DC MACHINES 15

141. In dc machines , the armature reaction mmf is
a) stationary with respect to armature
b) rotating with respect to stator
c) stationary with respect to stator
d) rotating with respect to brushes.
ans. c

142. The purpose of using interpoles in dc machines is to nullify
a) the demagnetising effect of armature mmf
b) the cross magnetising effect of armature mmf
c) the reactance voltage
d) both the cross-magnetizing mmf and the reactance voltage
ans. d

143. In dc generators, the polarity of interpoles is
a) same as that of main pole behind
b) same as that of mail pole ahead
c) opposite to that of the mail pole
d) none of these
ans. b

144. The object of using compensation winding in dc machines is to neutralize the
a) armature reaction in the interpolar zone
b) armature reaction in the commutating zone
c) armature reaction under the pole faces
d) cross-magnetizing armature reaction.
ans. c

145. The emf generated in a dc generator is
a) dynamically induced emf
b) statically induced emf
c) electrostatically induced emf
d) magnetically induced emf
ans. a

146. The yoke of generator is made of cast iron because
a) it is cheaper
b) it completes the magnetic path
c) it gives mechanical protection to the machine
d) all of these
ans. d

147. The field of self excited generator is excited by
a) dc
b) ac
c) its own current
d) ac and dc both
ans. c

148. The DC generator works on the principle of
a) Faraday's laws of electron magnetic induction
b) lenz's law
c) the current carrying conductor placed in the magnetic field an emf is produced .
d) induction.
ans. a

149. The carbon brushes are generally used as they
a) are self lubricating
b) are soft
c) have negative temperature coefficient
d) possess all the above three properties
ans. d

150. Armatures are made of laminated steel instead wood because it has
a) low permeability
b) high permeability
c) more mechanical strength
d) more mechanical strength and high permeability

DC MACHINES 14

131. Rosenberg generator is a
a) constant voltage generator at low speeds and voltage polarity is independent of the direction of rotation.
b) a constant voltage generator at low speeds and voltage polarity changes with the direction of rotation.
c) a constant current generator at high speeds and voltage polarity is independent of the direction of rotation.
d) a constant current generator at high speeds and voltage polarity changes with the direction of rotation.
ans. c

132. If the number of poles in the first stage of a Rototrol is 2, the number of poles in the second stage is
a) 2
b) 4
c) 6
d) 8
ans. c

133. The inductor of a 3-wire generator should be iron-cored to
a) increase the current through it
b) reduce the current through it
c) reduce the voltage ripple in the output circuit
d) improve the voltage regulation of generator.
ans. b

134. In a dc machines, the armature windings are placed on the rotor because of the necessity for
a) electromechanical energy conversion
b) generation of voltage
c) communication
d) development of torque
ans. c

135. In dc machines , the space distribution of air-gap flux density wave at no load is
a) sinusoidal
b) co-sinusoidal
c) flat-topped
d) rectangular
ans. c

136. A commutator in dc machines can convert
a) ac to dc
b) dc to ac
c) both ac to dc and dc to ac
d) none of these
ans. b

137. In dc machines the space waveform for the air-gap flux distribution effects
a) torque but not the voltage
b) voltage but not the torque
c) neither the voltage nor the torque
d) both the torque and voltage
ans. c

138. In dc machines, the axis of armature mmf is along the
a) direct axis
b) interpolar axis
c) midway between the direct and interpolar axes
d) none of these
ans. c

139. A 4-pole dc generator is running at 1500 rpm. The frequency of current in the armature winding is
a) 25 Hz
b) 50 Hz
c) zero Hz
d) 100 Hz
ans. b

140. The direction of rotation of a dc shunt motor can be reversed by interchanging
a) the supply terminals
b) the field terminals only
c) the armature terminals only
d) either field or the armature terminals
ans. d

DC MACHINES 13

121. A dc series motor is driving a load with a diverter connected across its armature. If the diverter resistance is decreased , the speed of the motor
a) increases
b) decreases
c) remains unchanged
d) becomes zero
ans. b

122. A dc series motor is running with a diverter connected across its field winding. If the diverter resistance is increased then the speed of the motor.
a) decreases
b) increases
c) remains unchanged
d) becomes excessively high
ans. a

