Apart from Regenerative Braking, another method of reversing the direction of torque and Braking the motor is dynamic braking. In this method of braking the motor which is at a running condition is disconnected from the source and connected across a resistance. When the motor is disconnected from the source, the rotor keeps rotating due to inertia and it works as a self excited generator. When the motor works as a generator the flow of the current and torque reverses. During braking to maintain the steady torque sectional resistances are cut out one by one.
It shall be noted that for Dynamic Braking the connection of Field coil is reversed in case of DC Series Motor so as to assist the residual flux. Thus the direction of current changes but the direction of field flux remains the same and this is because the direction of torque reverses.
In case of Separately Excited Motor, for fast Braking large value of RB is used but as the speed falls the value of RB is reduced to have high average torque as shown in figure below.
Another type of Braking is Plugging type Braking. In this method the terminals of supply are reversed for Separately Excited Motor, as a result the supply voltage assist the counter or back emf to reverse the current. This in turn demands external resistance into the circuit to limit the flowing current. The main disadvantage of this method is that regenerated power and supply power is wasted in the resistance. Therefore Plugging is most inefficient method of Braking.
As shown in figure above, for Series Motor, the supply voltage is not reversed but the armature terminal is reversed. This is done so that the direction of field current remains the same and hence the direction of field flux may remain same. Thus only the direction of current changes but field flux remains same and hence torque reverses.
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helpful information thanks