Transformer MCQs – Set 3

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Transformer MCQs – Set 3: Here is another Transformer MCQs – Set 3. Set 1 & 2 can be checked in previous posts.

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Transformer MCQs – Set 3

1. What happens to the primary current of an ideal transformer when the secondary load increases?

A) It decreases
B) It increases
C) It remains zero
D) It becomes constant

Correct Answer: B) It increases

2. The ratio of primary to secondary voltage in an ideal transformer is equal to the ratio of:

A) Secondary current to primary current
B) Primary turns to secondary turns
C) Secondary turns to primary turns
D) Primary resistance to secondary resistance

Correct Answer: B) Primary turns to secondary turns

3. If a transformer has a turns ratio of 10:1, it is most likely a:

A) Step-up transformer
B) Step-down transformer
C) Current transformer
D) Isolation transformer

Correct Answer: B) Step-down transformer

4. Which winding generally uses a thicker conductor in a step-down transformer?

A) High-voltage winding
B) Low-voltage winding
C) Both always use the same conductor
D) Core winding

Correct Answer: B) Low-voltage winding

5. Why does the low-voltage winding generally carry a higher current?

A) It has higher resistance
B) Power is approximately conserved in an ideal transformer
C) It has higher frequency
D) It has more core losses

Correct Answer: B) Power is approximately conserved in an ideal transformer

6. Which quantity remains approximately constant in a transformer under normal operation?

A) Core flux
B) Secondary current
C) Copper loss
D) Load power

Correct Answer: A) Core flux

7. Transformer core loss depends mainly on:

A) Load current only
B) Voltage, frequency, and core material
C) Secondary resistance only
D) Load power factor only

Correct Answer: B) Voltage, frequency, and core material

8. Which loss is directly proportional to the square of the current?

A) Hysteresis loss
B) Eddy-current loss
C) Copper loss
D) Dielectric loss

Correct Answer: C) Copper loss

9. If the load current of a transformer is doubled, its copper loss approximately becomes:

A) Half
B) Twice
C) Four times
D) Eight times

Correct Answer: C) Four times

Reason:

Pcu=I2RP_{cu}=I^2R

If II becomes 2I2I:

Pcu=(2I)2R=4I2RP_{cu}=(2I)^2R=4I^2R

10. A transformer has an efficiency of 95%. This means:

A) 95% of input power is lost
B) 95% of input power is converted to useful output
C) 5% of output power is useful
D) Input and output powers are equal

Correct Answer: B) 95% of input power is converted to useful output

11. Which condition generally gives zero voltage regulation?

A) Purely resistive load
B) Ideal transformer
C) Short-circuit condition
D) Open-circuit condition

Correct Answer: B) Ideal transformer

12. For a transformer supplying a lagging power-factor load, voltage regulation is generally:

A) Positive
B) Negative
C) Always zero
D) Infinite

Correct Answer: A) Positive

13. For a transformer supplying a sufficiently leading power-factor load, voltage regulation can become:

A) Positive only
B) Zero only
C) Negative
D) Infinite

Correct Answer: C) Negative

14. What is the purpose of the open-circuit test?

A) To determine mainly core loss and no-load parameters
B) To determine mechanical losses
C) To measure shaft torque
D) To determine insulation thickness only

Correct Answer: A) To determine mainly core loss and no-load parameters

15. During the short-circuit test, the applied voltage is usually:

A) Equal to rated voltage
B) Very low compared with rated voltage
C) Twice the rated voltage
D) Zero

Correct Answer: B) Very low compared with rated voltage

16. Why is a low voltage applied during the short-circuit test?

A) To prevent excessive current
B) To produce rated current with the secondary short-circuited
C) To eliminate copper loss
D) To increase core flux to maximum

Correct Answer: B) To produce rated current with the secondary short-circuited

17. If the percentage impedance of a transformer is increased, its short-circuit current for a given rated voltage will generally:

A) Increase
B) Decrease
C) Remain exactly the same
D) Become zero

Correct Answer: B) Decrease

18. A transformer has a rated voltage of 1100 V and an impedance of 5%. Its impedance voltage is:

A) 22 V
B) 55 V
C) 110 V
D) 220 V

Correct Answer: B) 55 V

Calculation:

VZ=5100×1100=55VV_Z=\frac{5}{100}\times1100=55V

19. If a transformer has an impedance voltage of 40 V and a rated current of 20 A, its equivalent impedance is:

A) 0.5 Ω
B) 1 Ω
C) 2 Ω
D) 4 Ω

Correct Answer: C) 2 Ω

Z=VI=4020=2ΩZ=\frac{V}{I}=\frac{40}{20}=2\Omega

20. Which instrument transformer is used to step down high voltage for measurement and protection?

A) Current transformer
B) Potential transformer
C) Autotransformer
D) Isolation transformer

Correct Answer: B) Potential transformer

21. The secondary of a current transformer should generally not be:

A) Connected to a measuring instrument
B) Connected to a protective relay
C) Open-circuited while the primary is energized
D) Connected to a burden

Correct Answer: C) Open-circuited while the primary is energized

22. What is one major advantage of an autotransformer compared with a two-winding transformer of similar rating?

A) Complete electrical isolation
B) Less copper and generally higher efficiency
C) No magnetic flux
D) It operates on DC

Correct Answer: B) Less copper and generally higher efficiency

23. A major disadvantage of an autotransformer is:

A) It cannot operate on AC
B) It has no electrical isolation between input and output
C) It always has very high losses
D) It requires a rotating shaft

Correct Answer: B) It has no electrical isolation between input and output

24. Which transformer connection is commonly associated with a closed three-phase delta path?

A) Star–Star
B) Delta–Delta
C) Star–Delta only
D) Star–Star without a neutral

Correct Answer: B) Delta–Delta

25. The main purpose of transformer insulation is to:

A) Increase frequency
B) Prevent unwanted electrical conduction between parts at different potentials
C) Increase copper loss
D) Reduce the number of turns

Correct Answer: B) Prevent unwanted electrical conduction between parts at different potentials

Quick Answer Key – Set 3

Q.No.AnswerQ.No.AnswerQ.No.Answer
1B10B19C
2B11B20B
3B12A21C
4B13C22B
5B14A23B
6A15B24B
7B16B25B
8C17B
9C18B

Read More on Transformer

  1. Best Books for Transformer Design and Electrical Machine Design
  2. Transformers MCQ questions set 1
  3. Transformers MCQ Questions- Set 2
  4. Transformer MCQ Questions Set 3
  5. Efficiency of Transformer|Equation & Example
  6. Losses in a Transformer
  7. [Solved]Transformer Voltage Regulation Problem
  8. LVDT|Linear Variable Differential Transformer
  9. Autotransformer|Application of Autotransformer
  10. Transformers Electrical Engineering Interview Questions
  11. Why Transformers Hum: 7 Shocking Causes and Expert Solutions You Need to Know
  12. Single-phase transformer Connections Series & parallel
  13. Hysterisis and Eddy current losses
  14. Special Transformers
  15. Static Electrification in transformers
  16. Magnetostriction and its ill effects

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