123. Two dc series motors connected in series are driving the same mechanical load. If the motors are now connected in parallel the speed becomes
a) slightly less than double
b) slightly less than half
c) slightly more than double
d) slightly more than half
ans. c

124. A dc series motor is running at rated speed with rated excitation. The motor has two resistance R1 and R2 connected across the armature and the field respectively. Speeds above the rated speed can be achieved by
a) decreasing R1 only
b) increasing R2 only
c) decreasing R1 and increasing R2
d) increasing R1 and decreasing R2
ans. d

125. A dc series motor is running at rated speed without any additional resistance in series. If an additional resistance is placed in series, the speed of the motor
a) increases
b) decreases
c) remains unchanged
d) oscillates around the rated speed.
ans. b

126. The most economic method of electrical braking is
a) regenerative braking
b) dynamic braking with self excitation
c) dynamic braking with separate excitation
d) plugging
ans. a

127. Plugging of dc motors is normally executed by
a) reversing the field polarity
b) reversing the armature polarity
c) reversing both armature and field polarity
d) connecting a resistance across the armature
ans. b

128. For non-reversing dc drives it is preferable to employ
a) regenerative braking
b) dynamic braking with separate excitation
c) dynamic braking with self excitation
d) plugging
ans. b

129. An amplitude has split poles to
a) provide space for Interpoles
b) increase amplification factor
c) increase efficiency
d) damp out mechanical oscillations.
ans. a

130. If the number of poles in a cross-field generator is 2, the number of Interpoles will be
a) 2
b) 4
c) 6
d) 8
ans. b

DC MACHINES 12

111. Direct-on-line starters are not suitable for starting large dc motors because
a) the motor may not start.
b) large voltage drop may occur in the supply.
c) the starting torque becomes very low.
d) the motor may run away.
ans. b

112. For dc shunt motor, speed control by armature resistance variations is best suited for
a) constant power drive
b) variable power drive
c) constant torque drive
d) variable torque drive
ans. c

113. For dc shunt motor, speed control by the variation of field flux is best suited for

a) constant power drive
b) variable power drive
c) constant torque drive
d) variable torque drive
ans. a

114. If the field circuit of a dc shunt motor running at rated speed gets open-circuited , then immediately after    this the speed of the motor would tend to
a) decrease
b) increase
c) remain unchanged
d) oscillate around the rated speed
ans. b

115. A dc shunt motor is running at rated speed with rated supply voltage. If the supply voltage is halved , then the speed of the motor becomes
a) half of the rated speed
b) double the rated speed
c) slightly less than the rated speed
d) slightly more than the rated speed
ans. c

116. A dc shunt motor is driving a constant torque load without any additional resistance in the armature circuit . If an additional resistance is placed in the armature circuit then the speed of the motor
a) increases
b) decreases
c) remains unchanged
d) becomes zero
ans. b

117. A dc shunt motor is driving a constant torque load with rated excitation. If the field current is halved then the speed of the motor
a) becomes half
b) becomes slightly more than half
c) becomes double
d) becomes slightly less than double.
ans. d

118. A dc shunt motor is running at rated speed with rated excitation, rated voltage  and with an additional resistance in the armature circuit .Speeds less than the rated speed can be achieved by
a) Reducing the supply voltage and increasing the field excitation.
b) Increasing the supply voltage and decreasing the armature circuit resistance
c) Decreasing the field excitation and increasing the supply voltage.
d) Decreasing the armature circuit resistance and decreasing the field excitation.
ans. a

119. In Ward-Leonard method of speed control the direction of rotation of the motor is reversed usually by
a) reversing the connections of the generator field terminals
b) reversing the connection of the generator armature terminals
c) reversing the connections of the motor armature terminals
d) none of these.
ans. a

120. A dc shunt motor is driving a mechanical load at rated voltage and the rated excitation. If the load torque becomes double then the speed of the motor
a) increase slightly
b) decreases slightly
c) becomes double
d) becomes half
ans. b

DC MACHINES 11

1. The brake test for the determination of efficiency of a dc machine is
a) an indirect method.
b) a regenerative method.
c) a direct method.
d) none of these
ans. c

2. In Swinburne's method for the determination of efficiency of a dc machine
a) the no-load losses are calculated and the copper losses are measured.
b) the no-load losses and measured copper losses are calculated.
c) both the no-load losses and the copper losses are measured.
d) both the no-load losses and the copper losses are calculated.
ans. b

