The electromagnetic torque of an induction motor changes with rotor speed because rotor speed determines slip.

A torque-speed characteristic plots the available electromagnetic torque against speed, or equivalently against slip.

Synchronous speed

At synchronous speed,

$$s=0,$$

so an ideal induction motor develops zero torque because no rotor current is induced.

Normal operating region

Just below synchronous speed, a modest increase in slip generally produces a roughly proportional increase in torque. This is the normal stable operating region for many motors.

If the load torque increases, the operating point moves to slightly lower speed and higher electromagnetic torque.

Starting torque

At standstill,

$$s=1.$$

The motor can develop finite starting torque, although its magnitude depends strongly on rotor design and electrical parameters.

Breakdown torque

As slip increases from the normal operating region, torque eventually reaches a maximum called the breakdown torque or pull-out torque.

Beyond this point, further slowing reduces available electromagnetic torque. If the load requires more than the breakdown torque, no stable running point exists and the motor can stall.

Operating point

Steady speed occurs where the motor torque curve intersects the load-torque curve. Stability depends not only on equality of torques but also on how the two torques change when speed is perturbed.