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DC Machines Question Bank on “Braking of DC Motors – 2”.
1. Regenerative braking is used when duty cycle ____________
a) Requires braking of machine
b) Requires accelerating of machine
c) Requires constancy of machine
d) Cannot comment on duty cycle
Answer: a
Clarification: Regenerative braking is used specially where the duty cycle requires the braking or slowing of the machine more frequently and is most useful in holding a descending load of high potential energy at a constant speed.
2. Regeneration is not easily possible for ____________
a) DC shunt motor
b) Separately excited motor
c) Compounding motor with weak series compounding
d) DC series motor
Answer: d
Clarification: Regeneration is possible with a shunt and separately excited motors and with compound motors with weak series compounding. Series motors need a reversal of either the field or the armature connections.
3. Which of the following method is not used for regeneration?
a) Increasing field current
b) Increasing armature speed
c) Increasing supply voltage
d) Reducing supply voltage
Answer: c
Clarification: Regeneration is achieved by either increasing field current, increasing armature speed, or by reducing supply voltage. Increasing supply voltage is not the method which is employed in regeneration process.
4. If the terminals of armature of DC motor are interchanged, this action will offer following kind of electrical braking ______________
a) Regenerative
b) Plugging
c) Dynamic braking
d) Depends on other parameters
Answer: b
Clarification: Plugging is electrical braking method, where field or armature connections are reversed technically. But field reversal is not employed as results obtained from field reversal are not good compare to armature reversal.
5. The plugging braking gives the _____________
a) Zero torque braking
b) Smallest torque braking
c) Highest torque braking
d) Variable torque braking
Answer: c
Clarification: In electrical braking called plugging direct reversal of connections is done, which causes maximum torque to act on shaft but in opposite direction. As the speed decreases this torque also starts decreasing.
6. Regenerative method of braking is based on ___________
a) Back emf is less than the applied voltage
b) Back emf is equal to the applied voltage
c) Back emf of rotor is more than the applied voltage
d) Cannot be determined
Answer: c
Clarification: The condition for regeneration is that the rotational emf is more than the applied voltage so that the current is reversed and the mode of operation changes from motoring to generating.
7. During regenerative braking of DC motors ____________
a) Motor will run as a generator
b) Motor will reverse in direction
c) Motor will run at reduced speed
d) Motor will run as free rotating shaft
Answer: a
Clarification: In regenerative method of electrical braking, motor is suddenly forced to act as a generator, all the energy then obtained is pushed back into the supply unlike in dynamic braking this energy is wasted.
8. Where dynamic braking is used?
a) Shunt motors
b) Series motors
c) Compound motors
d) All DC motors
Answer: d
Clarification: Dynamic braking is used in all DC motors though its implantation in series DC motor requires one more additional step of reversal of connections. Only care taken is, addition of braking resistance, armature resistance and series field resistance is lower than the critical resistance at that speed.
9. Which method of braking is generally used in elevators?
a) Plugging
b) Regenerative braking
c) Rheostatic braking
d) Mechanical braking
Answer: a
Clarification: Plugging braking provides maximum torque in opposite direction at the instant of braking, this characteristic of braking suits perfectly with the application that is in elevators. If switch is kept ON, we get reverse rotation also.
10. For which of the following motor dynamic braking is very effective?
a) Shunt motors
b) Separately excited motors
c) Series motors
d) Differential compound motors
Answer: b
Clarification: Dynamic braking is very effective for separately excited DC motors. As in separately excited motors the direction of field can be very easily altered by altering the terminals of the field, which is the condition in dynamic braking.
11. When is the dynamic braking is employed?
a) Non-reversing drive
b) Reversing drive
c) Both Reversing and Non-reversing
d) Cannot tell
Answer: c
Clarification: Dynamic braking is employed to brake both reversing drives and non-reversing drives.
In dynamic braking, the electrical energy generated during stopping action is released as heat through a voltage regulated transistor and resistor.