# DC Machines Questions and Answers – Braking of DC Motors – 1

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Braking of DC Motors – 1”.

1. Which of the following is the best braking method?
a) Friction
b) Electromechanical action
c) Eddy-currents
d) Electric braking

Explanation: Braking methods based on friction, electromechanical action, eddy-currents, etc. are independent of the motor but sometimes electric braking is better justified owing to its greater economy and absence of brake wear.

2. DC motor is still widely used in tractions due to its excellent braking properties.
a) True
b) False

Explanation: Dc motor is used in tractions because of its excellent braking characteristics and ability of smooth transition from the motor to the generator mode and vice versa. Also, characteristics suit perfectly for traction application.

3. Which of the following is not the method of electrical braking?
a) Plugging or counter-current
b) Dynamic or rheostatic
c) Regenerative
d) Eddy current

Explanation: Eddy current is the electrical effect or response of the system, which is reflected mechanically at brakes to reduce the speed of the motor. Thus, eddy current is not an electrical brake, it is mechanical one.

4. Which of the following is the plugging method of braking?
a) Reversal of field connections
b) Reversal of armature connections
c) Addition of equal and opposite field
d) Removal of field circuit from current machine circuit

Explanation: Plugging is method where connections are reversed at a given instant. Because of the problem of interrupting highly inductive field current and the time needed for the field current to build up in opposite direction, it is a common practice to reverse armature connections.

5. Which of the following is correct formula for braking torque in plugging?
a) n (ka2/Rb)
b) n2 (ka2/Rb)
c) n-1 (ka2/Rb)
d) (ka2/Rb)

Explanation: Braking torque is equal to braking power divided by speed of the motor.
T= [(nka )2/Rb]/n. As, Braking power is equal to Ea2/Rb/n. By solving for the braking torque from the above equation, we get n (ka2/Rb).
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6. Electrical braking of any variety becomes less effective as ________________
a) Speed increases
b) Speed decreases
c) Independent of speed
d) Depends on supply voltage

Explanation: Braking torque of the DC machine is given by n (ka2/Rb). Here, braking torque is directly proportional to the speed of the motor, so as the speed decreases the efficiency of electrical brakes which is dependent on braking torque decreases.

7. Plugging is applied in a motor, if we don’t make the switch OFF what will happen?
a) Motor will come to rest as a result of plugging
b) Motor will come to rest and will start rotating in another direction
c) Motor will burn
d) Nothing will happen

Explanation: If the switch is kept ON near to zero speed, motor will have braking torque acting in opposite direction greater than the electromechanical torque. Thus, motor will come to rest and for the next instant motor will start rotating in opposite direction.

8. Plugging is used in ____________
a) Small motors only
b) Small and medium powered
c) Only in large heavy machines
d) Everywhere

Explanation: Plugging is used in small scale applications only. The large initial current and high mechanical stress restrict the application of plugging in large machines. So, in order to balance stress this method is used in small machines only.

9. Which of the following is dynamic braking?
a) Reversal of field connections
b) Reversal of armature connections
c) Addition of equal and opposite field
d) Removal of armature circuit from current machine circuit

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

10. Braking time in the dynamic braking is the function of _____________
a) System inertia
c) Motor rating
d) All- system inertia, load torque and motor rating

Explanation: In dynamic braking, when brakes are applied the armature is disconnected from machine circuit and connected to the braking resistor. Now, at this point motor is driven by kinetic energy gained earlier, dissipating power in braking resistor.

11. In dynamic braking, when braking is applied system acts as ___________
a) Freely running machine
b) Motor with slow speed
c) Generator
d) Motor with same speed in opposite direction

Explanation: The armature is disconnected from the supply and then a braking resistor Rb is immediately connected across it. The motor acts as a generator, driven by the inertia and stored kinetic energy dissipating power in Rb. This is a simple method of bringing a motor nearly to a standstill.

12. In which of the following electrical braking method, energy is supplied back to the supply?
a) Plugging
b) Dynamic braking
c) Regenerative braking
d) In all electrical braking

Explanation: In plugging energy is wasted in braking resistance which is equal to starting resistance while running as a motor. In dynamic braking energy is generated but it is not fed back to supply. In regenerative method energy is sent back for reuse.

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