electrical

DC Motor MCQ Objective Questions and Answers Page - 2

1.
 if the load current and flux of a dc motor are held constant and voltage applied across it  armature is increased by 54% , the speed of the motor will A increase by 5% B reduce by 5% C remain un-alternated D depends on other factors Answer : A Explanation :
2.
 if the flux of a dc motor approaches zero,  its speed will A approach infinity B approach zero C remain unchanged D between 0 and ∞ Answer : A Explanation :
3.
 A series motor is working drawing a load current of 1A from the line if now the load is reduced such that the current drawn is halved, the speed of the machine (neglecting the saturation and armatu A unchanged B reduced by 50% C reduced by 100% D increased by 100% Answer : D Explanation :
4.
 A dc shunt motor runs at rated speed. If its field circuit gets open circuited, then soon after this the motor speed would tend to A Decrease B Remain unchanged C Increase D Fluctuate around its previous speed Answer : C Explanation :
5.
 The speed of a dc shunt motor ____from no load to full load A Falls slightly B Improves slightly C Remains unchanged D Falls rapidly Answer : A Explanation :
6.
 A dc shunt generator has an armature resistance of 0.1 and runs at 1200 rpm when delivering 50 kW at 250 V. The speed of the machine, when run as a motor fed from 250 V supply and drawing 50 kW wil A 1410 rpm B 1020 rpm C 1100 rpm D 1320 rpm Answer : B Explanation :
7.
 The speed regulation of a dc motor is given as A
8.
 A dc motor having full load speed of 750 rpm and speed regulation of 10% will have no load speed of A 825 rpm B 675 rpm C 800 rpm D 700 rpm Answer : A Explanation :
9.
 When  is the mechanical power developed by a dc motor maximum ? A Back emf is equal to applied voltage B Back emf is equal to zero C Back emf is equal to half the applied voltage D None of the above Answer : C Explanation :
10.
 If the applied voltage to a dc machine is 230 V , then the back emf, for maximum power developed is A 115V B 200V C 230V D 460 V Answer : A Explanation :
11.
 The efficiency of a dc motor when developing maximum power will be about A 100% B 50% C Less than 50% D More than 50% Answer : C Explanation :
12.
 A dc motor develop the maximum mechanical power when the armature current is such that A The back emf is equal to ¾ the of the applied emf B The back emf is equal to one half of the applied emf C The back emf is equal to 1/4th of the applied emf D The back emf is equal to 2/3 rd of the applied emf. Answer : B Explanation :
13.
 A 400 V shunt motor takes 5A at no load Ra=0.5 , Rf=200Ω what is the ratio of speed from full load to no load when the dc shunt mtoro takes 50 A on full load ? A 0.94 B 0.8 C 0.6 D 0.4 Answer : A Explanation :
14.
 The dc motor, which can provides zero speed regulation at full load without any controller is A Series B Shunt C Cumulative compound D Differential compound Answer : D Explanation :
15.
 A dc shunt motor having unsaturated magnetic circuit runs at 1,000rpm with rated voltage. If the applied voltage is half of the rated voltage, the motor  will run at A 2000 rpm B 1000 rpm C 750 rpm D 500 rpm Answer : B Explanation :
16.
 What happens to the speed when the flux is reduced by 10% in a 200 V dc shunt motor having an armature resistance of 0.2 ohm carrying a current of 50 A and running at 960 rpm prior to weakening of< A 1250 rpm B 1066 rpm C 920 rpm D 576 rpm Answer : B Explanation :
17.
 In a dc series motor, the torque developed in A Inversely proportional to armature current B Directly proportional to armature current C Proportional to the square of armature current D Proportional to the square root of the armature current Answer : C Explanation :
18.
 Torque developed by the armature of a dc motor is proportional to which one of the following A (Back emf) x (Armature current ) B (Magnetic flux per pole) x (Armature current ) C (Back emf) x (Magnetic flux per pole) D (Back emf) /  (Magnetic flux per pole) Answer : B Explanation :
19.
 A series motor drawing an armature current of  Ia is operating under saturated magnetic conditions The torque developed is the motor is proportional to A $\frac{1}{I_{a}}$ B $\dpi{100}&s C ${I_{a}}^{2}$ D $I_{a}$ Answer : D Explanation :
20.
 What is the increase in torque expressed as percentage of initial torque, if the current drawn by a dc series motor is increased from 10A to 12A (neglect saturation )  ? A 21% B 25% C 41% D 44% Answer : D Explanation :
21.
 ward Leonard control provides speed control A in one direction only B in both direction C above rated speed D below rated speed Answer : B Explanation :
22.
 For very sensitive and wide speed control, the preferable control methods is A Field control B Armature control C Multiple voltage control D Ward Leonard system Answer : D Explanation :
23.
 In Ward Leonard system, the lower limit of speed is imposed by A Armature resistance B Field resistance C Residual magnetism of the generator D None of the above Answer : C Explanation :
24.
