Plugging in Motors

Plugging is a method of braking used in induction motors.  Plugging involves interchanging the supply to two of the stator phase windings.  This method is used in applications which require immediate stop applications.  When the phase supply is reversed, torque is produced in the opposite direction.  This leads to braking of the electric motor.

Motors which are operated this way have a plugging switch.  This switch operates when the stop command is given to the motor circuit.  The operation of this switch applies reverses the supply to two of the windings.  When the motor comes to a complete halt, this reversed supply is disconnected. 

However, this method of braking generates a large amount of heat in the rotor, even greater than that produced by a locked rotor.  This is due to the kinetic energy of the rotor and the coupled load.

Plugging should not be used frequently as the heat produced in this method of braking can cause the rotor bars to overheat and, sometimes, melt.