3. In the Kapp's modificaiton of Hopkinson's efficiency test
a) the power losses in the two machine are supplied mechanically.
b) the power output of the generator is dissipated in a resistor.
c) the two machines are mechanically decoupled.
d) the power losses in the two machines are supplied electrically.
ans. d

4. Commutation conditions at full-load for a large dc machine can be checked by
a) the brake test
b) the swinburne's test
c) the Hopkinson's test
d) none of these
ans. c

5. The field's efficiency test of dc series motors overcomes the difficulty of obtaining readings relatively light loads by connecting
a) the series field of the generator in series with the motor armature.
b) the series field of the motor in series with the generator armature.
c) the armature of the generator in series with the motor.
d) the armature of the motor in series with the generator.
ans. a

6. DC motor starters are used
a) to increase the starting torque
b) to limit the starting current
c) both a and b
d) none of these
ans. b

7. For dc shunt motors the field excitation is kept at maximum value during starting to
a) increase acceleration time
b) decrease starting torque
c) reduce armature heating
d) prevent voltage dip in the supply mains
ans. c

8. DC motors should be stopped by opening the line switches and not by forcing the starter handle back to the off position
a) heavy sparking occurs at the brushes.
b) heavy sparking occurs at the first stud of the starting resistance steps.
c) both a and b
d) none of these.

9. A dc shunt motor has two additional resistances R1 and R2 in the field circuit and armature circuit respectively . The starting armature current can be kept to a minimum by keeping
a) R1 maximum R2 maximum
b) R1 minimum R2 maximum
c) R1 maximum R2 minimum
d) R1 minimum R2 minimum
ans. b

10. Three point starters of dc shunt motors are not used in applications where speed variation by field flux control is required because
a) the motor may stop at very high speeds
b) the motor may stop at very low speeds
c) the motor may stop both at very high and at very low speeds.
d) the motor may run away.
ans. a

DC MACHINES 10

 1. In a dc machine without interpoles to get improved commutation , the brush shift angle must be
a) varied with change in load
b) kept constant
c) 0 degree
d) none of these
ans. a

2. Compared to the air-gap under the field-poles, the inter-pole air-gap is made
a) larger for both the generators and the motors.
b) smaller for both the generators and larger for the motor.
c) smaller for generator and larger for the motors.
d) larger for generators and smaller for the motors.
ans. a

3. Compared to an uncompensated dc machine , the interpole as required in a compensated dc machine is
a) larger for the generators and smaller for the motors.
b) smaller for the generators and larger for the motors
c) smaller for both the generators and the motors.
d) larger for both the generators and the motors.
ans. c

4. Due to magnetic saturation , the flux per pole in a dc machine without brush shift
a) decrease the generators and increases in the motors with load.
b) decreases in both the generators and the motors with load.
c) increases in both the generators and the motors with load.
d) increases in the generators and decreases in the motors with load.
ans. b

5. The core losses in a dc machine occur due to
a) hysteresis only
b) eddy current only
c) armature current
d) both hysteresis and eddy currents.
ans. b

6. The core losses in a dc machine occur in
a) the armature only
b) the pole faces only
c) the yoke only
d) both the armature and the pole faces.
ans. d

7. The hysteresis loss per unit weight is
a) proportional to the frequency
b) inversely proportional to the frequency
c) inversely proportional to the flux density
d) proportional to the thickness of lamination's.
ans. a

8. If the thickness of lamination's is increased , then
a) the eddy current loss decreases
b) the eddy current loss increases
c) the hysteresis loss decreases
d) the hysteresis loss increases
ans. b

9. Ohmic losses in a dc machine occur in
a) the armature winding only
b) the field winding only
c) the brush contact only
d) the armature winding , the field winding and also in the brush contact.
ans. d

10. The efficiency of a dc machine is maximum when the variable losses equal to
a) the constant losses
b) the square of the constant losses.
c) the square root of the constant losses
d) zero.
ans. a

DC MACHINES 9

1. If the armature current of a dc motor is increased keeping the field flux constant, then the developed torque
a) increases proportionally.
b) decreases in inverse proportion.
c) remains constant.
d) increases proportionally to the square of the current.
ans. a

