## electrical

### DC Motor MCQ Objective Questions and Answers Page - 3

1.
 In an electric motor , the electromagnetic torque developed is A In opposite direction to both load torque and frictional torque B In the same direction as the load torque but in opposite direction to frictional torque C In the same direction is the load torque and frictional torque D In the same directional as the frictional torque but in the opposite direction to the load torque Answer : A Explanation :
2.
 The torque speed characteristic of a dc shunt motor is A A rectangular hyperbola B A drooping straight line C A parabola D None of the above Answer : B Explanation :
3.
 The horse power obtained from the motor shaft is called the A 1 HP B BHP C Useful output D None of these Answer : B Explanation :
4.
 In a dc motor , the shaft torque is less than armature torque . This is due to A Eddy current loss B Hysteresis loss C Stray loss D All of the above Answer : C Explanation :
5.
 The mechanical output available  at the dc motor shaft is equal to A Electrical power input to motor B Electrical power input less iron and friction losses C Electrical power input less copper losses D Electrical power input less total losses (copper loss, iron loss, friction and windage losses) Answer : D Explanation :
6.
 If Nis the speed of a dc series motor and Ia is the armature current then A
7.
 For small values of armature current Ia, the torque developed by a dc series motor is proportional to A $I_{a}$ B ${I_{a}}^{2}$ C $\sqrt{I_{a}}$ D $\frac{1}{I_{a}}$ Answer : B Explanation :
8.
 For higher values of armature current Ia, the torque developed by a dc series motor is proportional to A $I_{a}$ B ${I_{a}}^{2}$ C $\sqrt{I_{a}}$ D $\frac{1}{I_{a}}$ Answer : A Explanation :
9.
 For higher value of armature current, the speed of a dc series motor is A Proportional to armature current B Proportional to the square of armature current C Inversely proportional to armature current D Constant Answer : D Explanation :
10.
 T and P are the developed electromagnetic torque and the shaft power of a dc series motor. If losses are neglected, then for constant terminal voltage A $T\alpha&space;P$ B
11.
 In comparison to shunt and compound motors a dc series motor develops high torque at start. It is due to its comparatively A Few series turns B Stronger series field C Larger armature current D Smaller armature resistance Answer : B Explanation :
12.
 A fly wheel is normally fitted to A Dc series motor driving a constant torque load B Separately excited dc motor driving pulsed torque load C Cumulatively compound motor driving torque load D Differentially compound motor driving pulsed torque load Answer : C Explanation :
13.
 An electric motor with “constant output power ” will have a torque speed characteristic in the form of a A Straight line through the origin B Straight line parallel to speed axis C Circle about the origin D Rectangular hyperbola Answer : D Explanation :
14.
 T is the load torque of a dc series motor having linear magnetization and negligible armature resistance. Speed of the motor is A Inversely proportional to
15.
 An electric train employing a dc series motor is running at a fixed speed, when a sudden slight drop in the mains voltage occurs. This would result in A Drop in speed and rise in current B Rise in speed and drop in current C Rise in speed and rise in current D Drop in speed with current unaltered Answer : A Explanation :
16.
 In a dc series motor, if armature current is reduced to 70 % of its original value , the torque of the motor will be reduced by A 49% B 51% C 50% D 70% Answer : B Explanation :
17.
 When the torque of a dc series motor is tripled the power approximately increases by A 33% B 50% C 75% D 150% Answer : C Explanation :
18.
 When an electric train is moving down a hill, the dc motor will operate is a dc A Series motor B Series generator C Shunt motor D Shunt generator Answer : B Explanation :
19.
 A dc series motor must not be started on no load because it would A Not develop starting torque B Fail to start without load C Run at dangerously high speed D Draw more current without producing sufficient torque Answer : C Explanation :
20.
 In case of a dc series motor it is possible to have finite no load speed if a resistance is connected across its A Field terminals B Armature terminal C Field and armature together D It is always very high at no load Answer : B Explanation :
21.
 If an additional resistance is connected in series with a dc series motor operating at rated speed of the motor will A Reduce B Increase C Remain unaltered D Oscillate around the normal speed Answer : A Explanation :
22.
 If a resistance is connected across the armature of a dc series motor , it speed for a constant load torque will A Increase B Decrease C Remain unaltered D Become zero Answer : B Explanation :
23.
 Field diverter method of speed control of a dc series motor gives speeds above rated one due to reduction of A Field current B Armature current C Line current D None of these Answer : A Explanation :
24.
 The  field diverter resistance for a dc series motor is of the order of A 0.1 Ω B 10 Ω C 50 Ω D 100Ω Answer : A Explanation :
25.
 The current drawn by a dc motor at starting is A Zero B Very low C Normal D Very high Answer : D Explanation :
26.
