How does rotor speed interact with the magnetic field in an induction motor?

• A. The rotor speed is faster than the magnetic field
• B. The rotor speed is equal to the magnetic field
• C. The rotor speed is unrelated to the field
• D. The rotor speed is slower than the magnetic field's speed

Answer: (D)

In an induction motor, the stator creates a rotating magnetic field that turns at synchronous speed, set by the supply frequency and the number of poles. The rotor stays in step with this field only through induced currents in the rotor bars; this is what produces torque. But for those currents to exist, there must be a difference in speed between the rotor and the rotating field—some slip. If the rotor reached the same speed as the magnetic field, there would be no relative motion, no induced current, and no torque to drive the rotor further. If the rotor somehow ran faster than the field, the relative motion would reverse and the motor would oppose that motion rather than sustain it. In normal operation the rotor always lags behind the rotating field, so the rotor speed is slower than the magnetic field’s speed.