Why does the rotor of an induction motor turn more slowly than its rotating magnetic field?

The rotor of an induction motor must turn more slowly than its rotating magnetic field to maintain the motor's operation.

The magnetic field rotates relative to the rotor conductors, which induces a voltage in those conductors. Because the rotor conductors are short-circuited, this induced voltage causes currents to flow, creating the torque necessary to rotate the mechanical load. If the rotor were to spin at the exact same speed as the magnetic field (the synchronous speed), there would be no relative motion between them. Consequently, no voltage would be induced, no current would flow in the rotor, and no torque would be produced to drive the load.

Therefore, the rotor speed must remain lower than the field speed by an amount—often called slip—that is just sufficient to induce the voltage required to generate the necessary current and torque for the specific load.1