2. The direction of induced emf in an armature coil of a dc machine is
a) the same as that of the current for both the generator and the motor.
b) opposite to that of the current for the generator and the motor.
c) the same as that of current for the generator and opposite to that of the current for the motor.
d) none of these.
ans. c

3.In a dc machine the armature mmf is always directed along the
a) polar axis
b) brush axis
c) interpolar axis
d) none of these
ans. b

4. In a dc machine without any brush shift, the shift of the magnetic neutral axes due to armature is
a) in the direction of rotation for the generator and against the direction of rotation for the motor.
b) in the direction of rotation for both the generator and the motor.
c) against the direction of rotation for both the generator and the motor
d) against the direction of rotation for the generator and in the direction of rotation
ans. a

5. The wave form of the armature mmf in a dc machine is
a) square
b) rectangular
c) triangular
d) sinusoidal
ans. c

6. The commutator of a dc machine acts as a
a) full wave rectifier
b) half wave rectifier
c) controlled full-wave rectifier.
d) controlled half-wave rectifier.
ans. a

7. In a dc machine the armature reaction and the inductance of the commutating coils result in
a) over-commutation
b) sinusoidal commutation
c) linear commutation
d) under commutation
ans. d

8. Under commutation gives rise to
a) sparking at the leading edge of the brush
b) sparking at the trailing edge of the brush
c) no sparking at all
d) sparking at the middle of the brush
ans. b

9. For a dc generator if the brushes are given , a small amount forward shift the effect of armature reaction is
a) totally demagnetizing
b) totally magnetizing
c) partly demagnetizing and partly cross magnetizing
d) totally cross-magnetizing
ans. c

10. In a dc motor if the brushes are given backward shift , then
a) commutation is worsened and speed decreases
b) commutation is improved and speed decreases
c) commutation is unaffected and speed increases
d) commutation is improved and speed increases
ans. d

DC MACHINES 8

1. The main function of a commutator in dc motor is to
a) prevent sparking
b) convert ac to dc
c) reduce friction
d) convert dc to ac
ans. b

2. Which of the following statements is incorrect? If a starter is not used with large dc motor. It will draw a starting current which
a) is many times its full-load current
b) will produce excessive line voltage
c) will damage the commutator
d) will produce very low starting torque
ans. d

3. The factor which voltage does not cause decrease in the terminal voltage of a shunt generator is
a) armature resistance
b) armature reaction
c) reduction in field current
d) armature reactance
ans. d

4. A dc generator can be termed as
a) rotating amplifier.
b) prime mover.
c) power pump.
d) none of these
ans. a

5. The function of compensating windings placed in slots in the pole shoes is
a) to neutralize cross magnetizing effect
b) to neutralize the demagnetizing effect
c) to neutralize both the effects
d) to avoid flash over around the commutator
ans. d

6. In case of parallel operation of compound generators , for proper division of load from no load to full load , it is essential that
a) the regulation of each armature should be same
b) their series field resistances should be equal
c) their ratings should be identical
d) none of these
ans. a

7. The simplest way to shift load from one shunt generator to the other generator running in parallel is to
a) adjust their field rheostats.
b) use equalizer connection.
c) add resistance in their armature circuit
d) none of these.
ans. a

8. For parallel operation , the polarities of two generators
a) must oppose each other
b) must be same
c) may or may not be same
d) none of these
ans. a

9. The parts of the armature electric circuit which take active part in emf generation are
a) The coil sides inside the slots
b) the overhangs
c) both the coil sides inside the slots and the overhangs
d) the commutator segments
ans. a

10. The emf induced in the armature of a dc machine is
a) directly proportional to the flux and inversely proportional to the speed .
b) directly proportional to both the flux and speed.
c) inversely proportional to both the flux and the speed.
d) none of the above
ans. b

DC MACHINES 7

61. A dc shunt generator driven at a normal speed in the normal direction fails to build up armature voltage because
a) the resistance of the armature is high
b) there is no residual magnetism
c) the field current is too small
d) none of these
ans. b

62. An ideal dc generator has a regulation of
a) 0%
b) 50%
c) 75%
d) 100%
ans. a

63. Which of the following statements is incorrect in dc shunt generators about hysteresis loss?
a) It can be minimized by laminating the armature
b) It is independent of lamination thickness
c) It depends upon the supply frequency
d) none of these
ans. a