 In Ward Leonard method of speed control, the direction of rotation of the motor is reversed usually by reversing the connections of the A Generator field terminals B Generator armature terminals C Motor armature terminals D Motor field terminals Answer : A Explanation :
25.
 The main disadvantages (s) of the Ward Leonard system is/ are A High initial cost B Poor overall efficiency C High maintenance cost D All of the above Answer : D Explanation :
26.
 For a given torque, reducing diverter resistance of a dc series motor A Increases its speed but armature current remains the same B Increases its speed demanding more armature current decreases its speed demanding  less arma C Decreases its speed but armature current remains the same D Decreases speed but armature current remains the same Answer : B Explanation :
27.
 A dc series motor is operating at rated speed with rated excitation it has two resistance R1 and R2 connected across the filed and the armature respectively. Speeds above the rated speed can be obt A Increasing R1 only B Reducing R2 only C Reducing R1 and increasing R2 D Increasing R1 and R2 Answer : C Explanation :
28.
 In series parallel control of dc series motor, the torque produced in series arrangement will be …………..of that in parallel arrangement A Double B Four times C Half D One fourth Answer : B Explanation :
29.
 The speed of a cumulatively compounded dc motor can be controlled by varying A Armature voltage B Armature resistance C Shunt field resistance D Any of the above Answer : D Explanation :
30.
 The function of the filed  regulator in a a dc compound motor is to A Limit the armature current B Vary the armature resistance C Control the field current D None of the above Answer : C Explanation :
31.
 A direct on line starter is used for starting motor A up to 5 H.P B up to 10 H.P C up to 15 H.P D up to 20 H.P Answer : A Explanation :
32.
 If the speed of D.C shunt motor is increased the back e.m.f of the motor will A decreases B increases C remain same D become zero Answer : B Explanation :
33.
 What will happen if the back e.m.f of A D.C motor vanishes suddenly? A the motor will stop B the motor will continue to run C the armature may burn D the motor will run noisy Answer : C Explanation :
34.
 In case of D.C shunt motors the speed is dependent on back e.mf only because A back e.m.f is equal to armature drop B armature drop is negligible C flux is proportional to armature current D flux is practically constant in D.C shunt motors Answer : D Explanation :
35.
 In a D.c shunt motor, under the conditions of maximum power, the current in the armature will be A almost negligible B rated full-load current C less than full-load current D more than full-load current Answer : D Explanation :
36.
 Which DC motor will have least percentage increase of input current for the same percentage increase in the torque? A shunt motor B series motor C cumulative compound motor D separately excited motor Answer : B Explanation :
37.
 These days D.C motors are widely used in A pumping sets B air compressors C electric traction D machine shops Answer : C Explanation :
38.
 By looking at which part of the motor, it can easily confirm that a particular motor is D.C motor? A Frame B Shaft C Commutator D Stator Answer : C Explanation :
39.
 In which of the following applications D.C series motor is invariable tried? A Starter for a car B Drive for a water pump C Fan motor D Motor operation in A.C or D.C Answer : A Explanation :
40.
 In D.C machines fractional pitch winding is used A To improve cooling B To reduce copper losses C To increase the generated e.m.f D To reduce the sparking Answer : D Explanation :
41.
 A three-point starter is considered suitable for A Shunt motor B Shunt as well as compound motors C Shunt, compound and series motors D All D.C motors Answer : B Explanation :
42.
 Small D.C motors up to 5 H.P usually have A 2 poles B 4 poles C 6 poles D 8 poles Answer : A Explanation :
43.
 In case the conditions for maximum power for a D.C motor are established, the efficiency of the motor will be A 100% B Around 90% C Anywhere between 75% and 90% D Less than 50 % Answer : D Explanation :
44.
 A shearing machine has cyclic load consisting of intermittent light and hennery loads. Which of the following D.C motor will be suitable for5 this purpose? A Series motor B Shunt motor C Cumulative compound motor D Differential compound motor Answer : C Explanation :
45.
 The ratio of starting torque to full load torque is least in case of A Series motors B Shunt motors C Compound motors D None of the above Answer : B Explanation :
46.
 In D.C motor which of the following can sustain the maximum temperature rise? A Slip rings B Commutator C Field winding D Armature winding Answer : C Explanation :
47.
 Which of the following law/rule can be used to determine the direction of rotation of D.C motor? A Lenz’s law B Faraday’s law C Coulomb’s law D Fleming’s left-hand rule Answer : D Explanation :
48.
 Which of the following load normally needs starting torque more than the rated torque? A Blowers B Conveyors C Air compressors D Centrifugal pumps Answer : B Explanation :
49.
 The starting resistance of a D.C motor is generally A Low B Around 500Ω C 1000Ω D Infinitely large Answer : A Explanation :
50.
 The speed of a D.C series motor is A Proportional to the armature current B Proportional to the square of the armature current C Proportional to the field current D Inversely proportional to the armature current Answer : D Explanation :