All Free Electrical Engineering MCQs with Answers

• 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.

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

Explanation: From the transformer ratio, V is proportional to the number of turns, so the high-voltage winding requires more turns than the low-voltage…

• A. is always a low voltage winding
• B. is always a high voltage winding
• C. could either be a low voltage or high voltage winding
• D. none of the above

Explanation: The primary winding is the winding connected to the supply, so it may be either the high-voltage or low-voltage winding depending on the…

• A. there is no need to change the D.C. voltage
• B. a D.C. circuit has more losses
• C. Faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero
• D. none of the above

Explanation: A transformer requires changing magnetic flux to induce an emf, but steady DC produces essentially constant flux after switching.

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

Explanation: For the same power, the low-voltage winding carries higher current than the high-voltage winding.

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

Explanation: In the standard short-circuit test, the low-voltage winding is short-circuited and a reduced voltage is applied to the high-voltage…

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

Explanation: A conservator, breather, and Buchholz relay are associated with oil-immersed transformer installations.

• A. provide support to windings
• B. reduce hysteresis loss
• C. decrease the reluctance of the magnetic path
• D. reduce eddy current losses

Explanation: Iron has much higher permeability than air, so its inclusion greatly decreases the reluctance of the magnetic path and allows flux to pass…

• A. lags behind the voltage by about 75°
• B. leads the voltage by about 75°
• C. lags behind the voltage by about 15°
• D. leads the voltage by about 15°

Explanation: The no-load current is mainly magnetising current, which lags the applied voltage by nearly 90°, but the in-phase core-loss component…

• A. copper losses = hysteresis losses
• B. hysteresis losses = eddy current losses
• C. eddy current losses = copper losses
• D. copper losses = iron losses

Explanation: Transformer efficiency is maximum when variable copper loss equals the nearly constant iron loss.

• A. 1.7 Wb/m2
• B. 2.7 Wb/m2
• C. 3.7 Wb/m2
• D. 4.7 Wb/m2

Explanation: Cold-rolled grain-oriented steel commonly permits a maximum flux density of roughly 1.7 tesla, or 1.7 Wb/m².

• A. temperature
• B. stray losses
• C. all-day efficiency
• D. none of the above

Explanation: Sumpner’s, or back-to-back, test subjects two identical transformers to full-load conditions and is used to determine temperature rise as…

• A. lkV
• B. 33 kV
• C. 100 kV
• D. 330 kV

Explanation: Good transformer oil has a dielectric strength of approximately 30 to 35 kV for the standard test gap, so 33 kV is the suitable value.

• A. copper loss
• B. magnetising current
• C. magnetising current and loss
• D. efficiency of the transformer

Explanation: The no-load, or open-circuit, test measures the small no-load current and the power drawn, which is approximately the transformer’s iron…

• A. high resistance
• B. high reluctance
• C. low resistance
• D. low reluctance

Explanation: A magnetic circuit should offer low reluctance so that the required flux is produced with a small magnetising force.