64. Which of the following methods is most economical for finding the no-load losses of a large dc shunt motor?
a) Hopkinson's test
b) Swinburne's test
c) Retardation test
d) none of these
ans. b

65. The rated speed of a given dc shunt motor is 1050 rpm.To run this machine at 1200rpm , which of the following speed control scheme will be used
a) Armature current resistance control
b) Field resistance control
c) Ward-Leonard control
d) none of these
ans. b

66. The destroyed residual magnetism of a shunt generator may restored by connecting its shunt field to
a) earth
b) generator
c) a battery
d) none of these
ans. d

67. It is preferable to start a dc series motor with some mechanical load on it because
a) it will not run at no load
b) it acts as a starter to the motor
c) it may develop excessive speed and damage itself
d) none of these
ans. c

68. If the flux of a dc motor approaches zero, its speed will approach
a) infinity
b) zero
c) a stable value nearer to the rated speed
d) none of these
ans. a

69. As the load is increased, speed of a shunt motor
a) remains constant
b) increases slightly
c) reduces slightly
d) none of these
ans. c

70. A 230 V dc series motor is connected to 230 V ac it will
a) run slowly
b) not run at all
c) run with less efficiency
d) none of these
ans. c

DC MACHINES 6

51. Which of the following motors is used for rolling mills ?
a) D.C. shunt motor
b) D.C. cumulative compound motor.
c) D.C. series motor.
d) D.C. differential compound motor.
ans. b

52. Armature reaction is attributed to
a) the effect of magnetic field setup by armature current.
b) the effect of magnetic field setup by field current.
c) copper loss in the armature.
d) the effect of magnetic field setup by back emf.
ans. a

53. What will happen if the supply terminals of dc shunt motor are interchanged?
a) Motor will stop.
b) Motor will run at its normal speed in the same direction as it was running .
c) The direction of rotation will reverse.
d) Motor will run at a speed lower than the normal speed in the same direction.
ans. b

54. When the electric train is moving down a hill the dc motor acts as
a) dc series motor
b) dc shunt motor
c) dc series generator
d) dc shunt generator
ans.c

55. Which of the following dc motors is suitable for high starting torque?
a) shunt motor
b) cumulative compound motor
c) series motor
d) compound motor.
ans. c

56. For which of the following dc motors is the typical field of application mentioned?
a) shunt motor - electric trains
b) series motor - machine tools
c) series motor - belt drive
d) compound motor - fly wheel drive
ans. d

57. The generator of a power station produces an electric pressure by
a) conversion point
b) magnetic induction
c) conversion of light
d) mechanical pressure
ans. d

58. Why is the air gap between stator and armature of an electric motor kept as small as possible?
a) To get stronger magnetic field
b) To make the rotation easier.
c) To reach a higher speed of rotation.
d) To improve the air circulation.
ans. a

59. Which of the following types of generators gives constant voltage output at all loads?
a) series generator
b) shunt generator
c) short shunt compound generator
d) level compound generator
ans. d

60. Interpoles are meant for
a) increasing the speed of the motor
b) decreasing counter emf
c) reducing sparking at the commutator.
d) converting armature current to dc.
ans. c

DC MACHINES 5

41. The speed of d.c. series motor decreases if the flux in the field winding
a) remains constant
b) increases
c) decrease
d) None of the above.
ans. b

42. Which of the following motors is used to drive the constant speed line shafting lathes, blowers and fans?
a) D.C .shunt motor
b) D.C. series motor
c) Commutative compound motor.
d) None of these
ans. a

43. If the back emf is a d.c. motor is absent , then
a) motor will run at very high speed.
b) motor will run at very low speed.
c) motor will not run at all.
d) motor will burn.
ans. d

44. The field flux of a d.c. motor can be controlled to achieve
a) The speeds lowers than rated speed.
b) The speeds higher than rated speed.
c) the speeds at rated speed
d) None of the above .
ans. b

45. By providing a variable resistance across the series field in a d.c. series motor, speeds above normal can be obtained because
a) armature current decreases
b) flux is reduced
c) line current is decreased
d) None of these
ans. b

46. D.C. series motors are best suited for traction work because
a) torque is proportional to the square of armature current and speed is inversely proportional to torque.
b) torque is proportional to the square of armature current and speed is proportional to the torque.
c) torque and speed are proportional to square of armature current.
d) none of these
ans. a

