What is slip in an induction motor?

• A. The difference between synchronous speed and rotor speed.
• B. The ratio of rotor to synchronous speed.
• C. The frequency of rotor currents.
• D. The mechanical efficiency.

Answer: (A)

Slip is the difference between the speed of the rotating magnetic field in the stator (synchronous speed) and the actual rotor speed. The synchronous speed is set by the supply frequency and the motor’s pole count, while the rotor turns at Nr. The slip is typically expressed as a fraction or percentage: s = (Ns − Nr) / Ns, or simply as the difference Ns − Nr in rpm. This small speed gap is what induces currents in the rotor; without any slip (Nr = Ns) there’s no relative motion, no rotor current, and no torque. At standstill, Nr = 0 and slip is 1 (100%), giving the highest starting current and torque. As load increases, Nr falls further behind Ns, increasing slip; under normal running, slip remains a small percentage so the rotor current frequency f_r equals s times the supply frequency f.