All Free Electrical Engineering MCQs with Answers
Every Electrical Engineering question in the bank, across all chapters, each with the correct answer and a written explanation. Free and unlimited, with no account needed.
3,143 questions · page 72 of 158
- A. zero
- B. 10 Q
- C. 1000 Q
- D. infinity
Explanation: The primary and secondary windings must be electrically isolated, so their insulation resistance should ideally be infinite.
Correct answer: infinity- A. avoids core saturation and high voltage induction
- B. is safe to human beings
- C. protects the primary circuit
- D. none of the above
Explanation: An open-circuited CT secondary can develop dangerously high voltage and drive the core into saturation.
Correct answer: avoids core saturation and high voltage induction- A. high eddy current losses
- B. reduced magnetic leakage
- C. negligibly hysteresis losses
- D. none of the above
Explanation: In a shell-type transformer, the core surrounds much of the winding, giving a shorter and more confined magnetic-flux path.
Correct answer: reduced magnetic leakage- A. 1.5 MVA
- B. 5 MVA
- C. 15 MVA
- D. 50 MVA
Explanation: Natural-air cooling, or AN cooling, is generally used for relatively small transformers and is commonly limited to about 1.5 MVA.
Correct answer: 1.5 MVA- A. Step-up transformer
- B. Step-down transformer
- C. Potential transformer
- D. Current transformer
Explanation: A current transformer operates with its secondary nearly short-circuited because an open secondary can produce dangerously high voltage.
Correct answer: Current transformer- A. to provide oxygen inside the tank
- B. to cool the coils during reduced load
- C. to cool the transformer oil
- D. to arrest flow of moisture when outside air enters the transformer
Explanation: A transformer breather contains a moisture-absorbing material such as silica gel.
Correct answer: to arrest flow of moisture when outside air enters the transformer- A. about the same
- B. much smaller
- C. much higher
- D. somewhat smaller
- E. none of the above
Explanation: Transformers have no rotating or mechanical losses, so their efficiencies are commonly higher than those of motors of comparable rating.
Correct answer: much higher- A. low,low
- B. high,high
- C. low,high
- D. high,low
Explanation: Transformer oil should have low volatility so it does not evaporate readily and low viscosity so it circulates and transfers heat…
Correct answer: low,low- A. less than 15 A
- B. more than 15 A
- C. 15 A
- D. none of the above
Explanation: For an ideal step-down transformer, the primary current change is reduced in the inverse ratio of the voltage transformation: ΔI₁ =…
Correct answer: less than 15 A- A. same efficiency
- B. same polarity
- C. same kVA rating
- D. same number of turns on the secondary side.
Explanation: Transformers connected in parallel must have the same polarity so their secondary induced voltages aid rather than oppose each other.
Correct answer: same polarity- A. unity
- B. lagging
- C. leading
- D. zero
Explanation: For an inductive transformer, the reactive voltage-drop term is positive at lagging power factor but becomes negative at leading power…
Correct answer: leading- A. approximately equal to one
- B. less than one
- C. great than one
- D. none of the above
Explanation: An autotransformer uses the greatest fraction of its winding as common copper when its voltage ratio is close to unity, giving the largest…
Correct answer: approximately equal to one- A. hysteresis losses
- B. copper losses
- C. core losses
- D. eddy current losses
Explanation: In the short-circuit test, the applied voltage is low, so core flux and core loss are negligible.
Correct answer: copper losses- A. its power factor will deteriorate
- B. its power factor will increase
- C. its power factor will remain unaffected
- D. its power factor will be zero
Explanation: Excess voltage raises the core flux and may drive it toward saturation, causing a sharp increase in exciting current.
Correct answer: its power factor will deteriorate- A. hysteresis losses
- B. copper losses
- C. core losses
- D. eddy current losses
Explanation: With the secondary open, the input power is almost entirely the iron or core loss, made up mainly of hysteresis and eddy-current losses.
Correct answer: core losses- A. iron core
- B. copper winding
- C. winding insulation
- D. frame or case
- E. transformer tank
Explanation: Winding insulation is especially vulnerable because overheating accelerates its thermal ageing and can cause insulation breakdown.
Correct answer: winding insulation- A. primary is supplied rated voltage
- B. primary is supplied full-load current
- C. primary is supplied current at reduced voltage
- D. primary is supplied rated kVA
Explanation: The open-circuit test is performed at rated voltage so that the normal operating core flux and core loss are produced.
Correct answer: primary is supplied rated voltage- A. primary turns to secondary turns
- B. secondary current to primary current
- C. secondary induced e.m.f. to primary induced e.m.f
- D. secondary terminal voltage to primary applied voltage
Explanation: From the transformer emf equation, induced emf is proportional to the number of turns, so the voltage ratio is E₂/E₁ = N₂/N₁.
Correct answer: secondary induced e.m.f. to primary induced e.m.f- A. core loss
- B. copper loss
- C. efficiency
- D. magnetising current
- E. magnetising current and loss
Explanation: At no load, the transformer input supplies the core loss and the magnetising component of current, while copper loss is negligible.
Correct answer: magnetising current and loss- A. Circular type
- B. Sandwich type
- C. Cylindrical type
- D. Rectangular type
Explanation: Shell-type three-phase transformers commonly use interleaved sandwich windings, in which sections of high- and low-voltage windings are…
Correct answer: Sandwich type