic field

When a motor experiences slip, it means that the rotor is not rotating at the same speed as the magnetic field produced by the stator. Instead, it rotates at a slightly slower speed than the synchronous speed of the magnetic field. This difference in speed allows induced current to flow in the rotor, creating torque that enables the rotor to follow the rotating magnetic field. In an induction motor, for example, slip is necessary for torque production. If the rotor were to run at synchronous speed, there would be no relative motion between the rotor and the magnetic field, leading to no induced current and, as a result, no torque. Consequently, the motor would not be able to develop the mechanical power needed to perform work. Thus, the correct answer reflects the fundamental operation of induction motors where the rotor operates at a reduced speed due to the slip, which is essential for generating torque and maintaining the motor's functionality.