Free Electrical Machines MCQs with Answers

• A. asbestos fiber
• B. silica sand
• C. sodium chloride
• D. silica gel

Explanation: Silica gel absorbs moisture from the air entering the transformer conservator, preventing the transformer oil and insulation from becoming…

• A. ionizing air
• B. absorbing moisture
• C. cleansing the transformer oil
• D. cooling the transformer oil

Explanation: Breather material must absorb moisture from atmospheric air before it reaches the transformer oil.

• A. reduces weight per kVA
• B. reduces iron losses
• C. reduces copper losses
• D. increases part load efficiency

Explanation: For a given transformer rating, higher flux density permits a smaller iron core and therefore reduces the weight per kVA.

• A. primary side
• B. secondary side
• C. low voltage side
• D. high voltage side

Explanation: Tappings are normally placed on the high-voltage winding because it has more turns, allowing finer voltage adjustment, and carries lower…

• A. The power factor of the two trans-formers will be different from the power factor of common load
• B. Incorrect polarity will result in dead short circuit
• C. The transformers will not share load in proportion to their kVA ratings
• D. none of the above

Explanation: Connecting parallel transformers with opposite polarity places their induced voltages in opposition, producing a very large circulating…

• A. leakage reactance
• B. per unit impedance
• C. efficiencies
• D. ratings

Explanation: For parallel transformers, load division is governed mainly by their per-unit impedances.

• A. voltage device
• B. current device
• C. power device
• D. main flux device

Explanation: A transformer operates as a constant-flux device because its applied voltage, frequency, and turns determine the core flux, which remains…

• A. to provide coupling between primary and secondary
• B. to increase the magnitude of mutual flux
• C. to decrease the magnitude of mag-netizing current
• D. to provide all above features

Explanation: The iron core provides a low-reluctance path that links the primary and secondary, increases mutual flux, and reduces the magnetizing…

• A. less will be the secondary induced e.m.f.
• B. less will be the primary induced e.m.f.
• C. less will be the primary terminal voltage
• D. none of the above

Explanation: Greater leakage flux increases the leakage reactance and causes a larger voltage drop in the secondary winding, reducing its induced…

• A. to provide free passage to the cooling oil
• B. to insulate the coils from each other
• C. both A. and (b)
• D. none of the above

Explanation: Spacers between adjacent transformer coils form channels through which cooling oil can circulate freely.

• A. has high magnitude and low power factor
• B. has high magnitude and high power factor
• C. has small magnitude and high power factor
• D. has small magnitude and low power factor

Explanation: At no load, a transformer draws only a small exciting current, most of which is magnetizing current and therefore reactive.

• A. load on it increases
• B. load on it decreases
• C. load remains constant
• D. none of the above

Explanation: When the load decreases, transformer losses and oil temperature fall, so the oil contracts and its volume decreases.

• A. at no load
• B. at 50% full load
• C. at 80% full load
• D. at full load

Explanation: Distribution transformers remain energized continuously, so their design gives good efficiency at a moderate load rather than only at full…

• A. nearly full load
• B. 70% full load
• C. 50% full load
• D. no load

Explanation: Power transformers generally operate near their rated load, so they are designed to have maximum efficiency near full load.

• A. 3000 kVA
• B. 1000 kVA
• C. 500 kVA
• D. 250 kVA

Explanation: Natural oil circulation with natural air cooling is commonly used for transformers up to about 3000 kVA in standard textbook…

• A. provide fresh air for cooling the transformer
• B. supply cooling oil to transformer in time of need
• C. protect the transformer from damage when oil expends due to heating
• D. none of the above

Explanation: The conservator provides space for transformer oil to expand when heated, preventing excessive pressure and possible tank damage.

• A. supply frequency
• B. load current
• C. power factor of load
• D. both B. and (c)

Explanation: Routine, or all-day, efficiency depends on the load current and the load power factor because output energy and copper losses vary with…

• A. 180°
• B. 120″
• C. 90°
• D. 75°

Explanation: At no load, the transformer current is mainly the magnetizing component, which is nearly 90° behind the applied voltage.

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

Explanation: With applied voltage and frequency fixed, hysteresis and eddy-current losses depend mainly on the core flux and frequency, so they remain…

• A. 100 per cent
• B. 98 per cent
• C. 50 per cent
• D. 25 per cent

Explanation: Power transformers are highly efficient because they have no mechanical losses and are designed to minimize core and copper losses.