Quiz: Electrical Engineering 16 June 2021

Quiz: Electrical Engineering
Exam: PGCIL
Topic: Miscellaneous

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

Q1. For C coils and P poles, the distance between the coils connected by an equalizer ring is:
(a) C/2
(b) C/P
(c) C/2P
(d) 2C/P

Q2. A synchronous motor is operating at constant load, while its excitation is adjusted to get unity power factor. If the excitation is now increased, the power factor will be?
(a) Remains at unity
(b) Becomes zero
(c) Leading
(d) Lagging

Q3. The arc voltage produced in the circuit breaker is always
(a) Lagging the arc current by 90⁰
(b) In phase with arc current
(c) Leading the arc current by 90⁰
(d) None of the above

Q4. A parallel RLC circuit resonates at a frequency of 2 MHz. At a frequency 2.2 MHz, the nature of circuit impedance will be
(a) Resistive
(b) Inductive
(c) Capacitive
(d) None of the above

Q5. 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 %

Q6. If the conductor radius increases, inductance of the line-
(a) increases
(b) decreases
(c) remains same
(d) may increase or decrease

Q7. Surge impedance loading of a line is proportional to
(a) V
(b) V^2
(c) 1/V
(d) √V

Q8. As compared to DOL starting method, the star delta starting method of an induction motor should have
(a) High torque
(b) Low starting current
(c) High starting current
(d) Smooth acceleration

Q9. A sinusoidal current has a maximum value of 10 A. if signal is half rectified, its rms value will be:
(a) 7.07 A
(b) 14.14 A
(c) 10 A
(d) 5 A

Q10. Determine the ratio of weights of copper in an auto-transformer and a two-winding transformer if the transformation ratio is 3.
(a) 1/3
(b) 2/3
(c) 1/4
(d) 1/6

SOLUTIONS
S1. Ans.(d)
Sol.
The distance b/w equalizer ring(D)=2C/P

S2. Ans.(c)
Sol.
If excitation increases for given condition, then Power factor becomes leading.

S3. Ans.(b)
Sol.
The path of arc current is highly resistive. So, the arc voltage produced in the CB is always in phase of arc current.

S4. Ans.(c)
Sol.
As given frequency is greater than resonant frequency. So, nature will be capacitive for parallel RLC circuit.

S5. Ans.(c)
Sol.
η=√(Pc/Pcufl )=√(400/800) ×100=70.7 %

S6. Ans.(b)
Sol.
Inductance of a line.
L = 2×10-7 ln[GMD/GMR] H/m/phase
and. GMR α Radius of the conductor
If radius increases then GMR will increase and inductance will decrease.

S7. Ans.(b)
Sol.
Surge impedance loading: SIL=(KV)^2/Z_0 MW
i.e., SIL α V^2
Where; Z0=√(L/C)
SIL is independent of length of line.
SIL can be increased by increasing voltage level and vice-versa.

S8. Ans.(b)
Sol.
If the motor is started with star connection the phase voltage gets divided by √3 times i.e. (phase voltage = line voltage /√3). Since the voltage is reduced, the starting current is also reduced.

S9. Ans.(d)
Sol.
For HWR, I_rms=I_m/2=10/2=5 A

S10. Ans.(b)
Sol.
Required ratio=1-1/n
Where; n=transformation ratio
∴ Required ratio=1-1/n=1-1/3=2/3

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