DC Motors PDF

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DC Motors 19. EE Board Exam April 1992 The DC motor armature whose speed is to be controlled by Ward-Leonard system is fed by A. 3-phase motor B. DC generator C. Rectifier D. DC supply in parallel with the rheostat 20. REE Board Exam September 2000 What dc motor over-speeds at light loads? A. series motor B. shunt motor C. cumulative compound generator D. differential compound generator 21. REE Board Exam April 1997 DC motor starters are used for the following functions EXCEPT one. Which one is this? A. control the speed of the motor B. start large DC motors C. limit the starting current of the motor D. prevent large voltage drop during starting 22. REE Board Exam October 1997 If you try to start a DC compound motor the shunt field of which is open, what would happen? A. it will overspeed B. the direction of rotation will reverse C. the fuse will blow D. it will not start 23. EE Board Exam October 1994 The speed of a series motor at no load is A. 3,600 rpm B. infinity C. 300 rpm D. zero

24. EE Board Exam April 1994 The armature of a dc shunt motor is laminated to reduce the A. inductivity of the armature B. hysteresis loss C. mass of the armature D. eddy-current loss 25. EE Board Exam April 1994 A dc series motor is started without load the effect is a rapid increase in A. current drawn B. its emf C. speed D. torque 26. EE Board Exam April 1994 The rotation of a direct current motor may be reversed by A. reversing the line connection to the motor B. reversing either the armature or field C. reversing both the armature and field connection D. shifting the brushes opposite to the direction in which the motor is rotating 27. ECE Board Exam April 2000 It is a motor generator unit which is built to convert low voltage direct current to high voltage current both uses the same field windings and share armature. A. Vibrator B. Dynamotor C. Gen-set D. Electric generator 28. ECE Board Exam November 1998 _____ is called the electrical prime mover. A. Motor B. Armature C. Commutator D. Generator 29. ECE Board Exam April 2000

In D.C motors, type of armature consisting of coils actually wound around the armature core so that all coil surfaces are exposed to the magnetic field. A. Drum wound armature B. Compensating windings C. Gramme-ring armature D. Interpoles 30. ECE Board Exam November 1999 Type of motor where the field windings are connected in series with armature coil and the field strength varies with changes in armature current. A. Series motor B. Load motor C. Shunt motor D. Compound motor 31. ECE Board Exam November 1998 The speed of a dc-shunt motor can be increased under which of the conditions below. A. Increased field voltage B. Decreased armature voltage C. Increased armature voltage D. Increased armature resistance 32. ECE Board Exam April 1998 The speed of a dc shunt motor can be slowed down under which one of the conditions below. A. Increased armature resistance B. Decreased field voltage C. Increased field voltage D. Increased armature resistance 33. ECE Board Exam November 1999 The source of mechanical power to turn the rotors of alternators, large and small AC and DC A. Primer B. Prime mover C. Prime user D. Prime contractor

34. ECE Board Exam November 1997 It refers to a form of DC motor generator with common field coil for both motor and generator. A. Motor generator B. Dyna motor C. Polyphase motor D. Synchronous motor 35. ECE Board Exam November 2001 The power and the torque of an electric motor linked by _____. A. Load B. Shaft C. Speed D. Size 37. In a d.c. motor, unidirectional torque is produced with the help of A. brushes B. commutator C. end-plates D. both A and B 38. The counter e.m.f. of a d.c. motor A. often exceeds the supply voltage B. aids the applied voltage C. helps in energy conversion D. regulates its armature voltage 39. The normal value of the armature resistance of a d.c. motor is A. 0.005 ohm B. 0.5 ohm C. 10 ohms D. 100 ohms 40. The Eb/V ratio of a d.c. motor is an indication of its A. efficiency B. speed regulation C. starting torque D. running torque 41. The mechanical power developed by the armature of a d.c. motor is equal to

A. armature current multiplied by back e.m.f. B. power input minus losses C. power output multiplied by efficiency D. power output plus iron losses 42. The induced e.m.f. in the armature conductors of a d.c. motor is A. sinusoidal B. trapezoidal C. rectangular D. alternating 43. A d.c. motor can be looked upon as d.c. generator with the power flow A. reduced B. reversed C. increased D. modified 44. In a d.c. motor, the mechanical output power actually comes from A. field system B. air-gap flux C. back e.m.f. D. electrical input power 45. The maximum torque of d.c. motors is limited by A. commutation B. heating C. speed D. armature current 46. Which of the following quantity maintains the same direction whether a d.c. motor runs as generator or as a motor? A. induced e.m.f. B. armature current C. field current D. supply current 47. Under constant load conditions, the speed of a d.c. motor is affected by A. field flux B. armature current

