Page "Induction motor" Paragraph 13

The torque exerted by the motor as a function of slip is given by a torque curve.

Over a motor's normal load range, the torque line is close to a straight line, so the torque is proportional to slip.

As the load increases above the rated load, increases in slip provide less additional torque, so the torque line begins to curve over.

Finally at a slip of around 20 % the motor reaches its maximum torque, called the " breakdown torque ".

If the load torque reaches this value, the motor will stall.

At values of slip above this, the torque decreases.

In 3-phase motors the torque drops but still remains high at a slip of 100 % ( stationary rotor ), so these motors are self-starting.

The starting torque of an induction motor is less than other types of motor, but still around 300 % of rated torque.

In 2-pole single-phase motors, the torque goes to zero at 100 % slip ( zero speed ), so these require alterations to the stator such as shaded poles to provide starting torque.