DC Motor Braking MCQ || DC Motor Braking Questions and Answers

Answer:1. Dynamic or Rheostatic

Explanation: 

• In Rheostatic braking, the electric motor is disconnected from the supply during the braking period and is reconnected across the same electrical resistance.
• But field winding is continuously excited from the supply in the same direction.
• Thus, during the starts working as a generator during the braking period and all the kinetic energy of the rotating parts is converted into electric energy and is dissipated across the external resistance.
• In principle, a synchronous motor may be stopped by either plugging or rheostatic braking.
• But, the plugging method has certain disadvantages. Synchronous motors usually are of a large power rating and the line disturbances that occur during plugging are beyond the permissible limits.
• Also, since the plugging torques arise from the damper winding and unless it is specially designed for the purpose, the torques developed during plugging are smaller in magnitude than those obtained by rheostatic braking.
• Hence, rheostatic braking is almost exclusively used for braking synchronous motors.

As long as the field excitation is maintained, rheostatic braking can easily be applied to synchronous motors by disconnecting the armature from the supply and connecting it to resistances so that the machine acts as an alternator.

Answer: 2. Speed decreases

Explanation: 

Braking torque of the DC machine is given by

T_B = (ka²n)/R

If the excitation is constant, then the braking torque is directly proportional to the speed “n” so as the speed decreases the efficiency of electrical brakes which is dependent on braking torque decreases.

Answer: 4. Mechanical Braking

Explanation: 

In Mechanical braking, the kinetic energy of the rotating parts is dissipated in the form of heat by the brake shoes of the brake lining that rubs on the wheel of the vehicle or brake drum. Mechanical braking is used in the breaking of battery-operated scooters.

• Plugging is used where a series motor is used
• Rheostatic braking is used where shunt motor is used
• Regenerative braking is used in shunt motors where a load is in overhauling condition

Answer: 2. Motor will come to rest and will start rotating in another direction

Explanation: 

• In the plugging method when the connections of the motor are changed to run it in the reverse direction, that is, it starts developing a torque in the reverse direction to the previous.
• Previously it was developing a motoring torque i.e., the torque developed was in the same direction in which it is rotating so the torque was supporting the motion.

Now, when its connections are suddenly changed for a reversal but the machine is still running in its previous direction so this torque is now braking torque, it tries to quickly stop the machine, the machine slows down faster, if the supply is not switched off when the speed becomes zero then the motor will start running in the opposite direction. This method of braking is called plugging.

Answer:3. Returned to the supply mains

Explanation: 

• Regenerative braking is the most efficient method of braking to stop the motor. In other methods of braking, the kinetic energy of all rotating parts is wasted in an external braking resistor, and in the case of plugging extra energy is drawn from the supply during the braking period.
• But in this method of braking, no energy is drawn from the supply during the braking period and some of the energy is fed back to the supply system. Regenerative braking can be applied to both DC and AC motors.

Answer:1. Small motors only

Explanation: 

• In the Plugging method, the motor is reconnected to the supply in such a way that it has to develop torque in opposite direction to the movement of the rotor, it can be done by reversing the connections of the armature.
• The motor will decelerate until the speed is zero and then accelerate in opposite direction; Immediately, it is necessary to disconnect the motor from the supply as soon as the system comes to rest.
• The large initial current and the resultant high mechanical stress restrict the application of plugging to small motors only.

Answer:2. Highest

Explanation: 

In the plugging method of braking, the electric motor is reconnected to the supply in such a way that it has to develop torque in opposite direction to the movement of the rotor.

Now, the motor will decelerate until zero speed is zero and then accelerate in opposite direction. Immediately, it is necessary to disconnect the motor from the supply as soon as the system comes to rest.

• During plugging external resistance is also introduced into the circuit to limit the flowing current
• Thus, the armature current is reversed and high braking torque is produced
• It provides the highest braking torque in comparison to rheostatic and regenerative braking systems.