47. A series motor is started without load. The effect is that
a) the torque increases rapidly
b) the speed increases rapidly
c) current drawn increases rapidly
d) the back emf decreases.
ans. b

48. The direction of rotation of a dc series motor can be reversed by interchanging
a) the supply terminals only
b) the field terminals only
c) the supply as well as field terminals
d) None of these.
ans. b

49. The motor used for intermittent, high torque load is
a) dc shunt motor
b) dc series motor
c) differential compound motor
d) cumulative compound motor
ans. d

50. If the field of a dc shunt motor is opened.
a) it will continue to run at its rated speed.
b) the speed of the motor will become very high
c) the motor will stop
d) the speed of motor will decrease.
ans. b

DC MACHINES 4

31. The power stated on the name plate of any motor is always
a) the output power at the shaft.
b) the power drawn in kVA.
c) the power drawn in kW.
d) the gross power.
ans. a

32. The exciting coils of any d.c. machine are wound
a) in the machine
b) in the armature slots
c) around the poles
d) separately
ans. c

33. A starter is necessary to start a d.c. motor because
a) it starts the motor.
b) it limits the speed of the motor.
c) it limits the back emf to a safe value.
d) it limits the starting current to a safe value.
ans. d

34. The efficiency of any electrical machine will be high , if
a) its losses are minimum.
b) its losses are more.
c) voltage applied is constant.
d) the current drawn is minimum.
ans. a

35. The direction of rotation of a d.c. shunt motor is reversed by
a) reversing armature connections.
b) interchanging the armature and field connection.
c) adding resistance to the field circuit.
d) reversing supply connections.
ans. a

36. The armature of d.c.motors is laminated
a) to reduce the hysteresis losses.
b) to reduce the eddy current losses.
c) to reduce the inductivity of armature.
d) to reduce the mass of the armature.
ans. b

37. Which of the following is a correct statement about a series motor?
a) Its field winding consists of thicker wire and less turns.
b) It can run easily without load.
c) It has almost constant speed.
d) It has poor torque.
ans. a

38. Which of the following d.c. motors has the least drop in the speed between no load to nominal load?
a) Shunt motor with commutating poles.
b) Series motor without commutating poles.
c) Compound motor without commutating poles.
d) Series motor with commutating poles.
ans. a

39. What happens if the field winding of a running shunt motor suddenly breaks open?
a) Its speed slows down
b) Its speed becomes dangerously high.
c) It gives out sparks.
d) It stops at once.
ans. b

40. The speed of a series motor at no load is
a) zero
b) 3000 rpm.
c) 3600 rpm.
d) infinity.
ans. d

DC MACHINES 3

21. The function of a commutator in a d.c. generator is
a) to collect current from conductors.
b) to change d.c. to a.c.
c) to conduct the current to the brushes.
d) to change a.c. to d.c.
ans. d

22. The current flowing in the conductors of a d.c. motor is
a) a.c.
b) d.c.
c) a.c as well as d.c.
d) transients.
ans. a

23. As the load is increased the speed of a d.c. shunt motor will
a) remain constant.
b) increase proportionately.
c) increase slightly.
d) reduce slightly.
ans. d

24. If the current in the armature of d.c. series motor is reduced to 5% , the torque of the motor will become
a) 50% of the previous value.
b) 25% of the previous value.
c) 150% of the previous value.
d) 125% of the previous value.
ans. b

25. In d.c. shunt motor, if the terminal voltage is reduced to half and torque remains the same. then,
a) speed will be half and armature current also will be half.
b) speed will be half but armature current remains the same.
c) speed will be half and armature current becomes double.
d) speed and armature current will remain the same.
ans. c

26. One d.c.motor drives another d.c.motor . The second d.c. motor when excited and driven,it
a) runs
b) does not run as generator.
c) runs as a generator
d) also runs as a motor.
ans. c

27. A 4-pole lap-wound armature has 480 conductors and a flux per pole of 25 m Wb. The emf generated, when running at 600 rpm, will be
a) 240 V
b) 120 V
c) 60 V
d) 30 V
ans. b

28. The torque of a motor is
a) force is N-m acting on the rotor.
b) the product of tangential force on the motor and its radius.
c) the electrical power in kW.
d) the power given to load being driven by the motor
ans. b

29. The armature current drawn by any d.c. motor is proportional to
a) the voltage applied to the motor.
b) the torque required .
c) the flux required in the motor.
d) the speed of the motor.
ans.