C. back e.m.f. D. both B and C 48. It is possible to increase the field flux and, at the same time, increase the speed of a d.c. motor provided its _____ is held constant. A. applied voltage B. torque C. armature circuit resistance D. armature current 50. The shaft torque of a d.c. motor is less than its armature torque because of ______ losses. A. copper B. mechanical C. iron D. rotational 53. If load on a d.c. shunt motor is increased, its speed is decreased due primarily to A. increase in its flux B. decrease in back e.m.f. C. increase in armature current D. increase in brush drop 54. If the load current and flux of a d.c. motor are held constant and voltage applied across its armature is increased by 10 %, its speed will A. decrease by about 10% B. remain unchanged C. increase by about 10% D. increase by 20% 55. If the pole flux of a d.c. motor approaches zero, its speed will A. approach zero B. approach infinity C. no change due to corresponding change in back e.m.f. D. approach a stable value somewhere between zero and infinity 56. If the field current of a loaded shunt motor is suddenly opened A. it would race to almost infinite speed

B. it would draw abnormally high armature current C. circuit breaker or fuse will open the circuit before too much damage is done to the motor D. torque developed by the motor would be reduced to zero

62. The Ta/Ia graph of a d.c. series motor is a A. parabola from no-load to overload B. straight line throughout C. parabola throughout D. parabola up to full-load and a straight line at overloads

57. Which of the following d.c. motor would be suitable for drives requiring high starting torque but only fairly constant speed such as crushers? A. shunt B. series C. compound D. permanent magnet

63. As compared to shunt and compound motors, series motor has the highest torque because of its comparatively ____ at the start. A. lower armature resistance B. stronger series field C. fewer series turns D. larger armature current

58. A d.c. shunt motor is found suitable to drive fans because they require A. small torque at start up B. large torque at high speeds C. practically constant voltage D. both A and B

64. Unlike shunt motor, it is difficult for a series motor to stall under heavy loading because A. it develops high overload torque B. its flux remains constant C. it slows down considerably D. its back e.m.f is reduced to almost zero

59. Which of the following load would be best driven by a d.c. compound motor? A. reciprocating pump B. centrifugal pump C. electric locomotive D. fan 60. As the load is increased, the speed of a d.c. shunt motor A. increases proportionately B. remains constant C. increase slightly D. reduces slightly 61. Between no-load and full-load, ____ motor develops the least torque A. series B. shunt C. cumulative compound D. differential compound

65. When load is removed, _____ the motor will run at the highest speed. A. shunt B. cumulative compound C. differential compound D. series 66. A series motor is best suited for driving A. lathes B. cranes and hoists C. shears and punches D. machine tools 68. The d.c. series motor should never be switched on at no load because A. the field current is zero B. the machine does not pickup C. the speed becomes dangerously high D. it will take too long to accelerate

69. A shunt d.c. motor works on a.c. mains A. unsatisfactory B. satisfactorily C. none at all D. none of these 70. A 200 V, 10 A motor could be rewound for 100 V, 20 A by using ____ as many turns per coil of water having ____ cross-sectional area. A. twice, half B. thrice, one-third C. half, twice D. four times, one-fourth 71. The speed of a d.c. motor can be controlled by varying A. its flux per pole B. resistance of armature circuit C. applied voltage D. all of these 72. The most efficient method of increasing the speed of a 3.75 kW d.c. shunt motor would be the ____ method. A. armature control B. flux control C. Ward-Leonard D. tapped field control 73. Regarding Ward-Leonard system of speed control which statement is false? A. it is usually used where wide ad very sensitive speed control is required B. it is used for motors having ratings from 750 kW to 4000 kW C. capital outlay involved in the system is right since it is uses two extra machine D. it has low overall efficiency especially at light loads 74. In the rheostatic method of speed control for a d.c. shunt motor, used of armature diverter makes the method A. less wasteful B. unsuitable for changing loads

