Plugging

Plugging : It is a simple method of electric braking, In this method reversing the connections of the armature terminals of a motor, so as  to reverse its direction of rotation which will oppose the original direction of rotation of the motor and will bring it to zero speed then mechanical brakes can be applied. At the end of the braking period the supply to the motor is automatically cut off.

DC Series Motor:

DC Shunt Motor:

In the normal running position the back emf is nearly equal to applied voltage and opposite in direction, so that a small voltage acts across the armature circuit to drive the normal current through a small resistance of the motor. In the plugging position, When the armature terminal connections are reversed, emf of the machine is in the same direction as that of the applied voltage. The total voltage of V+Eb is available across the armature terminals cause a current I to flow around the circuit. When Eb = V, then the voltage across the armature is 2V, twice of supply voltage acts across the machine circuit and heavy current would flow (about twice the current drawn by the stationary motor on normal rated voltage). Hence to avoid flow of heavy current and limit it to the safer value, it is necessary that switching performing the plugging operation may also re-insert starting resistance and some additional resistance in series with the armature circuit of the motor.

Plugging of Three Phase Induction Motor

Plugging braking of induction motor is done by interchanging connections of any two phases of stator with respect of supply terminals. Plugging induction motor braking is done by reversing the phase sequence of the motor, the direction of rotating magnetic field in the stator can be reversed.  The rotor also try to rotate in the reverse direction. Thus an opposite torque is produced in the rotor and first comes to rest. Just before the zero speed instant the supply to the motor should be stopped. Ie, the motor starts accelerating in the reverse direction. This method also known as counter current braking.