2. DC ELECTRICAL MACHINES
11. If the number of poles in a lap wounded generator is increased by a factor of 2, the generated emf will A. remain the same B. increase by a factor of 2 C. decrease by a factor of 2 D. increase by a factor of 4
A
13. If speed of a dc shunt motor increases, the back emf A. increases B. decreases C. remains constant D. decreases and then increases
A
14. Brushes are always placed along A. magnetic neutral axis (MNA) B. geometric neutral axis (GNA) C. perpendicular to MNA D. perpendicular to GNA
A
14. The current flowing in the conductors of a dc motor is A. ac B. dc C. ac as well as dc D. transients
A
19. In the commutation process it is the A. current which is getting reversed B. voltage which is getting reversed C. both of these D. none of these
A
2. A smooth output from a generator is obtained by A. design of the armature winding B. changing the form of the commutator C. changing the field strength D. design of the commutator
A
24. Interpoles are connected in_______with the armature and compensating windings are connected in ________ with the armature. A. series, series B. series, shunt C. shunt, series D. shunt, shunt
A
25. The power stated on the name plate of any motor is always A. the output power of the shaft B. the power drawn in kVA C. the power drawn in kW D. the gross power
A
27. The efficiency of any electrical machine/apparatus 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
A
27. Which of the following is most suitable for running in parallel operation? A. shunt generator B. series generator C. compound generator D. all of these are equally suitable
A
43. Which of the following helps in reducing the effect of armature reaction in dc generator? A. compensating windings B. interpoles C. both A and B D. neither A nor B
C
46. 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
C
47. Which of the following motors is suitable for high starting torque? A. shunt motor B. cumulative compound motor C. series motor D. compound motor
C
50. Interpoles are meant for A. increasing the speed of the motor B. decreasing the counter emf C. reducing sparking at the commutator D. converting armature current to dc
C
53. 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
C
55. 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. run faster than its rated speed
C
8.The highest speed attained by a dc shunt motor is A.equal to infinity of 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
C
9.For all symbols having their usual meaning, the emf generated in the lap winding is given by A. E = (φLN/60) x (P/2) B. E = φZNA/2P C. E = ZPφN/60a D. E = φZN/120 x (A/P)
C
1. The output voltage of a single loop generator is a A. steady d.c. B. steady a.c. C. pulsating a.c. D. pulsating d.c.
D
12. Electric dc generators have normally, an overall efficiency of the order of A. 60-80% B. 75-85% C. 80-90% D. 85-95%
D
12. The speed of a dc shunt motor is required to be more than F.L. speed. This is possible by A.increasing the armature B.decreasing the armature current C.increasing the excitation current D. reducing the field current
D
13. The armature magnetic field has effect that A. it demagnetizes or weakens the main flux B. it cross magnetizes, or distorts it C. it magnetizes or strengthens the main flux D. A and B
D
15. As the load is increased the speed of a dc shunt motor will A. remain constant B. increase proportionately C. increase slightly D. reduce slightly
D
3. The output voltage of a generator is given by the equation A. E = V - IaRa B. E = V + IaRa C. E = −L di/dt D. E = N dφ/dt
B
28. The direction of rotation of a dc 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 connection
A
30. Equalizer connections are required when paralleling two A. compound generators B. series generators C. shunt generators D. long shunt generators
A
30. 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 a load C. it has an almost constant speed D. it has a poor torque
A
31. The function of an interpole is A. neutralize cross-field of armature reaction and obtain ideal commutation B. increase the armature reaction and thus obtain ideal commutation C. neutralize cross field of the armature reaction and thus obtain under commutation D. increase the armature reaction and thus obtain over commutation
A
31. Which of the following dc motors has the least drop in speed between no load and 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
A
32. Interpoles are normally connected in A. parallel with field B. series with the field C. parallel with the load D. series with the load
A
34. DC generators preferred for charging automobile batteries is A. shunt generator B. series generator C. long shunt compound generator D. short shunt generator
A
34. Which of the following motors is used to derive the constant speed line shafting lathes, blowers and fans? A. DC shunt motor B. DC sreis motor C. cumulative compound motor D. differential compound motor
A
38. DC 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 torque. C. torque and speed is proportional to the square of armature current. D. none of these
A
39. Which loss in dc generator varies with load A. copper loss B. eddy current loss C. hysteresis loss D. windage loss
