DC GENERATOR

 

1. The DC generator works on the principle of

A) Faraday's laws of electro magnetic induction
B) The current carrying conductor placed in the magnetic field an emf is produced
C) Lenz's law
D) Mutual induction

ANSWER= (A)
Explain:- The DC generator works on the principle of Faraday's laws of electro magnetic induction.

 

2. The emf generated in a dc generator is

A) Statically induced emf
B) Electrostatically induced emf
C) Dynamically induced emf
D) Magnetically induced emf

ANSWER= (C)
Explain:- The emf generated in a dc generator is dynamically induced emf.

 

3. The yoke of generator is made of cast iron because

A) It gives mechanical strength to the machine
B) It is cheaper
C) It completes the magnetic path
D) All of the above

ANSWER= (D)
Explain:- The yoke of generator is made of cast iron because it has all these properties.

 

4. The field of self excited dc generator is excited by

A) Separate AC Supply
B) Separate DC Supply
C) Its own current
D) All of the above

ANSWER= (C)
Explain:- The field of self excited dc generator is excited by its own current.

 

5. Armatures are made of laminated steel instead wood because it has

A) More mechanical strength
B) Low permeability
C) High permeability
D) Both A and C

ANSWER= (D)
Explain:- Armatures are made of laminated steel instead wood because it has more mechanical strength and high permeability.

 

6. A commutator in dc machines can convert

A) AC to DC
B) DC to AC
C) Both A and B
D) None of the above

ANSWER= (C)
Explain:- A commutator in dc generator converts AC into DC and in dc motor it converts DC into AC.

 

7. In dc machines , the space distribution of air-gap flux density wave at no load is

A) Co-sinusoidal
B) Sinusoidal
C) Rectangular
D) Flat-topped

ANSWER= (D)
Explain:- In dc machines , the space distribution of air-gap flux density wave at no load is flat-topped.

 

8. A 4-pole dc generator is running at 1500 rpm. The frequency of current in the armature winding is

A) 2.5 Hz
B) 50 Hzl
C) Zero Hz
D) 6000 Hz

ANSWER= (B)
Explain:-Ns = 120*f/P.

 

9. The efficiency of a dc machine is maximum when the variable losses equal to the

A) Constant losses
B) Zero
C) Square of the constant losses
D) Square root of the constant losses

ANSWER= (A)
Explain:- The efficiency of a dc machine is maximum when the variable losses equal to the constant losses.

 

10. In a dc machine the armature mmf is always directed along the

A) Brush axis
B) Polar axis
C) Interpolar axis
D) None of the above

ANSWER= (A)
Explain:- In a dc machine the armature mmf is always directed along the brush axis.

 

11. In dc machines , the armature reaction mmf is

A) Stationary with respect to armature
B) Rotating with respect to stator
C) Rotating with respect to brushes
D) Stationary with respect to stator

ANSWER= (D)
Explain:- In dc machines , the armature reaction mmf is stationary with respect to stator (field poles) but rotating with respect to armature.

 

12. The carbon brushes are generally used as they

A) Are soft
B) Have negative temperature coefficient
C) Are self lubricating
D) All of the above

ANSWER= (D)
Explain:- The carbon brushes are generally used as they have all these properties.

 

13. The armature mmf in a dc machine is 

 1. Stationary with respect to field poles 

 2. Stationary with respect to armature 

 3. Rotating with respect to field poles 

 4. Rotating with respect to armature 

 From these, the correct answer is

A) 1 & 2
B) 1 & 4
C) 1 & 3
D) 2 & 4

ANSWER= (B)
Explain:- The armature mmf in a dc machine is stationary with respect to field poles but rotating with respect to armature.

 

14. In dc generators, the polarity of interpoles is

A) Same as that of main pole ahead
B) Same as that of main pole behind
C) Opposite to that of the mail pole
D) None of these

ANSWER= (A)
Explain:- In dc generators, the polarity of interpoles is same as that of main pole ahead.

 

15. The purpose of using interpoles in dc machines is to nullify

A) The cross magnetising effect of armature mmf
B) The reactance voltage
C) The demagnetising effect of armature mmf
D) Both A and B

ANSWER= (D)
Explain:- The purpose of using interpoles in dc machines is to nullify both the cross-magnetizing mmf and the reactance voltage.

