Electrical Machines(DC Machines) Part-02

Electrical Machines(DC Machines) Part-02:

26. The back to back test is best suited for

a. Small machines
b. Medium size machines
c. Large machines
d. All of these

27. For performing Hopkinson’s test two identical machines are required which are mechanically coupled. The iron losses in the two machines

a. Can be obtained separately
b. Cannot be separated

28. The test which can be performed on the d.c. series motor is

a. Brake test
b. Hokinson’s test
c. Swinburne’s test
d. Field test

29. To perform field test on d.c. machines two motors are required. The motors used are

a. Series motors
b. One will be series motor and other will be shunt motors
c. Shunt motors
d. None of these

30. The difference between field test and regenerative test is that

a. Field test requires two identical machines while regenerative test requires only one machine
b. In field test iron losses can be separated while in regenerative test it cannot
c. In field test generator output is not fed back to the motor while in regenerative test generator output is fed back to the motor
d. All of these

31. The most common method used to check for shorted windings is to perform

a. Field test
b. Drop test
c. Regenerative test
d. Brake test

32. To have an induced emf in the d.c. generator, there should be relative motion between the conductor and flux. The plane of rotation and plane of flux

a. Should be parallel to each other
b. Should not be parallel to each other
c. Both (a) & (b)
d. None of these

33. To have d.c. voltage, a device is used in a d.c. generator to convert alternating emf to unidirectional emf. This device is called

a. Armature
b. Commutator
c. Brushes
d. All of these

34. Yoke in d.c. machine serves the outermost cover. For magnetic flux it provides

a. A low reluctance path
b. A high reluctance path
c. Both (a) & (b)
d. Does not provide path for magnetic flux

35. Functions of commutator in d.c. machines are

a. To facilitate the collection of current from armature conductors
b. To convert internally developed induced emf to unidirectional emf
c. To produce unidirectional torque in case of motors
d. All of these
e. None of these

36. Brushes in d.c. machines are made up of

a. Cast iron
b. Mild steel
c. Copper
d. Carbon

37. If A is the number of parallel paths and P is the number of poles, then the number of parallel path in lap winding and in wave winding is

a. A = P, A = 2
b. A = 2P, A = P
c. A = 2, A = P
d. A = P, A = 2P

38. A 4 pole, d.c. generator has a wave wound armature with 812 conductors. The flux per pole is 0.014 Wb. The speed at which it should be run to generate 240 V on no load is

a. 623.23 rpm
b. 633.35 rpm
c. 643.36 rpm
d. 645.53 rpm

39. A 4 pole, lap wound d.c. generators has 40 coils with 8 turns per coils. It is driven at 1200 rpm. If the flux per pole is 0.022 Wb, then the generated emf is

a. 265.8 V
b. 276.3 V
c. 281.6 V
d. 287.9 V

40. In a d.c. machine, the current rating and voltage rating of wave winding is

a. Low, high
b. High, low
c. High, high
d. Low, low

41. For the construction of the armature of a d.c. machine, the best suited material is

a. Cast iron
b. Silicon steel
c. Carbon
d. All of these

42. In a d.c. machine, the form of armature reaction mmf is

a. Triangular
b. Sinusoidal
c. Saw tooth
d. Rectangular

43. For low reluctance path for the flux in armature, the permeability of the material should be

a. High
b. Low
c. Both (a) and (b)
d. None of these

44. In a d.c. machine, the eddy current losses mainly occurs in

a. Armature
b. Windings
c. Yoke
d. At brush contact

45. The conductors which will cause distortion in the main field flux of d.c. machine are

a. Demagnetizing armature conductors
b. Cross magnetizing armature conductors
c. Both (a) and (b)
d. None of these

46. If the brush shift angle is given in electrical degrees then for converting it into mechanical degree by using the relation

a. θ_electrical / P
b. 2θ_electrical / P
c. θ_electrical / 2P
d. 2P / θ_electrical

47. A 6 pole wave wound d.c. generator has 460 armature conductors carrying armature current of 180 A. The armature reaction demagnetizing and cross magnetizing ampere turns per pole if brushes are shifted by 6 degree electrical from G.N.A are

a. 3220, 230
b. 3220, 260
c. 260, 3220
d. 230, 3220

48. For neutralizing the demagnetizing effect of armature reaction, an extra number of winding turns are provided on each pole. To calculate number of extra turns per pole the equation used is

a. (AT_d / I) * leakage coefficient
b. (AT_d / I) / leakage coefficient
c. Leakage coefficient / (AT_d / I)
d. (AT_d / I)