SSC Junior Engineer Electrical Online Exam 2018 CPWD/CWC/MES Electrical Engineering 22-01-2018 Morning6

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51. In dc machine iron losses cause ?




... Answer is D)
Types of losses in dc machine
1-Electrical or Copper losses
2-Core Losses or Iron Losses
3-Brush Losses
4-Mechanical Losses
5-Stray Losses



In dc machine iron losses cause heating in core
rise in temperature of ventilating air
less in efficiency
.

52. Armature current at starting can be reduced by keeping?




... Answer is D)
Ra represents armature resistance
and Rf - field circuit resistance
Armature current at starting can be reduced by keeping Ra maximum and Rf minimum.

53. In dc motors, under leading pole tips flux density will?




... Answer is A)
leading pole tips:-
The tip of the pole from where the armature conductors come into influence is called leading tip and the other tip opposite in direction to it will be the trailing tip.


Main flux and armature flux will aid each other under leading pole tips and for motors it will oppose each other in the trailing pole tips.


In a D.C. generator, the armature reaction results in demagnetisation of the leading pole tip and magnetisation of the trailing pole tip

.

54. The field winding will be drawn at what degree?




... Answer is No correct answer)
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows Basically armature winding is responsible for torque and field winding is responsible for flux in the machine. So armature current and field current should be independent of each other. This can be achieved only when the two windings namely armature and field winding are perpendicular to each other. .

55. Compensating winding is placed in the?




... Answer is A)
Compensating winding is placed in the pole shoe or pole face. They are connected in series with the armature having same axis as armature but opposite polarity of the pole. .

56. Stepped cores are used in transformers in order to reduce?




... Answer is B)

the ratings of transformers are increased stepped or multi-stepped cores are preferred
which improve mechanical stability, reduce the length of mean turn and result in copper savings besides increasing efficiency and improving regulation..

57. In a transformer the resistance between its primary and secondary is?




... Answer is D)
In a transformer the coils are not electricaly connected therefore the resistance is ideally infinite. But an autotransformer does the same using a single coil as primary with one or more taps for secondary in different parts of the coil. In this case the resistance will ideally be ZERO, or a short-circuit if you will.

58. In a transformer zero voltage regulation is achieved at a load power factor which is?




... Answer is B)


Voltage Regulation of Transformer at Unity Power Factor
positive
Voltage Regulation of Transformer at Lagging Power Factor
positive

Voltage Regulation of Transformer at Leading Power Factor
Zero and negative
.

59. In the induction motors the torque is directly proportional to the?




... Answer is D)
torque is directly proportional to lower slip like when motor is running in full speed. torque is inversely proportional to higher slip values slip like when motor starting.

60. In induction motor If we remove on phase for instance, two terminals will have 'Y' phase and one will have 'B' phase called?




... Answer is A)
AC Dynamic Braking-

This type of induction motor braking is obtained when the motor is made to run on a single phase supply by disconnecting any one of the three phase from the source, and it is either left open or it is connected with another phase.






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