Percent slip of an induction motor refers to the:

• A. Ratio of synchronous speed to rotor speed
• B. Difference between synchronous speed and rotor speed as a percentage of synchronous speed
• C. Rotor speed relative to synchronous speed
• D. Motor efficiency

Answer: (B)

Slip in an induction motor is the difference between the speed of the stator’s rotating magnetic field (the synchronous speed) and the actual rotor speed, expressed as a percentage of the synchronous speed. The percent slip is calculated as s = (Ns − Nr) / Ns × 100%. This shows how far the rotor lags behind the rotating field, which is necessary for torque production. When the rotor is stationary, slip is 100%; as the motor speeds up toward synchronous speed, slip decreases toward 0. The other ideas describe a ratio of speeds or refer to efficiency, which do not capture the definition of slip.