A
44. Armature reaction is attributed to A. the effect of magnetic field set-up by the armature current B. the effect of magnetic field set-up by the field current C. copper loss in the armature D. the effect of magnetic field set-up by back emf
A
44. With dc generator, which of the following regulation is preferred A. 1% regulation B. 50% regulation C. infinite regulation D. 100% regulation
A
49. Why is the air gap between stator and armature of an electric motor kept as small as possible? A.to get a stronger magnetic field B. to make the rotation easier C. to reach a higher speed of rotation D. to improve the air circulation
A
5. Voltage equation for dc motor is A. V = Eb + IaRa B. V = Eb - IaR a C. Eb = V + IaR a D. Eb = V - IaR a
A
54. 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. unity
A
7. The speed of a dc 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
A
7.What classification is given to a d.c. generator that receives its field excitation current from internal source? A. self excited B. controlled excitation C. separately excited D. internally excited
A
9.In a dc motor iron losses occur in A. the yoke B. the armature C. the field D. none of these
A
36. The field flux of a dc motor can be controlled to achieve A. the speeds lower than rated speed B. the speeds higher than rated speed C. the speeds at rated speed D. none of these
B
36. In dc generator the ripples in the direct emf and generated are reduced by A. using equalizer rings B. using commutator with large number of segments C. using carbon brushes of superior quality D. using carbon brushes of excellent quality
B
1.What is the effect produced by the electric motor? A.magnetic effect only B. magnetic as well as heating effect C. heating effect only D. heating as well as chemical effect
B
10. Residual magnetism is necessary in a A. separately excited generator B. self excited generator C. both of these D. none of these
B
11. An external resistance is added in the series with the field of a dc 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
B
15. Magnetic neutral axis makes an angle of ______with the flux passing through the armature. A. 0 deg B. 90 deg C. 45 deg D. 180 deg
B
17. If the current in the armature of a dc 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
B
17. If Өm is the forward lead angle in radians, Z is the total number of conductors and I is the current in each armature pole, the demagnetizing ampere-turns/pole are given as A. ATa = ZI x (θm/180) B. ATa = ZI x (θm/2π) C. ATa = ZI x (1/P - 2θm/3π) D. none of these
B
18. The shunt current flowing in short shunt compound-wound motor is given by A. Ish = V/ Rsh B. Ish = (V+ IscRsc)/Rsh C. Ish = IscRsc/Rsh D. none of these
B
20. In any dc 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 is maximum C. flux linked with conductor is maximum D. flux linked with conductor is minimum
B
21. A simple method of increasing the voltage of a dc 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
B
22. The torque of a motor is A. the force in N-m acting on the rotor B. the product of tangential force on the rotor and its radius C. the electrical power in kW D. the power given to load being driven by the motor
B
23. The armature current drawn by any dc 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
B
24. 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
B
28. Critical resistance of a dc generator R c A. Rc α (speed) ^ -1 B. Rc α (speed) C. Rc α (speed)^ 2 D. Rc α (speed)^ -2
B
29. The armature of dc motor 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
B
37. By providing a variable resistance across the series field (diverter) in a dc series motor, speeds above normal can be obtained because A. armature current decreases B. flux is reduced C. line current decreased D. none of these
B
37. Which of the following generators are preferred for parallel operations? A. series generator B. shunt generator C. compound generator D. all of the above
B
39. A series motor is started without load. The effect is that A. the torque increases rapidly B. the speed increase rapidly C. the current drawn increases rapidly D. the back emf decreases
B
4.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
B
40. 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
B
42. 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 the motor will decrease
B
43. Which of the following motors is used for rolling mills? A. DC shunt motor B. DC cumulative compound motor C. DC series motor D. DC differential compound motor
B
45. What will happen if the supply terminals of a 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
B
5. In most generators the output voltage is induced by A. rotating magnetic field past stationary coils B. relative motion between the field and the armature coils C. converting electrical energy into mechanical energy D. converting chemical energy into electrical energy
B
51. Which of the following methods is most economical for finding the no-load losses of a large dc shunt motor is A. Hopkinson's test B. Swinburne's test C. retardation test D. none of these
B
