Free Electrical Machines MCQs with Answers
1,061 Electrical Machines MCQs from Electrical Engineering, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
Last updated
1,061 questions · page 53 of 54
- 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…
Correct answer: High voltage winding- 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…
Correct answer: could either be a low voltage or high voltage winding- 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.
Correct answer: Faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero- 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.
Correct answer: Low 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…
Correct answer: Low voltage side- A. Conservator
- B. Breather
- C. Buchholz relay
- D. Exciter
Explanation: A conservator, breather, and Buchholz relay are associated with oil-immersed transformer installations.
Correct answer: Exciter- 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…
Correct answer: decrease the reluctance of the magnetic path- 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…
Correct answer: lags behind the voltage by about 75°- 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.
Correct answer: copper losses = iron losses- 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.
Correct answer: 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…
Correct answer: temperature- 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.
Correct answer: 33 kV- 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…
Correct answer: magnetising current and loss- 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.
Correct answer: low reluctance- A. 0.2 to 0.5 per cent
- B. 2 to 5 per cent
- C. 12 to 15 per cent
- D. 20 to 30 per cent
Explanation: Transformer no-load current is normally only about 2 to 5 percent of full-load current, although it can vary with transformer size and…
Correct answer: 2 to 5 per cent- A. tightness of clamping
- B. gauge of laminations
- C. size of laminations
- D. all of the above
Explanation: Transformer lamination vibration is affected by clamping tightness, lamination thickness or gauge, and lamination size.
Correct answer: all of the above- A. reduce hysteresis loss
- B. reduce eddy current losses
- C. reduce copper losses
- D. reduce all above losses
Explanation: Thin insulated laminations interrupt the circulating paths in the iron core, thereby reducing eddy-current loss.
Correct answer: reduce eddy current losses- A. through cooling coil
- B. through air
- C. by the flux
- D. none of the above
Explanation: The alternating primary current produces magnetic flux in the core, and changing mutual flux induces the secondary voltage, transferring…
Correct answer: by the flux- A. Current
- B. Voltage
- C. Frequency
- D. All of the above
Explanation: A transformer changes voltage and current according to its turns ratio, but it operates at the same frequency as the applied supply.
Correct answer: Frequency- A. Input power
- B. Losses
- C. True component of power
- D. kWh consumed
- E. Ratio of output to input
Explanation: Efficiency is output power divided by input power, so reducing losses allows a larger fraction of the input to appear as useful output.
Correct answer: Losses