 

16. The object of using compensation winding in dc machines is to neutralize the

A) Armature reaction under the pole faces
B) Armature reaction in the interpolar zone
C) Armature reaction in the commutating zone
D) Cross-magnetizing armature reaction

ANSWER= (A)
Explain:- The object of using compensation winding in dc machines is to neutralize the armature reaction under the pole faces.

 

17. Rosenberg generator is

A) A constant voltage generator at low speeds and voltage polarity is independent of the direction of rotation
B) A constant current generator at high speeds and voltage polarity is independent of the direction of rotation
C) A constant current generator at high speeds and voltage polarity changes with the direction of rotation
D) A constant voltage generator at low speeds and voltage polarity changes with the direction of rotation

ANSWER= (B)
Explain:- Rosenberg generator is a constant current generator at high speeds and voltage polarity is independent of the direction of rotation.

 

18. In dc machines the space waveform for the air-gap flux distribution effects

A) Voltage but not the torque
B) Torque but not the voltage
C) Neither the voltage nor the torque
D) Both the torque and voltage

ANSWER= (B)
Explain:- In dc machines the space waveform for the air-gap flux distribution effects neither the voltage nor the torque.

 

19. In dc machines, the axis of armature mmf is along the

A) Interpolar axis
B) Direct axis
C) Midway between the direct and interpolar axis
D) None of the above

ANSWER= (C)
Explain:- In dc machines, the axis of armature mmf is along the midway between the direct and interpolar axis.

  

20. The exciting coils of any d.c. machine are wound

A) In the machine
B) In the armature slots
C) Separately
D) Around the poles

ANSWER= (D)
Explain:- The exciting coils of any d.c. machine are wound around the poles.

21. The function of a commutator in a d.c. generator is

A) To change d.c. into a.c
B) To change a.c.into d.c
C) To collect current from conductors
D) To conduct the current to the brushes

ANSWER= (B)
Explain :- The function of a commutator in a d.c. generator is to change a.c. into d.c.

 

22. In any d.c. generator the emf generated in the armature is maximum when

A) Rate of change of flux linked is maximum
B) Flux linked with conductor is maximum
C) Rate of change of flux linked is minimum
D) Flux linked with conductor is minimum

ANSWER= (A)
Explain :- In any d.c. generator the emf generated in the armature is maximum when rate of change of flux linked is maximum.

 

23. Which one of the following is used for charging batteries?

A) DC Shunt Generator
B) DC Series Generator
C) DC Compound Generator
D) Tacho Generator

ANSWER= (A)
Explain :- DC Shunt Generator is used for charging batteries.

 

24. D.C. shunt generator has

A) Constant voltage characteristics
B) Slightly drooping characteristics
C) Appreciably rising characteristics
D) Appreciably falling characteristics

ANSWER= (B)
Explain :- D.C. shunt generator has slightly drooping characteristics.

 

25. If the speed of d.c. generator is increased, the generated emf

A) Decreases
B) Increases
C) Remains constant
D) Increases then Decreases

ANSWER= (B)
Explain :- If the speed of a d.c. generator is increased, the generated emf increases.

 

26. The wave form of the armature mmf in a dc machine is

A) Square
B) Triangular
C) Rectangular
D) Sinusoidal

ANSWER= (B)
Explain :- The wave form of the armature mmf in a dc machine is triangular. 

 

27. The commutator of a dc generator acts as a

A) Half wave rectifier
B) Full wave rectifier
C) Controlled half-wave rectifier
D) Controlled full-wave rectifier

ANSWER= (B)
Explain:- The commutator of a dc generator acts as a full wave rectifier.

 

28. In a dc machine the armature reaction and the inductance of the commutating coils result in

A) Sinusoidal commutation
B) Under commutation
C) Over commutation
D) Linear commutation

ANSWER= (B)
Explain:- In a dc machine the armature reaction and the inductance of the commutating coils result in under commutation.

 

29. For parallel operation , the polarities of two generators

A) Must be same
B) Must oppose each other
C) May or may not be same
D) None of the above

ANSWER= (B)
Explain:- For parallel operation , the polarities of two generators must be same.
Conditions for Parallel Operation of DC Generators :
To connects the generators in parallel to common bus-bars, the generators should satisfy the following conditions.
1. The incoming generator voltage should be the same as the bus-bar voltage.
2. The +ve and -ve terminals ( i.e. polarity ) of generators must be connected to +ve and -ve of bus-bars otherwise a serious short-circuit will occur.
3. Equalizer bar should be used for compound and series generators.
4. Induced e.m.fs of generators should be preferably the same otherwise circulating currents result.
5. The voltage regulation of all the generators should be same.
6. The current shared by each generator should be inversely proportional to the armature resistance of that generator.

