Motor slip is defined as

• A. An inherent inefficiency within the motor
• B. The difference between actual rotor speed and synchronous speed
• C. The rotor speed divided by the synchronous speed
• D. None of the above

Answer: (B)

In an induction motor, torque is produced because the rotor must lag behind the rotating magnetic field. That lag is called slip. Slip is defined as the difference between the synchronous speed (the speed of the rotating magnetic field) and the actual rotor speed. It can also be expressed as a percentage of the synchronous speed, but the essential idea is the gap between where the field is rotating and where the rotor is actually turning.

So the correct description is the difference between the rotor speed and the synchronous speed. The other options don’t fit: slipping is not simply an inherent inefficiency, and it’s not the rotor speed divided by the synchronous speed (that ratio relates to speed, not the slip itself).