In a typical induction motor, rotor speed is less than the synchronous speed due to slip.

In a typical induction motor, rotor speed is less than the synchronous speed due to slip.

• A. True
• B. False
• C. Only at start
• D. Only at overload

Answer: (A)

The rotating magnetic field in the stator runs at synchronous speed, and the rotor must lag behind it to produce the currents that generate torque. If the rotor reached the same speed as the field, there would be no relative motion, no induced current, and no torque, so the motor wouldn’t be able to accelerate further. That’s why, in normal operation, the rotor runs slightly slower than the synchronous speed; this difference is called slip. Mathematically, slip is s = (Ns − Nr)/Ns, where Ns is synchronous speed and Nr is rotor speed, so Nr = (1 − s)Ns and Nr < Ns as long as s > 0. At start, Nr = 0 and slip is 1 (100%). Under light load, slip is small; under heavier load, slip increases to produce more torque. This is why rotor speed is less than synchronous speed due to slip.

The rotating magnetic field in the stator runs at synchronous speed, and the rotor must lag behind it to produce the currents that generate torque. If the rotor reached the same speed as the field, there would be no relative motion, no induced current, and no torque, so the motor wouldn’t be able to accelerate further. That’s why, in normal operation, the rotor runs slightly slower than the synchronous speed; this difference is called slip. Mathematically, slip is s = (Ns − Nr)/Ns, where Ns is synchronous speed and Nr is rotor speed, so Nr = (1 − s)Ns and Nr < Ns as long as s > 0. At start, Nr = 0 and slip is 1 (100%). Under light load, slip is small; under heavier load, slip increases to produce more torque. This is why rotor speed is less than synchronous speed due to slip.