# Quiz: Electrical Engineering 09 June 2021

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Miscellaneous

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency
(a) Remains unchanged
(b) Will increase
(c) Will decrease
(d) May increase or decrease depending on the line parameters

Q2. A 200 KV transmission line has a maximum power transfer capacity of P. if it is operated at 100 KV with the series reactance unchanged, then its power transfer capacity is approximately
(a) P
(b) P/4
(c) 2P
(d) P/2

Q3. Oscillators use following feedback.
(a) Negative
(b) Positive
(c) Both negative and positive
(d) None of the above

Q4. Moving iron and PMMC instruments can be distinguished from each other by looking at
(a) Pointer
(b) Terminal size
(c) Scale
(d) Scale range

Q5. The average power delivered to an impedance (4 – j3) Ω by a current 5cos (100πt +100) A is
(a) 42.5 W
(b) 50 W
(c) 62.5 W
(d) 125 W

Q6. Ward-Leonard control is basically a:
(a) Voltage control method
(b) field diverter method
(c) field control method
(d) armature resistance control method

Q7. What can be the approximate efficiency of a 3-phase, 50 Hz, 4-pole induction motor, which is running at 1470 rpm?
(a) 92 %
(b) 95 %
(c) 96 %
(d) 98 %

Q8. Slip of a 3-phase induction motor may be expressed as
(a) Rotor power input/rotor copper loss
(b) Rotor copper input/rotor core loss
(c) Rotor copper loss/rotor power input
(d) Rotor copper loss/ input power

Q9. A 40 KVA transformer has a core loss of 400 W and full-load copper loss of 800 W. The fraction of rated load at maximum efficiency is
(a) 50 %
(b) 62.3 %
(c) 70.7 %
(d) 100 %

Q10. Laplace transform of damped sine wave e^(-at) cosωt is __
(a) ω/((s+a)^2+ω^2 )
(b) (s+a)/((S+a)^2+ω^2 )
(c) s/((S+a)^2+ω^2 )
(d) 1/((S+a)^2+ω^2 )

SOLUTIONS

S1. Ans.(a)
Sol. The string efficiency is independent of transmission system frequency.

S2. Ans.(b)
Sol. P_m=V^2/X
As series reactance unchanged,
∴P_max α V^2
∴P2=P1×(V2/V1 )^2=P×(100/200)^2=P/4

S3. Ans.(b)
Sol. Oscillators use positive feedback.

S4. Ans.(c)
Sol. A PMMC instruments has linear scale whereas moving iron instrument has non-linear /non-uniform scale.

S5. Ans.(b)
Sol. Given that, Z=4-j3=R_L-jXC;RL=4 Ω and IL=5cos (100πt +100) A
∴P=1/2 Im^2 RL=1/2×25×4=50 W

S6. Ans.(a)
Sol. Ward-Leonard controlled dc drives are generally used for “Heavy duty excavators.
Ward-Leonard control is basically a voltage control method, in which motor (drives) is controlled with the help of the field current of generator its output voltage can be controlled thus DC drives.

S7. Ans.(d)
Sol. N_s=120f/P=(120×50)/4=1500 rpm
∴efficiency(η)=(1-s)=N_r/N_s =1470/1500×100=98 %

S8. Ans.(c)
Sol. rotor power input= air gap power=(3I^2 R2)/s
Rotor copper loss=3I^2 R2
∴SLIP(s)=(3I^2 R2)/((3I^2 R2)/s)=(Rotor copper loss)/(rotor power input)

S9. Ans.(c)
Sol. η=√(P_I/P_cufl )=√(400/800) ×100=70.7 %

S10. Ans.(b)
Sol. cos ωt (→LT ) s/(S^2+ω^2 )
e^(-at) cosωt (→LT ) ((s+a))/((s+a)^2+ω^2 )

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