C. less expensive D. suitable for rapidly changing loads 75. The chief advantage of WardLeonard system of d.c. motor speed is that it A. can be used for small motors B. has high overall efficiency at all speed C. gives smooth, sensitive and wide speed control D. uses a flywheel to reduce fluctuations in power demand 78. In practice, regenerative braking is used when A. quick motor reversal is desired B. load has overhauling characteristics C. controlling elevators, rolling mills and printing presses D. other method cannot be used 79. Statement 1. A direct-on-line (DOL) starter is used to start a small d.c. motor because Statement 2. it limits initial current drawn by the armature circuit. A. both statements 1 and 2 are incorrect B. both statements 1 and 2 are correct C. statement 1 is correct but 2 is wrong D. statement 2 is correct but 1 is wrong 80. Ward-Leonard system of speed control is NOT recommended for A. wide speed range B. constant speed operation C. frequent motor reversals D. very low speeds 81. Thyristor chopper circuits are employed for A. lowering the level of a d.c. voltage B. rectifying the a.c. voltage C. frequency conversion D. providing commutation circuitry

82. An inverter circuit is employed to convert A. a.c. voltage into d.c. voltage B. d.c. voltage into a.c. voltage C. high frequency into law frequency D. low frequency into high frequency 83. The phase control rectifiers used for speed of d.c. motors to convert fixed a.c. supply of voltage into A. variable d.c. supply voltage B. variable a.c. supply voltage C. full-rectified a.c voltage D. half-rectified a.c. voltage 84. If some of the switching devices in a converter are controlled devices and some are diodes, the converter is called A. full converter B. semi converter C. solid-state copper D. d.c. converter 85. A solid-state chopper converts a fixed-voltage d.c. supply into a A. variable-voltage a.c. supply B. variable-voltage d.c. supply C. higher-voltage d.c. supply D. lower-voltage a.c. supply 86. The d.c. motor terminal voltage supplied by a solid-state chopper for speed control purposes varies with the duty ratio of the chopper A. inversely B. linearly C. indirectly D. parabolically 87. One of the main advantages of Swinburne’s test is that it A. is applicable both to shunt and compound motors B. needs one running test C. is very economical and convenient D. ignores any change in iron loss

88. The main disadvantage of Hopkinson’s test for finding efficiency of shunt d.c. motors is that it A. requires full load power B. ignores any change in loss in iron loss C. needs one motor and one generator D. requires two identical shunt machines 89. The most economical method of finding noload losses of a large d.c. shunt motor is A. Hopkinson’s B. Swinburne’s C. retardation D. Field’s 90. Retardation test on a dc shunt motor is used for finding ____ losses. A. stray B. copper C. friction D. iron 91. The main thing common between Hopkinson’s test and field’s test is that both A. require two electrically-coupled series motors B. need two similar mechanicallycoupled motors C. use negligible power D. are regenerative test 92. The usual test for determining the efficiency of a traction number is the _____ test. A. Field’s B. retardation C. Hopkinson’s D. Swinburne’s 93. A dc motor is used to A. generate power B. change mechanical energy to electrical energy

C. change electrical energy to mechanical energy D. increase energy put into it 94. A dc motor is still used in industrial application because it A. is cheap B. is simple in construction C. provides fine speed control D. none of these 95. Carbon brushes are preferable to copper brushes because A. they have longer life B. they reduce armature reaction C. they have lower resistance D. they reduce sparking 96. The field poles and armature of a dc machine are laminated to A. reduce the weight of the machine B. decrease the speed C. reduce eddy currents D. reduce armature reaction 97. The back emf in a dc motor A. opposes the applied voltage B. aids the applied voltage C. aids the armature current D. none of these 98. The value of back emf (Eb) in a dc motor is maximum at A. no load B. full load C. half full-load D. none of these 99. The motor equation is given by A. V = Eb – IaRa B. V = Eb + IaRa C. Eb = IaRa – V D. none of these 102. The mechanical power developed in a dc motor is maximum when back emf (Eb) is equal to ____ the applied voltage (V).

