Free Electrical Machines MCQs with Answers

• A. poor insulation
• B. Overload
• C. loose connections
• D. Core saturation

Explanation: A saturated core draws a nonsinusoidal magnetizing current, and nonsinusoidal current contains harmonics.

• A. low power factor wattmeter
• B. unity power factor wattmeter
• C. frequency meter
• D. any type of wattmeter

Explanation: In the open-circuit test, the wattmeter reading is mainly the iron loss, while the power factor is very low because the exciting current…

• A. Core loss
• B. Friction loss
• C. Eddy current loss
• D. Hysteresis loss

Explanation: A transformer has no rotating mechanical parts, so it has no friction loss even at full load.

• A. capacitive only
• B. inductive only
• C. inductive or resistive
• D. none of the above

Explanation: A leading, usually capacitive, load can make the reactive voltage-drop component oppose the resistive drop, producing negative regulation.

• A. low voltage side of high kVA transformers
• B. high frequency transformers
• C. high voltage side of small capacity transformers
• D. high voltage side of high kVA rating transformers

Explanation: Helical windings are suited to low-voltage, high-current windings because their construction provides good mechanical strength and…

• A. Core insulation voltage test
• B. Impedance test
• C. Radio interference test
• D. Polarity test

Explanation: Radio interference testing is generally a special test performed when specified, not part of the standard routine tests applied to every…

• A. leading power factor
• B. lagging power factor
• C. unity power factor
• D. zero power factor

Explanation: The resistive voltage drop and reactive voltage drop can cancel when the load power factor is leading, giving zero regulation.

• A. value of transformation ratio remains constant
• B. permeability of transformer core remains constant
• C. core flux remains practically constant
• D. primary voltage remains constant
• E. secondary voltage remains constant

Explanation: With rated voltage and frequency held constant, the core flux is approximately constant from no load to full load.

• A. flux density remains unaffected
• B. iron losses are reduced
• C. core flux density is reduced
• D. core flux density is increased

Explanation: Transformer flux is approximately proportional to voltage divided by frequency.

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

Explanation: Low hysteresis loss, high permeability, and adequate mechanical strength are important core-material properties.

• A. load current
• B. load current and voltage
• C. load current, voltage and frequency
• D. load current, voltage, frequency and power factor

Explanation: Load current produces the ampere-turns that establish leakage flux, so leakage flux increases approximately with load current.

• A. no losses and magnetic leakage
• B. interleaved primary and secondary windings
• C. a common core for its primary and secondary windings
• D. core of stainless steel and winding of pure copper metal
• E. none of the above

Explanation: An ideal transformer assumes zero winding and core losses, infinite permeability, and no leakage flux.

• A. Frequency
• B. Voltage
• C. Current
• D. Any of the above

Explanation: An ordinary transformer changes voltage and current according to its turns ratio, but it does not change the supply frequency.

• A. Conservator
• B. Breather
• C. Bushings
• D. Buchholz relay

Explanation: The conservator provides space for transformer oil to expand and contract as its temperature changes.

• 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…