The rotor in an induction motor produces a magnetic field when current flows through the conductors in the squirrel cage. There is no electrical connection to the rotor. Therefore all flux producing magnetic currents in the rotor must be induced by cutting flux lines produced by the rotating stator field.
When the rotor is stopped and the stator is first energized, the slip is maximum and a 60 Hz current is induced in the rotor. This will produce starting torque for the motor and the rotor will accelerate.
As the rotor speeds up, its slip reduces and the current induced in the rotor reduces in frequency. The rotor continues to accelerate. Acceleration torque continues to increase slowly.
When the rotor reaches about 95% of synchronous speed, the induced rotor current frequency has been reduced to 5% of 60 Hz (3 Hz). This is the speed at which the rotor produces the maximum torque called the breakdown torque.
As the motor continues to accelerate, the slip decreases. There is very little slip and very little induced rotor current. Torque falls off until induced current and torque reach zero with no slip.
The chart above shows the current induced in the rotor while a motor is started across the line. No Variable Speed Drive is used. Note the frequency decreases with speed. The pulsations in current will produce pulsations in torque.
Feedback - If you think you know how this chart was measured, please leave a comment.