In an induction motor, the stator rotating field induces rotor currents. Their interaction with the field produces torque in the direction that reduces the relative motion between rotor and field.

Starting

At standstill the slip is

$$s=1,$$

so the rotor sees the full electrical frequency of the rotating field. Rotor EMF and current are induced and the machine can develop starting torque.

Acceleration

Electromagnetic torque accelerates the rotor toward synchronous speed. As speed rises, slip falls and the rotor electrical conditions change.

The rotor cannot reach exact synchronous speed while supplying steady load torque, because at

$$s=0$$

there would be no induced rotor current and therefore no induction torque.

Loaded steady operation

The motor settles at a speed slightly below synchronous when electromagnetic torque balances load and loss torques.

If the load increases, the rotor slows slightly. Slip rises, strengthening the induction process and increasing electromagnetic torque until a new balance is reached, provided the required torque remains within the machine's stable operating range.

Speed is nearly, but not exactly, constant

Because normal operating slip is usually small, an induction motor often behaves approximately as a constant-speed machine. Its small speed change with load is nevertheless essential to producing the torque adjustment.