30. The output power of any electrical motor is taken from
a) the armature
b) the coupling mounted on the shaft
c) the conductors.
d) the poles.
ans. b

DC MACHINES 2

11.  In a d.c. motor the iron losses occur in
a) the yoke
b) the armature
c) the field.
d) None of these.
ans. a

12. The dummy coil in d.c. machines used to
a) eliminate reactance voltage.
b) eliminate armature reaction.
c) bring about mechanical balance of armature.
d) eliminate harmonics developed in the machine.
ans. c

13. A simple method of increasing the voltage of an d.c. generator is
a) to decrease the air gap flux density
b) to increase the speed of rotation.
c) to decrease the speed of rotation.
d) to increase the length of the armature.
ans. b

14. The function of starter in a d.c. machine is
a) to avoid the excessive current at starting
b) to control the speed.
c) to avoid armature reaction.
d) to increase the length of the armature.
ans. b

15. An external resistance is added in the series with the field of a d.c. shunt motor. When the motor runs, the effect of resistance is
a) to reduce the speed of the motor
b) to increase the speed of the motor.
c) to reduce the armature current drawn by the motor
d) to reduce the losses.
ans. b

16. An external resistance is added in the field of a d.c. shunt generator will
a) increase the speed of the generator.
b) increase the voltage of the generator.
c) decrease the voltage of the generator.
d) increase the power delivered.
ans. c

17. The speed of a d.c. shunt motor is required to be more than F.L. speed. This is possible by
a) increasing the armature current.
b) decreasing the armature current.
c) increasing the excitation current.
d) reducing the field current.
ans. d

18. An emf is induced in the windings of an armature of a d.c. generator when the armature rotates in
a) alternating magnetic flux.
b) magnetic field.
c) electrostatic field.
d) electromagnetic flux.
ans. b

19. If the speed of  d.c. generator is increased, the generated emf
a) increases
b) decreases
c) remains constant
d) decreases and then increases.
ans. a

20. If the speed of a d.c. shunt motor increases, the back emf
a) increases
b) decreases
c) remains constant
d) first decreases and then increases.
ans. a

DC MACHINES

1. What is the effect produced by the electric current in an electric motor?
a) magnetic effect only.
b) magnetic as well as heating effect.
c) heating effect only.
d) heating as well as chemical effect.
ans. b

2. The rotating part of a d.c. motors is known as
a) pole
b) stator
c) armature
d) carbon brush.
ans. c

3. Why is the airgap between the yoke and armature of an electric motor kept smaller?
a) To achieve a stronger magnetic field.
b) To avoid overheating of the machine.
c) To make station easier.
d) None of these.
ans. a

4. The function of the commutator in a d.c. machine is
a) to change alternating current to direct current.
b) to improve commutation.
c) for easy speed control.
d) to change alternating voltage to direct voltage.
ans. d

5. Carbon brushes are used in electric motors to
a) brush off carbon deposits on the commutator.
b) provide a path for flow of current.
c) prevent overheating of armature windings.
d) prevent sparking during commutation.
ans. b

6. D.C. shunt generator has
a) slightly drooping characteristics.
b) appreciably rising characteristics.
c) constant voltage characteristics.
d) appreciably falling characteristics.
ans. a

7. which one of the following is used for charging batteries?
a) compound generator.
b) shunt generator.
c) series generator.
d) tacho generator.
ans. b

8. The speed of a d.c. motor is
a) directly proportional to back emf and inversely proportional to flux.
b) inversely proportional to back emf and directly proportional to flux.
c) directly proportional to emf as well as flux.
d) inversely proportional to emf as well as flux.
ans. a

9. In any d.c. generator the emf generated in the armature is maximum when
a) rate of change of flux linked is minimum.
b) rate of change of flux linked is maximum.
c) flux linked with conductor is maximum.
d) flux linked with conductor is minimum.
ans. b

10. The highest speed attained by a d.c. shunt motor is
a) equal to infinity at rated flux.
b) higher than no load speed at rated flux.
c) equal to no load speed at rated flux.
d) lower than no load speed at rated flux.
ans. c