 

30. For the parallel operation of two or more D.C. compound generators, we should ensure that

A) Polarity of incoming generator should be same as that of bus bar and voltage of the incoming generator should be same as that of bus bar
B) All- the series fields should be run in parallel by means of equilizer connection
C) Voltage of the incoming generator should be same as that of bus bar
D) All of the above

ANSWER= (D)
Explain:- For the parallel operation of two or more D.C. compound generators, we should ensure that all conditions should be satisfied.

 

31. The emf induced in the armature of a dc machine is

A) Directly proportional to both the flux and speed
B) Directly proportional to the flux and inversely proportional to the speed
C) Directly proportional to the speed and inversely proportional to the flux
D) Inversely proportional to both the flux and speed

ANSWER= (A)
Explain:- The emf induced in the armature of a dc machine is directly proportional to both the flux and speed.
E = (N*P* Φ *Z)/(60*A)

 

32. A dc shunt generator driven at a normal speed in the normal direction fails to build up armature voltage because

A) The field current is too small
B) The resistance of the armature is high
C) There is no residual magnetism
D) None of these

ANSWER= (C)
Explain:- A dc shunt generator driven at a normal speed in the normal direction fails to build up armature voltage because there is no residual magnetism.

 

33. The generator of a power station produces an electric pressure by

A) Magnetic induction
B) Mechanical pressure
C) Conversion of light
D) None of the above

ANSWER= (B)
Explain:- The generator of a power station produces an electric pressure by mechanical pressure.

 

34. Which of the following type of generator gives constant voltage output at all loads?

A) Shunt generator
B) Level compound generator
C) Long shunt compound generator
D) Short shunt compound generator

ANSWER= (B)
Explain:- Level compound generator gives constant voltage output at all loads.

 

35. In dc machines, the armature windings are placed on the rotor because of the necessity for

A) Commutation
B) Generation of voltage
C) Electromechanical energy conversion
D) Development of torque

ANSWER= (A)
Explain:- In dc machines, the armature windings are placed on the rotor because of the necessity for commutation.

 

36. The brake test for the determination of efficiency of a dc machine is

A) A regenerative method
B) A direct method
C) An indirect method
D) None of these

ANSWER= (B)
Explain:- The brake test for the determination of efficiency of a dc machine is a direct method.

 

37. The Swinburne's method for the determination of efficiency of a dc machine is

A) A regenerative method
B) A direct method
C) An indirect method
D) None of these

ANSWER= (C)
Explain:- The Swinburne's method for the determination of efficiency of a dc machine is indirect method.

 

38. In Swinburne's method for the determination of efficiency of a dc machine is

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) None of these

ANSWER= (B)
Explain:- In Swinburne's method for the determination of efficiency of a dc machine is the no-load losses and measured copper losses are calculated.

 

39. In the Kapp's modificaiton of Hopkinson's efficiency test

A) The power output of the generator is dissipated in a resistor
B) The power losses in the two machine are supplied mechanically
C) The power losses in the two machines are supplied electrically
D) The two machines are mechanically decoupled

ANSWER= (C)
Explain:- In the Kapp's modificaiton of Hopkinson's efficiency test the power losses in the two machines are supplied electrically.

 

40. Commutation conditions at full-load for a large dc machine can be checked by

A) The swinburne's test
B) The Hopkinson's test
C) The brake test
D) None of these

ANSWER= (B)
Explain:- Commutation conditions at full-load for a large dc machine can be checked by the Hopkinson's test.

 

41. In a dc machine without interpoles to get improved commutation , the brush shift angle must be

A) Kept constant
B) Zero degree
C) Varied with change in load
D) None of these

ANSWER= (C)
Explain:- In a dc machine without interpoles to get improved commutation , the brush shift angle must be varied with change in load.

 

42. 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 generator and larger for the motors
C) Smaller for both the generators and larger for the motor
D) Larger for generators and smaller for the motors

ANSWER= (A)
Explain:- Compared to the air-gap under the field-poles, the inter-pole air-gap is made larger for both the generators and the motors.