 High inrush of current during a starting of a dc motor can  efficiently be restricted to a safer value by increasing the ……..resistance A Shunt field B Armature circuit C Both a and b D None of these Answer : B Explanation :
27.
 The dc motor draws high current at the starting instant due to absence of back emf . In order to prevent this high inrush of current at start, a resistance is connected A Across the field B Across the armature C In series with the armature D In series with the filed Answer : C Explanation :
28.
 For efficient operation of a dc motor, the additional resistance inserted in he armature circuit should be A Increased in steps B Cut out in steps C Left in the armature circuit D Removed immediately after start Answer : B Explanation :
29.
 Three point starters are not suitable for application where speed variation by flux control is required because the motor may A Not start B Run away C Stop at very high speed D Stop at very slow speed Answer : C Explanation :
30.
 If the no –voltage release in a dc motor starter fails to operate on failure of supply, on resumption of supply the motor will A Not start automatically B Start automatically without trouble C Develop very low starting torque D Get damaged due to flow of excessive current in the armature Answer : D Explanation :
31.
 In a D.C series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to A 100% of the previous value B 50% of the previous value C 10% of the previous value D None of the above Answer : C Explanation :
32.
 The current drawn by the armature of D.C motor is directly proportional to the A The torque required B The speed of the motor C The voltage across the terminals D None of the above Answer : A Explanation :
33.
 The power mentioned on the name plate of an electric motor indicates A The power drawn in kW B The power drawn in kVA C The gross power D The output power available at the shaft Answer : D Explanation :
34.
 In Ward Leonard method of speed control of a D.C motor, change in speed of the motor is obtained by the A Change in armature voltage of D.C motor B Change in field excitation of the D.C motor C Change in supply voltage D All of the above Answer : A Explanation :
35.
 Which D.C motor has got maximum self-loading property? A Series motor B Shunt motor C Cumulatively compounded motor D Differentially compounded motor Answer : D Explanation :
36.
 Which D.C motor will be suitable along with flywheel for intermittent light and heavy loads? A Series motor B Shunt motor C Cumulatively compounded motor D Differentially compounded motor Answer : C Explanation :
37.
 If a D.C shunt motor is working at no load and if shunt field circuit suddenly opens A Nothing will happen to the motor B This will make armature to take heavy current, possibly burning it C This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses D Motor will run at very slow speed Answer : C Explanation :
38.
 D.C series motors are used A Where load is constant frequently B Where load changes frequently C Where constant operating speed is needed D In none of the above situations Answer : D Explanation :
39.
 For the same H.P rating and full load speed. Following motor has poor starting torque A Shunt B Series C Differentially compounded D Cumulatively compounded Answer : C Explanation :
40.
 Sparking at the commutator of a D.C motor may result in A Damage to commutator segments B Damage to commutator insulations C Increased power consumption D All of the above Answer : D Explanation :
41.
 Which of the following motor is preferred for operation in highly explosive atmosphere? A Series motor B Shunt motor C Air motor D Battery operated motor Answer : C Explanation :
42.
 If the supply voltage for a D.C motor is increased, which of the following will decrease? A Starting torque B Operating torque C Full-load current D All of the above Answer : C Explanation :
43.
 When the speed of a D.C motor increases A Back e.m.f increases and current drawn decreases B Back e.m.f as well as current drawn both increases C Back e.m.f as well as current drawn both decrease D Back e.m.f decreases and current drawn increase Answer : A Explanation :
44.
 As compared to an induction motor, the air gap in a D.C motor is A Less than 50% B Between 505 and 80% C Same D More Answer : D Explanation :
45.
 Field winding of a D.C series motor is usually provided with track wire A To provide large flux B To reduce the use of insulating materials C As it carries large load current D In order to reduce eddy current Answer : C Explanation :
46.
 Which one of the following is not the function of pole shoes in a D.C machine? A To reduce eddy current loss B To support the field coil C To spread out flux for better uniformity D To reduce the reluctance of the magnetic path Answer : A Explanation :
47.
 The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f to applied voltage is A 4.0 B 2.0 C 1.0 D 0.5 Answer : D Explanation :
48.
 The condition for maximum power in case of D.C motor is A Back e.m.f = 2 x supply voltage B Back e.m.f = ½ x supply voltage C Supply volateg = ½ x back e.m.f D Supply voltage = back e.m.f Answer : B Explanation :
49.
 For which of the following applications a D.C motor is preferred over an A.C motor? A Low speed operation B High speed operation C Variable speed operation D Fixed speed operation Answer : C Explanation :
50.
 In D.C machines the residual magnetism is of the order of A 2 to 3 percent B 10 to 15 percent C 20 to 25 percent D 50 to 75 percent Answer : A Explanation :