A. B. C. D.

twice half one-third none of these

103. When the speed of a dc shunt motor increases, its armature current A. increases B. decreases C. remains constant D. none of these 104. The amount of back emf of a shunt motor will increase when A. the load is increased B. the field is weakened C. the field is strengthened D. none of these 105. The speed of a dc motor is A. directly proportional to flux per pole B. inversely proportional to flux per pole C. inversely proportional to applied voltage D. none of these 106. The torque developed by a dc motor is directly proportional to A. flux per pole x armature current B. armature resistance x applied voltage C. armature resistance x armature current D. none of these 107. The shaft torque (Tsh) in a dc motor is less than total armature torque (Ta) because of A. Cu losses B. field losses C. iron and friction losses D. none of these 108. Armature reaction in a dc motor is increased A. when the armature current increases

B. when the armature current decreases C. when the field current decreases D. by interpoles 109. With respect to the direction of rotation, interpoles on a dc motor must have the same polarity as the main poles A. ahead of them B. behind them C. along them C. none of these 110. In a dc motor, the brushes are shifted from the mechanical neutral plane in direction opposite to the rotation to A. decrease speed B. increase speed C. reduce sparking D. produce flat characteristics 111. In very large dc motors with severe heavy duty, armature reaction effects are corrected by A. using interpoles only B. using compensatory windings in addition to interpoles C. shifting the brush position D. none of these 112. The speed of a ____ motor is practically constant. A. cummulatively compounded B. series C. differentially compounded D. shunt 113. ____ motor is a variable speed motor. A. series B. shunt C. cummulatively compounded D. differentially compounded

114. The most commonly used method of speed control of a dc motor is by varying A. voltage applied to the motor B. field strength C. effective number of conductors in series D. armature circuit resistance 115. The running speed of a dc series motor is basically determined by A. field excitation B. load C. armature resistance D. none of these 116. After a shunt motor is up to speed, the speed may be increased considerably by A. increasing field circuit resistance B. decreasing field circuit resistance C. increasing armature circuit resistance D. reducing the load 117. ____ motor has the best speed regulation. A. series B. cummulatively compounded C. shunt motor D. differentially compounded 118. The deciding factor in the selection of a dc motor for a particular application is its ______ characteristics. A. speed-torque B. speed-armature current C. torque-armature current D. none of these 119. The demand for a large increase in torque of a dc shunt motor is met by a A. large decrease in speed B. large increase in speed C. large increase in current D. small increase in current

120. The demand for a large increase in torque of a dc series motor is met by a A. large decrease in current B. large decrease in speed C. large increase in speed D. small decrease in speed 121. A series motor will over-speed when A. the load is increased B. the field is opened C. the armature circuit is opened D. load is removed 122. As the load increases, a ____ motor will speed up. A. series B. shunt C. cummulatively compounded D. differentially compounded 123. A cummulatively compounded motor does not run at dangerous speed at light loads because of the presence of A. shunt winding B. series winding C. interpoles D. compensating winding 124. DC shunt motors are used in applications where ____ is required. A. high starting torque B. practically constant speed C. high no-load speed D. variable speed 125. DC series motors are used in applications where ______ is required. A. high starting torque B. constant speed C. low no-load speed D. none of these 126. Cummulatively compounded motors are used where we require A. variable speed B. poor speed regulation

C. sudden heavy loads for short duration D. none of these 127. For the same rating, ____ motor has the highest starting torque. A. shunt B. cummulatively compounded C. differentially compounded D. series 128. _____ motors never use beltconnected loads. A. series B. shunt C. cummulatively compounded D. differentially compounded 129. For 20% increase in current, the motor that will give the greatest increase in torque is ____ motor. A. shunt B. series C. cummulatively compounded D. differentially compounded 130. The ____ motor is used to start heavy loads. A. series B. shunt C. differentially compounded D. none of these 131. When load is removed, the motor that will run at the highest speed is A. shunt B. series C. cummulatively compounded D. differentially compounded 132. For the same rating, ____ motor has the least starting torque. A. cummulatively compounded B. series C. shunt D. none of these

133. A dc series motor is most suitable for A. cranes B. pump C. lathes D. punch presses 134. ____ is most suitable for punch presses. A. shunt B. series C. differentially compounded D. cummulatively compounded 135. In a vacuum cleaner, we generally used ____ motor. A. shunt B. cummulatively compounded C. series D. differentially compounded 136. The most suitable motor for elevators is the ____ motor. A. series B. shunt C. differentially compounded D. cummulatively compounded 137. The friction and windage losses in a dc motor depends upon A. speed B. flux C. armature current D. field and armature resistance 138. The iron losses in a dc motor depend upon A. flux only B. speed only C. both speed and flux D. none of these 139. The greatest percentage of power loss in a dc motor is due to A. windage loss B. copper loss C. core loss

D. friction loss 140. A motor takes a large current at starting because A. the armature resistance is high B. back emf is low C. shunt field is producing weak field D. none of these 141. Excessive sparking at the brushes may be cause ...