DC Motor Questions and Answers pdf

1. The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is

(a) 2.36%
(b) 4.76%
(c) 6.71%
(d) 8.84%

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2. The armature voltage control of D.C. motor provides

(a) constant torque drive
(b) constant voltage drive
(c) constant current drive
(d) none of the above

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3. The speed of a D.C. shunt motor can be increased by

(a) increasing the resistance in armature circuit
(b) increasing the resistance in field circuit
(c) reducing the resistance in the field circuit
(d) reducing the resistance in the armature circuit

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4. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will

(a) remain unchanged
(b) reduce to one-fourth value
(c) increase four folds
(d) be doubled

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5. At the instant of starting when a D.C. motor is put on supply, it behaves like

(a) a highly resistive circuit
(b) a low resistance circuit
(c) capacitive circuit
(d) none of the above

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6. Which motor has the poorest speed control?

(a) Differentially compounded motor
(b) Cumulatively compounded motor
(c) Shunt motor
(d) Series motor

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7. The plugging gives the

(a) zero torque braking
(b) smallest torque braking
(c) highest torque braking
(d) none of the above

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8. If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking

(a) regenerative
(b) plugging
(c) dynamic braking
(d) none of the above

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9. Which of the following motors is usually used in household refrigerators ?

(a) D.C. shunt motor
(b) D.C. series motor
(c) Single phase induction motor (split phase start or induction run motor)
(d) Reluctance motor
(e) Synchronous motor

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10. Ward-Leonard control is basically a

(a) voltage control method
(b) field diverter method
(c) fled control method
(d) armature resistance control method

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11. For constant torque drive which speed control method is preferred ?

(a) Field control
(b) Armature voltage control
(c) Shunt armature control
(d) Mechanical loading system

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12. In Ward-Leonard control the lower limit of speed is imposed by

(a) residual magnetism of the generator
(b) core losses of motor
(c) mechanical losses of motor and generator together
(d) all of the above

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13. The main disadvantage of the Ward Leonard control method is

(a) high initial cost
(b) high maintenance cost
(c) low efficiency at light loads
(d) all of the above

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14. The losses occurring in a D.C. generator are given below. Which loss is likely to have the least proportion ?

(a) Magnetic losses
(b) Armature copper losses
(c) Mechanical losses
(d) Field copper losses

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15. In a D.C. generator all of the following could be the effects of iron losses except

(a) Loss of efficiency
(b) Excessive heating of core
(c) Increase in terminal voltage
(d) Rise in temperature of ventilating air

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16. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?

(a) hysteresis loss
(b) field copper loss
(c) armature copper loss
(d) eddy current loss

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17. If ‘t’ be the thickness of the laminations, then eddy current loss in a generator will vary as

(a) 1/t
(b) t
(c) t2
(d) 1/t2

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18. If Bmax is the maximum flux density, then eddy current loss will vary as

(a) Bmax
(b) Bmax2
(c) Bmax1.2
(d) Bmax2.4

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19. The hysteresis loss in a D.C. generator varies with the frequency of magnetic reversals as

(a) 1/f
(b) f
(c) f1.6
(d) f2

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20. Which of the following methods of braking is used in rolling mills ?

(a) Dynamic braking
(b) Plugging
(c) Regenerative braking
(d) Mechanical brakes

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21. Regenerative method of braking is based on that

(a) backe.m.f.is less than the applied voltage
(b) back e.m.f. is equal to the applied voltage
(c) backe.m.f. of rotor is more than the applied voltage
(d) none of the above

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22. The retardation test is applicable to shunt motors and generators and is used to find

(a) the copper losses
(b) the stray losses
(c) the friction losses
(d) the eddy current losses

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23. Compensating winding in a D.C. machine is placed

(a) on yoke in the pole faces
(b) on yoke in the interpole gap
(c) on armature
(d) none of the above

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24. Torque developed by a D.C. motor depends upon

(a) magnetic field
(b) active length of the conductor
(c) current flow through the conductors
(d) number of conductors
(e) radius of armature
(f) all above factors

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25. In a manual shunt motor starter

(a) over load relay is connected in series and no-volt relay in parallel with the load
(b) over load relay is connected in parallel and no volt relay in series with the load
(c) over load relay and no volt relay are both connected in series with the load
(d) over load relay and no volt relay are both connected in parallel with the load

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26. Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?

(a) Providing laminations in armature core
(b) Providing laminations in stator
(c) Using non-magnetic material for frame
(d) Using material of low hysteresis co-efficient for armature core material

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27. Which loss in a D.C. generator does not vary with load as well as flux density ?

(a) Copper loss
(b) Eddy current loss
(c) Hysteresis loss
(d) Windage loss

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28. The total losses in a well designed D.C. generator of 10 kW will be nearly

(a) 100V
(b) 500 W
(c) 1000 W
(d) 1500 W

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29. The condition for maximum efficiency for a D.C. generator is

(a) eddy current losses = stray losses
(b) hysteresis losses = eddy current losses
(c) variable losses = constant losses

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30. D.C. generators are normally designed for maximum efficiency around

(a) full-load
(b) rated r.p.m.
(c) rated voltage
(d) all of the above

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