This type of braking is possible in normal stator fed Induction motors, if the no. of poles of machine can be changed during running condition by using a special arrangement.
àThis is obvious that this would be possible in squirrel cage induction motor.
No. of poles in stator= No. of poles in rotor (Not necessary no. of phases)
àThus when braking is desired the no. of poles is increased usually by a factor of 2.
àThe synchrounous speed of revolving field thus becomes half and therefore the induction motor goes into generating mode as the slip becomes negative.
Consequently the speed reduces and when it reaches near the new synchronous speed, Power supplt to the motor is switched off.
Subsequently it requires mechanical braking to bring the motor to rest.
àSince electrical power is fed to the main at the loss of kinetic energy of the rotor this method is known as regenerative braking.
àWith much progress in potential energy during the last cayed frequency drives are available for induction motors of the squirrel cage type as well as for slip ring type.
àTherefore even if the no. of poles remain unchanged the frequency of supply can be reduced keeping V/f variable constant to avoid the over fluxing.
àThis will reduce the synchronous speed causing regenerative braking as explained above thus the variable frequency supply regenerative braking must be possible to almost standstill & would be applicable to the squirrel cage as well as slip ring motors.
Regenerative braking characteristics is shown as below:-