Answer:4. Removal of armature circuit from current machine circuit

Explanation: 

In the dynamic braking method, the electric motor is disconnected from the supply during the braking period and is reconnected across the same electrical resistance. But field winding is continuously excited from the supply in the same direction.

Thus, during the starts working as a generator during the braking period and all the kinetic energy of the rotating parts is converted into electric energy and is dissipated across the external resistance.

Reversal of the connections of the armature is the method called plugging. In dynamic braking, we remove the armature circuit and connect it to the different resistors, with the field circuit still connected to the external supply.

Answer:3. AC dynamic braking

Explanation:

AC dynamic braking is not a braking method of a dc motor.

Regenerative braking:

• In this method, no energy is drawn from the supply during the braking period and some of the energy fed back to the supply system
• The braking torque generated in the motor is in the opposite direction of rotation of the rotor
• It can be applied to both DC and AC motors
• The net energy saved during the regenerative braking of an electric train is high with a high down gradient.

Plugging type braking or Reverse current braking:

• Plugging is used to reduce the speed of motors
• The supply voltage V and the induced voltage Eb (back emf) will act in the same direction
• The effective voltage across the armature will be (V + E which is almost twice the supply voltage
• During plugging external resistance is also introduced into the circuit to limit the flowing current
• Thus, the armature current is reversed and high braking torque is produced

Dynamic braking:

• In Dynamic Braking, the armature of the DC motor is disconnected from the supply mains and a braking resistor Rb is connected across the armature
• The motor now works as a generator, producing the braking torque

Answer:4. All-system inertia, load torque, and motor rating

Explanation: 

In the dynamic braking method, the electric motor is disconnected from the supply during the braking period and is reconnected across the same electrical resistance.

But field winding is continuously excited from the supply in the same direction. Thus, during the starts working as a generator during the braking period and all the kinetic energy of the rotating parts is converted into electric energy and is dissipated across the external resistance.

The braking time is a function of the system inertia, load torque, and motor rating. The field circuit is left connected to the supply.

Answer:3. Hydraulic

Explanation: 

In electric braking, the kinetic energy of the rotating parts of the motor is converted into electrical energy which in turn is dissipated as heat energy in resistance or in sometimes, electrical energy is returned to the supply. Here, no energy is dissipated in brake shoes.

Types of Electric Braking in DC Motor:

(i) Rheostatic or Dynamic braking
(ii) Plugging
(iii) Regenerative braking

Answer: 3. Generator

Explanation: 

In dynamic braking, the Supply is disconnected to the armature but the field is usually maintained. An external resistance (rheostat) is connected across the armature. The machine now runs as a generator.

The input to the machine is the kinetic energy of the rotating armature which gets converted to electrical energy and is dissipated as heat in the resistance. The braking torque reduces as speed decreases and becomes zero at zero speed.

Answer: 3. Regenerative braking

Explanation: 

• Regenerative braking is the most efficient method of braking to stop the motor. In other methods of braking, the kinetic energy of all rotating parts is wasted in an external braking resistor, and in the case of plugging extra energy is drawn from the supply during the braking period.
• But in this method of braking, no energy is drawn from the supply during the braking period and some of the energy is fed back to the supply system. Regenerative braking can be applied to both DC and AC motors.

Answer: 3. Both AC and DC

Explanation: 

• Regenerative braking is the most efficient method of braking to stop the motor.
• In other methods of braking, the kinetic energy of all rotating parts is wasted in an external braking resistor, and in the case of plugging extra energy is drawn from the supply during the braking period.
• But in this method of braking, no energy is drawn from the supply during the braking period and some of the energy is fed back to the supply system.
• Regenerative braking can be applied to both DC and AC motors.

Answer: 4. All of the above

Explanation: 

Disadvantages of electric Braking

• During the braking period, the traction motor acts generator and electric brakes can almost stop the motor but it cannot hold stationary. Hence, it is necessary to employ mechanical braking in addition to electric braking.
• The traction motor has to work as a generator during the braking period. So, the motor has to select in such a way that it should have suitable braking characteristics.
• The initial cost of the electric braking equipment is costlier.