In AC motors, what is slip?

• A. The difference between rotor speed and synchronous speed
• B. The ratio of rotor speed to synchronous speed
• C. The difference between stator speed and rotor speed
• D. The sum of rotor and synchronous speeds

Answer: (A)

Slip is the difference between the rotor’s mechanical speed and the speed of the rotating magnetic field created by the stator. In an AC induction motor, the stator field rotates at synchronous speed Ns = 120 f / P. The rotor tends to chase this field but always lags a little due to induced currents, and that lag is what we call slip. It’s often described with s = (Ns − Nr) / Ns, expressed as a percentage as 100 × (Ns − Nr) / Ns, where Nr is the rotor (mechanical) speed.

This slipping motion is what enables energy transfer from the stator to the rotor and generates torque; without any slip (Nr equal to Ns), there would be no rotor current and no torque. In normal operation, slip is small—just a few percent. If slip grows too large, the motor loses torque efficiently and heats up.

The other ideas don’t fit because the ratio Nr/Ns isn’t slip, the difference between stator speed and rotor speed isn’t meaningful since the stator’s speed is the rotating magnetic field speed, and adding rotor and synchronous speeds doesn’t describe a real physical quantity.