Electric Braking Of DC Motors
A running motor may be brought to rest quickly by either mechanical braking or electrical braking. The mechanical braking is applied by means of mechanical break shoes. Hence the smoothness of mechanical braking is dependent on the surface and physical condition of brakes. Smooth braking of a motor can be achieved by electric braking.
(i) Rheostatic or dynamic braking:
In case of DC shunt motors, armature is disconnected from the supply and a rheostat (variable resistor) is connected across it. The field winding is left connected across the supply. Obviously, now armature is driven by the inertia and hence machine starts acting as a generator. Thus the machine will now feed the current to the connected rheostat and heat will dissipate at the rate of I2R. Braking effect is controlled by varying the resistance connected across the armature.
In case of DC series motor, motor is disconnected from the supply and field connections are reversed and a rheostat is connected in series. The field connections are reversed to make sure that the current through field winding will flow in the same direction as before.
(ii) Plugging or Reverse current braking:
Plugging gives greater braking torque as compared to rheostatic braking. This method is generally used in controlling elevators, machine tools, printing presses etc.