In an induction motor, when the rotor runs faster than the stator field’s synchronous speed, the slip becomes negative and the machine generates power.
Whenever the motor has a tendency to run faster than the rotating field, regenerative braking occurs and the kinetic energy of the rotating parts is returned to the mains.
The speed torque curve extends to the second quadrant as shown in Figure 4.17.
As the speed of the motor decreases, the braking torque makes the motor run at a constant speed and arrests its tendency to rotate faster. Due to the effects of stator resistance, the maximum torque developed during regeneration is greater than the maximum torque during motoring.
For example, in cranes and hoists, the motor has a tendency to run faster than the synchronous speed. This situation can occur when a hoist is raising an empty cage. Due to counter weight, the cage may acquire dangerous speeds. The transition takes place almost automatically, a torque is developed to arrest the acceleration, and regeneration takes place. Automatic regeneration arrests any undue acceleration. In such cases, the rotor resistance control could be employed to get a better braking torque.
Regenerative braking is also possible with a pole change motor, when the speed is changed from high to low. It can also be easily accomplished, in variable frequency drives by decreasing the frequency of the motor momentarily – the synchronous speed decreases and conditions favorable to regeneration are created.
As the motor speed decreases, the frequency is continuously reduced so that the braking takes place at a constant torque and stator current, until the motor comes to zero speed. During regenerative braking, there is a possibility of dangerous speeds, if the operating point during the braking falls in the unstable region of the characteristic. This happens if the load torque is greater than the breakdown torque of the motor. The torque developed cannot break the motor and undue acceleration takes place. This possibility can be eliminated by means of a high resistance in the rotor.