09-10-2017, 11:35 AM
The electric braking of a DC motor is of three types: (i) dynamic or dynamic braking, (ii) reverse current braking or braking and (iii) regenerative rotation.
(i) Dynamic or dynamic braking:
In the case of DC bypass motors, the armature is disconnected from the power supply and a rheostat (variable resistor) is connected through it. The field winding is connected through the power supply. Obviously, now the armature is driven by inertia and therefore the machine starts acting as a generator. In this way, the machine will now feed the current to the connected rheostat and the heat will dissipate at the speed of I2R. The braking effect is controlled by varying the resistance connected through the armature.
In the case of the DC series motor, the motor is disconnected from the power supply and the field connections are inverted and a rheostat is connected in series. The field connections are reversed to ensure that the current through field winding will flow in the same direction as before.
(ii) Blocking or reverse current braking:
In this method, the armature connections are reversed and therefore the motor tends to run in the opposite direction. Due to the inversion of the armature terminals, the applied voltage V and the back emf Eb starts to act in the same direction and, therefore, the total armature current exceeds. To limit this armature current, a variable resistor is connected through the armature. This is similar for both serial and bypass injury methods.
Capping provides greater braking torque compared to braking. This method is generally used to control elevators, machine tools, printing presses, etc.
(iii) Regenerative braking:
Regenerative braking is used where, the load on the engine has very high inertia (for example in electric trains). When the voltage applied to the motor is reduced to less than the emf Eb, obviously the armature current Ia will be inverted, and therefore the armature pair is inverted. Thus, the speed decreases. As the emf generated is greater than the applied voltage (the machine is acting as a DC generator), the energy will be returned to the line, this action is called regeneration. The velocity continues to fall, the emf Eb also drops until it becomes lower than the applied voltage and the direction of the armature current is again opposite to Eb.
(i) Dynamic or dynamic braking:
In the case of DC bypass motors, the armature is disconnected from the power supply and a rheostat (variable resistor) is connected through it. The field winding is connected through the power supply. Obviously, now the armature is driven by inertia and therefore the machine starts acting as a generator. In this way, the machine will now feed the current to the connected rheostat and the heat will dissipate at the speed of I2R. The braking effect is controlled by varying the resistance connected through the armature.
In the case of the DC series motor, the motor is disconnected from the power supply and the field connections are inverted and a rheostat is connected in series. The field connections are reversed to ensure that the current through field winding will flow in the same direction as before.
(ii) Blocking or reverse current braking:
In this method, the armature connections are reversed and therefore the motor tends to run in the opposite direction. Due to the inversion of the armature terminals, the applied voltage V and the back emf Eb starts to act in the same direction and, therefore, the total armature current exceeds. To limit this armature current, a variable resistor is connected through the armature. This is similar for both serial and bypass injury methods.
Capping provides greater braking torque compared to braking. This method is generally used to control elevators, machine tools, printing presses, etc.
(iii) Regenerative braking:
Regenerative braking is used where, the load on the engine has very high inertia (for example in electric trains). When the voltage applied to the motor is reduced to less than the emf Eb, obviously the armature current Ia will be inverted, and therefore the armature pair is inverted. Thus, the speed decreases. As the emf generated is greater than the applied voltage (the machine is acting as a DC generator), the energy will be returned to the line, this action is called regeneration. The velocity continues to fall, the emf Eb also drops until it becomes lower than the applied voltage and the direction of the armature current is again opposite to Eb.