52. The rated speed of a given dc shunt motor is 1050 rpm. To run this machine at 1200 rpm. 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
B
6.Condition for maximum power output for a dc motor is A. Eb = V B. Eb = V/2 C. Eb = IaR a D. Eb = 1/2 * IaRa
B
8.The shunt field of a compound generator is connected across both the series field and the armature. This connection is known as A. short shunt B. long shunt C. differential compound D. cumulative compound
B
10. The dummy coil in dc machines is used to A. eliminate reactance voltage B. eliminate armature reaction C. bring about mechanical balance of armature D. eliminate harmonics developed in the machine
C
16. In a dc generator, the flux is seen to be ________ at the trailing pole tips and also at the leading pole tips. A. weakened, weakened B. weakened, crowded C. crowded, weakened D. same, same
C
16. The current drawn by armature of a dc motor is A. V / Ra B. Eb/Ra C. (V - Eb) / R a D. (Eb − V)/ Ra
C
18. The function of a compensating winding is to neutralize the A. reactance voltage generated as a consequence of commutation B. cross magnetize the effect of armature reaction C. demagnetizing effect of armature reaction D. all of these
C
19. In dc 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 and armature current remains the same C. speed will be half and armature current becomes double D. speed and armature current will remain the same
C
2. The rotating part of a dc motor is know as A. pole B. stator C. armature D. carbon brush
C
20. One dc motor drives another dc motor. The second dc motor when excited and driven A.runs B. does not run as a generator D. runs as a generator D. also runs as a motor
C
21. Armature torque of a dc motor is given by A. T = 0.159φZIa(P/A) kg-m B. T= 0.1620/2Ia (P/A) N-m C. T= 0.159Eb Ia /N N-m D. T = P / A kg-m
C
25. The function of an equalizer ring is A. to avoid unequal distribution of current at the brushes B. to help to get sparkless commutation C. both of these D. none of these
C
33. In DC generators, lap winding is used for A. low voltage, low current B. high voltage, high current C. low voltage, high current D. high voltage, low current
C
4. What effect on current flow does reversing the direction of movement of a conductor in the magnetic field of a generator have A. neutralizes the current flow B. has no effect on current C. it reverses the direction of current flow D. has no effect on the voltage
C
41. In a shunt generator, the voltage built up generally restricted by A. speed limitation B. armature heating C. saturation of iron D. brush wear
C
42. The armature of a dc generator is laminated to A. reduce the bulk B. insulate the core C. reduce eddy current loss D. provide passage for cooling air
C
22. The method which can be used to improve commutation process is A. increasing the resistance of the brushes B. reducing the number of turns in the armature coil and hence the number of segments in the commutator C. to neutralize the reactance voltage by producing a reverse emf in the short circuit coil under commutation D. all of the above
D
23. The function of interpole or compole is A. to neutralize the reactance voltage and help in commutation process B. to neutralize the cross-magnetizing effect of armature reaction C. to neutralize the demagnetizing effect of the armature D. A and B
D
26. A starter is necessary to start a dc 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 safe value
D
26. Generators are often preferred to be run in parallel because of having the advantages of A. great reliability B. generator efficiency C. meeting greater load demand D. all of the above
D
29. Shunt generators are used where the main requirement is a constant A. current and voltage B. current C. voltage over a wide load range D. voltage over a narrow load range
D
3. The function of the commutator in a dc 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
D
32. The speed of a series motor at no load is A. zero B. 3000 rpm C. 3600 rpm D. infinity
D
33. The speed of dc series motor decreases if the flux in the field winding A. remains constant B. increases C. decreases D. none of these
D
35. If the back emf in a dc 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
D
35. Which of the following component of a dc generator plays vital role for providing direct current of a dc generator? A. dummy coils B. equalizer rings C. eye bolt D. commutator
D
38. Full load efficiency of the generator will be A. 68.5% B. 73.5% C. 80.8% D. 92.51%
D
40. In dc generator, the cause of rapid brush wear may be A. imperfect contact B. sever sparking C. rough commutator surface D. any of the above
D
41. The motor used for intermittent, high torque is A. dc shunt motor B. dc series motor C. differential compound motor D. cumulative compound motor
D
48. 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
D
56. The main function of a commutator in a dc motor is to A. prevent sparking B. convert ac to dc C. reduce friction D. convert dc to ac
D
6.How would you determine, from visual observation of the armature winding, whether a generator is lap or wave wound A. connection to the field winding B. connection to the commutator C. connection to the brushes D. the direction of the end connection
D