In the case of a 3 phase induction motor, plugging is done by:
Right Answer is:
Interchanging connections of any two phases of the stator for quick stopping
It is known that the rotor of a polyphase induction motor develops torque in the same direction as the rotating magnetic ﬁeld set up by the stator winding. Also if any two stator leads are reversed, the rotating magnetic ﬁeld is also reversed. If therefore, the pair of stator leads are reversed while a motor is rotating, torque is suddenly produced opposite to the original direction of rotation. This reverse torque causes rotation in the opposite direction as soon as the motor stops, therefore provision must be made to disconnect the stator completely from the supply lines when the motor stops.
Hence Plugging in induction motor braking is applied by just reversing the supply phase sequence by interchanging connections of any two phases of the stator, we can attain plugging braking of an induction motor. Due to the reversal of phase sequence, the direction of the rotating magnetic field gets reversed. This produces a torque in the reverse direction and the motor tries to rotate in opposite direction.
This opposite flux acts as the brake and it slows down the motor. During plugging the slip is (2 – s), if the original slip of the running motor is s.
Slip during motoring = s =(Ns – Nr)/Ns
Nr/Ns = (1 – s)
Slip = spl = (-Ns – Nr)/-Ns = (Ns + Nr)/Ns = (2 – s)
Note: The method can be applied to both squirrel cage as well as wound rotor induction motors.