Free Electrical Machines MCQs with Answers

• A. Its secondary current is 5 A
• B. Its secondary voltage is 400 V
• C. Its rating is 2 kVA
• D. Its secondary current is 20 A
• E. It is a step-up transformer

Explanation: For an ideal transformer, V₂/V₁ = N₂/N₁ = 2, so V₂ = 400 V, while current changes inversely to 5 A.

• A. hysteresis and eddy current losses
• B. friction and windage losses
• C. copper losses
• D. none of the above

Explanation: At fixed applied voltage and frequency, hysteresis and eddy-current losses depend mainly on the core flux and remain nearly constant as…

• A. friction and windage losses
• B. magnetic losses
• C. hysteresis and eddy current losses
• D. copper losses

Explanation: Friction and windage losses require moving mechanical parts, so they occur in rotating machines but not in a stationary transformer.

• A. eddy current loss will decrease
• B. eddy current loss will increase
• C. eddy current loss will remain unchanged
• D. none of the above

Explanation: With applied voltage fixed, core flux is approximately proportional to 1/frequency.

• A. to withstand the high voltage drop due to line surge produced by the shunting capacitance of the end turns
• B. to absorb the line surge voltage and save the winding of transformer from damage
• C. to reflect the line surge and save the winding of a transformer from damage
• D. none of the above

Explanation: A line surge produces a nonuniform voltage distribution in the end turns because of the winding's shunt capacitance, placing excessive…

• A. core built-up of laminations of cold rolled grain oriented steel
• B. core built-up of laminations of hot rolled sheet
• C. either of the above
• D. none of the above

Explanation: Cold-rolled grain-oriented steel provides high permeability along the grain direction and lower hysteresis and eddy-current losses than…

• A. zoom
• B. hum
• C. ringing
• D. buzz

Explanation: The familiar low-frequency noise of a transformer is called hum, mainly caused by magnetostriction and the resulting vibration of the core…

• A. Bushings
• B. Core
• C. Primary winding
• D. Secondary winding

Explanation: Bushings pass the transformer conductors safely through the earthed tank and are mounted externally, so they can be seen from outside.

• A. Cellulose
• B. Asbestos
• C. Mica
• D. Glass fibre

Explanation: Mica has excellent dielectric strength and a very high temperature capability, commonly associated with Class C insulation.

• A. short-circuit test
• B. back-to-back test
• C. open circuit test
• D. any of the above

Explanation: The back-to-back, or Sumpner, test uses two identical transformers to reproduce full-load conditions while measuring losses, temperature…

• A. Horn gaps
• B. Thermal overload relays
• C. Breather
• D. Conservator

Explanation: Horn gaps provide a low-resistance path for lightning and switching surges, diverting them safely to earth.

• A. A good voltage regulation of a transformer means
• B. output voltage fluctuation from no load to full load is least
• C. output voltage fluctuation with power factor is least
• D. difference between primary and secondary voltage is least
• E. difference between primary and secondary voltage is maximum

Explanation: Good voltage regulation means the secondary-terminal voltage changes very little between no-load and full-load conditions.

• A. 0.8 leading power factor
• B. 0.8 lagging power factor
• C. zero power factor
• D. unity power factor

Explanation: With load current fixed, transformer losses are essentially fixed, while useful output is proportional to power factor.

• A. on full-load
• B. on overload
• C. on leading power factor
• D. on zero power factor

Explanation: At a leading power factor, the voltage drop caused by transformer leakage reactance can be offset by the resistive drop, bringing…

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.