All Free Electrical Engineering MCQs with Answers

• A. small air gap
• B. large leakage flux
• C. laminated silicon steel core
• D. fewer rotating parts

Explanation: A transformer core has a very small air gap and high permeability, so only a small magnetising current is needed to establish the flux.

• A. electrical fault inside the transformer itself
• B. electrical fault outside the transformer in outgoing feeder
• C. for both outside and inside faults
• D. none of the above

Explanation: A Buchholz relay is installed in the oil pipe between the transformer tank and conservator, so it detects gas formation and oil surges…

• A. short-circuiting of the secondaries
• B. power factor of one of the transformers is leading while that of the other lagging
• C. transformers having higher copper losses will have negligible core losses
• D. loading of the transformers not in proportion to their kVA ratings

Explanation: When parallel transformers have unequal percentage impedances, the lower-impedance transformer takes a greater share of the load.

• A. the current on secondary side is negligible
• B. the voltage on secondary side does not vary
• C. the voltage applied on primary side is low
• D. full-load current is not supplied to the transformer

Explanation: In the short-circuit test, only a small primary voltage is needed to circulate rated current because the winding impedance is low.

• A. Hysteresis losses are reduced
• B. Saving in winding material
• C. Copper losses are negligible
• D. Eddy losses are totally eliminated

Explanation: An autotransformer uses part of the same winding for both primary and secondary, so less copper and conductor material are required than…

• A. no-load
• B. half-load
• C. near full-load
• D. 10% overload

Explanation: Power transformers operate near full load for much of their service, so they are designed for maximum efficiency near full-load…

• A. Primary winding
• B. Secondary winding
• C. Low voltage winding
• D. High voltage winding

Explanation: For approximately equal power, the high-voltage winding carries lower current than the low-voltage winding.

• A. hysteresis loss
• B. eddy current losses
• C. copper losses
• D. all of the above

Explanation: Adding silicon to steel increases its electrical resistivity and reduces hysteresis loss, while the use of thin laminations reduces…

• A. load is balanced only
• B. load is unbalanced only
• C. on balanced as well as unbalanced loads
• D. none of the above

Explanation: A delta winding provides a closed path for triplen-harmonic and zero-sequence currents, while the star winding can supply an unbalanced…

• A. load is unbalanced only
• B. load is balanced only
• C. on balanced as well as unbalanced loads
• D. none of the above

Explanation: A star/star transformer is normally suitable for balanced loads, but unbalanced loading and third-harmonic currents can cause neutral…

• A. Mechanical strength
• B. Low hysteresis loss
• C. High thermal conductivity
• D. High permeability

Explanation: Transformer-core material should have high permeability, low hysteresis loss, and adequate mechanical strength.

• A. 90% load
• B. zero load
• C. 25% load
• D. 50% load

Explanation: Distribution transformers remain energized continuously and are designed with core loss in mind, so their maximum efficiency is commonly…

• A. average value
• B. r.m.s. value
• C. maximum value
• D. instantaneous value

Explanation: The transformer emf equation is written as E = 4.44 f N Φm, where Φm is the maximum value of sinusoidal flux.

• A. fluctuating load
• B. poor insulation
• C. mechanical vibrations
• D. saturation of core

Explanation: When the transformer core approaches saturation, the magnetizing current becomes non-sinusoidal and contains harmonic components.

• A. 50°C
• B. 80°C
• C. 100°C
• D. 150°C

Explanation: Transformer oil generally begins significant thermal dissociation and gas formation only at temperatures above about 150°C.

• A. 6400 W
• B. 1600 W
• C. 800 W
• D. 400 W

Explanation: Copper loss varies as the square of load current: Pcu ∝ I². At half-load it is (1/2)² × 1600 = 400 W, so the loss becomes one-fourth of…

• A. A Buchholz relay can be installed on
• B. auto-transformers
• C. air-cooled transformers
• D. welding transformers
• E. oil cooled transformers

Explanation: A Buchholz relay is fitted in the oil pipe between the main tank and conservator of an oil-immersed transformer, where it detects gas…

• A. sludge
• B. odour
• C. gases
• D. moisture

Explanation: Moisture sharply reduces transformer oil's dielectric strength and can damage the insulation system, so oil must be kept dry.

• A. copper core
• B. cost iron core
• C. air core
• D. mild steel core

Explanation: At high frequencies, an air core avoids the large hysteresis and eddy-current losses associated with iron cores.

• A. zero
• B. 1 ohm
• C. 1000 ohms
• D. infinite

Explanation: The primary and secondary windings are electrically insulated from each other, so their insulation resistance is ideally infinite.