Free Electrical Machines MCQs with Answers

• 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.

• 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…

• 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 =…

• 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.

• 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…

• 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…

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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…

• A. insulation resistance
• B. copper loss
• C. core loss
• D. total loss
• E. efficiency
• F. none of the above

Explanation: The open-circuit test is made at rated voltage with the secondary open, so the wattmeter reading mainly represents hysteresis and…

• A. mica strip
• B. thin coat of varnish
• C. paper
• D. any of the above

Explanation: Transformer laminations are coated with a thin insulating varnish before assembly.

• A. hot because of heavy iron losses taking place in it due to high flux density
• B. hot because primary will carry heavy current
• C. cool as there is no secondary current
• D. none of above will happen

Explanation: Opening a CT secondary removes the opposing secondary ampere-turns, causing the core flux and induced secondary voltage to rise…

• A. copper loss becomes high in proportion to the output
• B. iron loss is increased considerably
• C. voltage drop both in primary and secondary becomes large
• D. secondary output is much less as compared to primary input

Explanation: Copper loss varies approximately with the square of load current, while output rises roughly in proportion to load current.

• A. load changes
• B. oil in the transformer
• C. magnetostriction
• D. mechanical vibrations

Explanation: Alternating magnetization causes repeated expansion and contraction of the transformer core, a phenomenon called magnetostriction.

• A. core
• B. windings
• C. tank
• D. any of the above

Explanation: The windings generally have the highest temperature rise because they produce I²R copper loss and are surrounded by insulation that…