Electrical Engineering ⇒ Topic : Electric braking of shunt motors

Electric braking of shunt motors

(1) Plugging or counter current braking :

In this method, connection to the armature terminals are reversed so that motor tends to run in the opposite direction (Fig. (a) ). Due to reversal of armature connections, applied voltage V and E_b start acting in the same direction around the circuit. In order to limit Shunt field the armature current to reasonable value, it Winding is necessary to insert a resistor in the circuit while reversing armature connection.

                                            Fig. (a)  Plugging or counter current braking

This method is commonly used in controlling :

(1) Printing presses

(2) Rolling mills

(3) Machine tools

(4) Elevators etc

(2) Rheostatic or dynamic braking. In this method of electric braking of shunt motors,the armature of the shunt motor is disconnected from the supply and is connected across a variable resistance R as shown in Fig. (b). The field winding is, however, left connected across the supply undisturbed. The braking effect is controlled by varying the series resistance R.

                                                Figure  Rheostatic or dynamic braking

(3) Regenerative braking. Refer Fig. (a). Regenerative braking method is used when the load on the motor has overhauling characteristic as in the lowering of the cage of a hoist or downgrade motion of an electric train. Regeneration place when E_b becomes greater than V This happens when the overhauling load acts as a prime mover and so drives the machine as a generator. Consequently, direction of I_a and hence of armature torque is reversed and speed falls until F becomes less than V.

                                                    Figure  Regenerative braking