Quiz: Electrical Engineering 10 June 2021

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Miscellaneous

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

Q1. Transformer for constant voltage applications is considered good if its voltage regulation is
(a) High
(b) Low
(c) Zero
(d) None of the above

Q2. The purpose of performing Sumpner’s test is mainly to find out’
(a) Efficiency of the transformer
(b) Regulation of the transformer
(c) The temperature rises on full-load economically
(d) None of the above

Q3. Which of the following are constant losses in transformers?
(a) Winding losses
(b) Core losses
(c) Both Winding & core losses
(d) None of these

Q4. On which of the following do the eddy current losses depend?
I. Nature of the material
II. Maximum flux density
III. Frequency
Choose the correct answer from the options given below:
(a) I, II and III
(b) II and III
(c) I and II
(d) I and III

Q5. What is the value of charging current in short transmission lines?
(a) Less than medium transmission lines
(b) Equal to medium transmission lines
(c) More than medium transmission lines
(d) More than long transmission lines

Q6. Voltage regulation of a transmission line should be __
(a) Minimum
(b) Maximum
(c) Greater than 50 %
(d) Less than 50 %

Q7. Gain cross-over frequency is defined as one at which |G(jω) H(jω)| equals
(a) 0
(b) 1
(c) √2
(d) 1/√2

Q8. Which of the following theorems can be applied to any network linear or non-linear, active or passive, time-variant or time-invariant?
(a) Norton theorem
(b) Thevenin theorem
(c) Superposition theorem
(d) Tellegen theorem

Q9. When used as a voltage regulator, a Zener diode is normally
(a) Not biased
(b) Forward biased
(c) Reverse biased
(d) None of the above

Q10. What are the average and RMS values of half wave rectified output if peak voltage is 5 V?
(a) 1.59 V, 2.5 V
(b) 1.59 V, 5 V
(c) 3.18 V, 2.5 V
(d) 3.18 V, 5 V

SOLUTIONS

S1. Ans.(b)
Sol. Voltage regulation is defined as rise in the voltage when the transformer is thrown off from full load condition to no-load condition. Thus, least voltage regulation means output fluctuations depending on the load are very less.
So, transformer for constant voltage applications is considered good if its voltage regulation is low.

S2. Ans.(c)
Sol. Sumpner’s test on transformer –

1. to check steady state temperature rise if the transformer was fully loaded continuously.
2. in this, two identical transformers needed.
3. Primary of both in parallel and secondaries are connected in series such that their polarity should be opposite.

S3. Ans.(b)
Sol. core loss or iron loss = Eddy + hysteresis loss
P_i=P_e+P_h

S4. Ans.(a)
Sol. eddy current loss (Pe)= Ke f^2 Bm ² t²
Where, t = thickness of material

S5. Ans.(a)
Sol. Line to Earth capacitance of short transmission line is less than that in medium and long transmission line and is negligible. Because of this the charging current of short transmission line is less than that in medium and long transmission line and is negligible.

S6. Ans.(a)
Sol. More value of voltage regulation means more voltage fluctuations and this is undesirable. So, the value of voltage regulation should be as low as possible.

S7. Ans.(b)
Sol. At gain cross-over frequency, magnitude of G(jω) is 1.
Or at `ω=ω_gc`; |G(jω) H(jω)|=1
Phase margin is calculated at gain cross over frequency.

S8. Ans.(d)
Sol. Tellegen theorem can be applied to any network linear or non-linear, active or passive, time-variant or time-invariant.

S9. Ans.(c)
Sol. Zener diode shows regulator action in reverse biased condition only.

S10. Ans.(a)
Sol. For half wave rectifier: Vavg=Vm/π=3/3.14=1.59 V
And Vrms=Vm/2=5/2=2.5 V

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