Advantages of Electrical Braking over Mechanical Braking:
- Mechanical Braking due to excessive wear on the braking blocks or braking lining, needs frequent and costly replacement and also due to wear and tear increase the maintenance cost
- In some cases of electrical braking, we can return about 60 to 80% of the kinetic and potential energy of the apparatus back to the line which is not possible with mechanical braking
- By employing electrical braking, system capacity can be increased by way of higher speeds and haulage of the heavy loads
- In mechanical braking heat is produced at the brake blocks or brake lining which may be the source of failure of the brake. In electrical braking heat is produced at convenient place which is no way harmful to the braking system
- Electrical braking is smooth and without any snatching compared to mechanical braking
Types of DC Motor Electrical Braking:
- Plugging or reverse current braking
- Rheostatic or dynamic braking
- Regenerative Braking
Plugging or Reverse Current Braking:
This method of braking involves reconnection of dc motor to supply in such a way that dc motor now develops torque in opposite direction to the movement of the rotor. System speed will decrease to till dc motor attains zero speed and if the connection is unchanged dc motor will accelerate in opposite direction. Therefore, it is necessary to disconnect the supply as soon as the system comes to rest. This method of braking is most inefficient as not only the kinetic energy of the moving mass is wasted but it requires additional energy from supply for the development of the counter torque. In case of failure of the supply, plugging or reverse current braking becomes ineffective. In order to reverse the torque developed, either magnetic field or armature current has to be reversed. Since field winding will have large time constant because of the heavy inductance, it is usual practice to reverse the armature current. On reversing the armature current, back emf of the armature no longer oppose the applied voltage but both back emf and applied voltage will be in the same direction. As this condition results in flow of high current into the armature winding an additional resistance is inserted simultaneously with the operation of reversing the connections of the armature. Braking torque generated can be regulated by varying the magnitude of the resistance inserted in the armature circuit. Braking torque of motor decreases linearly with the speed of the dc motor.
Rheostatic or dynamic braking:
In rheostatic or dynamic braking, motor is made to work as generator and all the kinetic energy of the moving mass is converted to electrical energy which is dissipated in the resistance connected as electrical load. In case of shunt motor armature is disconnected from the supply and reconnected across the resistance. DC motor now act as separately excited generator. In case of failure of electrical supply, this method of braking of shunt motors becoming ineffective. The magnitude of braking torque can be regulated by the magnitude of the armature resistance.
In case of braking dc series motor, care should be taken to see that direction of current flow through field does not change. It should also consider that value of the the external resistance connected should be less than the critical value, otherwise there will be no excitation of the generator. The braking torque decreases linearly with speed till it ceases at zero speed.
In regenerative braking with out disconnected from the supply, dc motor is made to operate as dc generator and feedback or deliver back the power to the supply. Magnetic drag produced on account of generation action offers the required braking torque. Regenerative braking is most efficient. In many cases, the transition from motor action to generation action is smooth and without switching any switching operation. As soon as the overhauling load drives the motor, dc motor acts as generator.
In case of dc machines, current can flow from the machine to supply line only when the emf generated by the machine is more than the supply voltage.
If the motor is separately excited and controlled by ward leonard method of speed control, we can reduce the applied voltage. The back emf corresponding to high operating speed will be more than the applied voltage and hence the dc motor will supply power back to the line
If the motor is connected to the constant voltage supply, regeneration braking can be made by increasing the field excitation in case of shunt and separately excited motors
Regeneration braking is also possible if load overhauls the motor and drives it at higher speed than no load speed
In case of separately excited motor, braking torque can be made to remain constant, even if the speed drops by gradually increasing the field excitation. But there is a limit to the increase in the field excitation due to saturation of the field circuits. After maximum field current is attained, braking torque decreases linearly with the speed till emf generated and terminal voltage are equal
In case of dc series motors, increase in excitation results in decrease in the speed. The product of speed and excitation therefore remains constant. Therefore it is not possible to attain emf higher than the terminal voltage. And also, it is not possible to make field current more than the armature current. Therefore regenerative braking with series motor is not possible. Regenerative braking in case of series motor can only be applied after connecting the field as separately excited and also by adopting measures to make it more stable.