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NHPC-JE’21 EE: Daily Practices Quiz 10-NOV-2021

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NHPC-JE’21 EE: Daily Practices Quiz

NHPC-JE’21 EE: Daily Practices Quiz 10-NOV-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 06 min.

Q1. In a dc generator, sparking at brushes results due to
(a) Winding distribution
(b) Improper connection of winding
(c) High constant resistance of the brushes
(d) Armature reaction and commutation

Q2. The maximum number of brushes which may be used in an electrical machine is equal to
(a) Number of poles in the machine
(b) 2
(c) Either (a) or (b)
(d) 4

Q3. Which of the following distribution systems are more economical?
(a) DC system
(b) Single Phase ac system
(c) Three phase 3 wire ac system
(d) Three Phase 4 wire ac system

Q4. In Overhead transmission systems, the conductors used are:
(a) Copper Conductors
(b) All Aluminium Conductors
(c) ACSR conductors
(d) Aluminium and Zinc alloy Conductors

Q5. No eddy current and hysteresis losses occur in
(a) Electrostatic instruments
(b) PMMC instruments
(c) Moving iron instruments
(d) Electro-dynamo meter instruments

Q6. The deflection expression θ ∝ V^2 dc/dθ, applies to the:
(a) Moving iron type instrument
(b) Electrodynamic type instrument
(c) Electrostatic type instrument
(d) Induction type instrument

SOLUTIONS

S1. Ans.(d)
Sol. Due to armature reaction, MNA is shifted in the direction of rotation in case of generator and against the direction of rotation in case of motor. As a result, rotational emfs are induced in the coils undergoing commutation which leads to sparking.
NOTE: In a DC generator in order to reduce sparking at brushes, the self-induced emf in the coil is neutralized by dummy coils, shifting of axis of brushes or with compensating winding.

S2. Ans.(c)
Sol. The maximum number of brushes which may be used in an electrical machine is:
For wave connected it is generally 2.
For LAP connected, it is P (number of poles).
Generally, number of brushes in DC machine=number of parallel paths.

S3. Ans.(a)
Sol. DC distribution is more economical compared to cost wise because it requires two conductors, no inductance capacitance phase displacement and surge problem, no skin effect in dc hence cross section of the conductor is completely utilized, no charging currents, and saving in the copper.

S4. Ans.(c)
Sol. In overhead lines the conductors used are ACSR conductors’ (Aluminium Conductor Steel Reinforced), consists of galvanised steel strand surrounded by a number of aluminium strands.

S5. Ans.(a)
Sol. No eddy current and hysteresis losses occur in Electrostatic instruments.

S6. Ans.(c)
Sol. The deflection expression θ ∝ v^2 dc/dθ, applies to the electrostatic instruments.

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