 

43. The direction of induced emf in an armature coil of a dc machine is

A) Opposite to that of the current for the generator and the motor
B) The same as that of the current for both 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 the above

ANSWER= (C)
Explain:- The direction of induced emf in an armature coil of a dc machine is the same as that of current for the generator and opposite to that of the current for the motor.

 

44. For a dc generator if the brushes are given , a small amount forward shift the effect of armature reaction is

A) Totally cross-magnetizing
B) Partly demagnetizing and partly cross magnetizing
C) Totally demagnetizing
D) Totally magnetizing

ANSWER= (B)
Explain:- For a dc generator if the brushes are given , a small amount forward shift the effect of armature reaction is partly demagnetizing and partly cross magnetizing.

 

45. Under commutation gives rise to

A) No sparking at all
B) Sparking at the middle of the brush
C) Sparking at the trailing edge of the brush
D) Sparking at the leading edge of the brush

ANSWER= (C)
Explain:- Under commutation gives rise to sparking at the trailing edge of the brush.

 

46. A simple method of increasing the voltage of an d.c. generator is to

A) Decrease the speed of rotation
B) Decrease the air gap flux density
C) Increase the speed of rotation
D) None of the above

ANSWER= (C)
Explain:- A simple method of increasing the voltage of an d.c. generator is to increase the speed of rotation.

 

47. The destroyed residual magnetism of a shunt generator may restored by connecting its shunt field to

A) An Earth
B) A Battery
C) An Alternator
D) An Induction Generator

ANSWER= (B)
Explain:- The destroyed residual magnetism of a shunt generator may restored by connecting its shunt field to a Battery.

 

48. Which of the following statements is incorrect in dc shunt generators about hysteresis loss?

A) It is independent of lamination thickness
B) It depends upon the supply frequency
C) It can be minimized by laminating the armature
D) All of the above

ANSWER= (C)
Explain:- In dc shunt generators hysteresis loss is independent of lamination of armature core.

 

49. An ideal dc generator has a regulation of

A) 100 %
B) 0 %
C) 50 %
D) 80 %

ANSWER= (B)
Explain:- An ideal dc generator has a regulation of Zero percentage.

 

50. The parts of the armature electric circuit which take active part in emf generation are

A) The coil sides inside the slots
B) The commutator segments
C) The overhangs
D) All of the above

ANSWER= (A)
Explain:- The parts of the armature electric circuit which take active part in emf generation are The coil sides inside the slot.

 

51. The simplest way to shift load from one shunt generator to the other generator running in parallel is to

A) Use equalizer connection
B) Add resistance in their armature circuit
C) Adjust their field rheostats
D) None of the above

ANSWER= (C)
Explain:- The simplest way to shift load from one shunt generator to the other generator running in parallel is to adjust their field rheostats.

 

52. A dc generator can be termed as

A) Prime mover
B) Rotating amplifier
C) Power pump
D) Power amplifier

ANSWER= (B)
Explain:- A dc generator can be termed as rotating amplifier.

 

53. The terminal voltage of a D.C. shunt generator drops on load because of all of the following reasons except

A) Armature resistance drop
B) Commutation
C) Armature reaction
D) Field weakening due to armature reaction and armature

ANSWER= (B)
Explain:- The terminal voltage of a D.C. shunt generator drops on load dur to armature resistance drop,armature reaction and field weakening due to armature reaction & armature.

 

54. 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 both the generator and the motor
B) Against the direction of rotation for both the generator and the motor
C) In the direction of rotation for the generator and against the direction of rotation for the motor
D) Against the direction of rotation for the generator and against the direction of rotation for the motor

ANSWER= (C)
Explain:- In a dc machine without any brush shift, the shift of the magnetic neutral axes due to armature is in the direction of rotation for the generator and against the direction of rotation for the motor.

 

55. Ohmic losses in a dc machine occur in

A) The field winding only
B) The armature winding only
C) The brush contact only
D) All of the above

ANSWER= (D)
Explain:- Ohmic losses in a dc machine occur in the armature winding , the field winding and also in the brush contact.

 

56. If the thickness of laminations is increased , then

A) The eddy current loss increases
B) The eddy current loss decreases
C) The hysteresis loss increases
D) The hysteresis loss decreases

ANSWER= (A)
Explain:- If the thickness of lamination's is increased , then the eddy current loss increases.