Plugging
The fig below shows that the plugging of a DC Shunt Motor. In this technique the motor inclines to rotate in opposite direction by reversing the armature connection, hence given that the essential breaking effect. If the motor comes to initial position the supply connection should be disconnected otherwise the will continuously rotates in opposite direction.
Though the armature connections are reversed, the connections in the field windings are retained same. Therefore the armature current reverses. When the motor runs at normal speed, the back EMF opposes the applied voltage. During the connection in the armature are reversed the back EMF and the applied voltage acts in the similar direction all over the circuit. Thus a voltage across the armature circuit is equal to V+Eb is impressed. Since Eb is equal to Source voltage, the voltage applied across the armature is 2V. An adjustable resistor R is placed in the circuit when changing the connections of armature to limit the current to safe value.
Now will see how braking torque depends on the motor speed.
From the above fig (plugging)
Armature Current Ia = (V+Eb) / (R=Ra)
Ia = (V/R+Ra) + (K1NΦ/R+Ra)
∵ Eb ∝ NΦ
Braking Torque TB = K2IaΦ
TB = K2Φ {(V/R+Ra) + (K1NΦ / R+Ra)}
∴ TB = K3NΦ + K4NΦ2
For a DC Shunt Motor, Φ is constant.
Thus Braking torque ∴ TB = K5 + K6N
Therefore braking torque reduces as the motor speed slows down. Even when the motor speed reduced to zero there will be certain braking torque (TB = K5)
Regenerative Braking
The Fig above shows that the Regenerative braking of a DC Shunt Motor. In this method the motor runs as a generator. As a consequence of the motor, the kinetic energy is converted into electrical energy and back to the supply. There are two methods can be used of regenerative braking for a DC Shunt motor.
In the first method the field windings is cut off from the source and the field current is raised by exciting it from separate source. Therefore the induced EMF goes beyond exceeds the source voltage and the machine feed energy into the source. Hence the braking torque is applied up to the speed of the motor till the induced EMF and the supply voltage becomes equal. Once the Machine speed drops down, then it is not possible to maintain the induced EMF at high value than the supply voltage. Hence this technique is used only a limited range of speed.
In another technique Instead of changing the field excitation, the load causes the motor to run beyond the rated speed (I.e. reducing the load on a hoist). As an outcome, the induced EMF E is increased higher than the source voltage. Therefore the current in the armature reverses and the current in the shunt field remain unaltered. Thus the torque reversed and the motor speed slows down until the induced EMF E becomes lesser than the supply voltage.
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