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.
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- A. sometimes 2-layer type
- B. never 2-layer type
- C. always 2-layer type
- D. none of the above
Explanation: D.C. windings in transformer terminology are formed as two-layer windings, with the conductors arranged in two radial layers.
Correct answer: always 2-layer type- A. Cylindrical winding with circular conductors
- B. Cross-over winding with either circular or small rectangular conductors
- C. Continuous disc type winding with rectangular conductors
- D. All of the above types
Explanation: High-voltage transformer windings may be cylindrical with circular conductors, crossover type with circular or small rectangular…
Correct answer: All of the above types- A. 0.5 to 0.8 Wb/m2
- B. 0.8 to 1.0 Wb/m2
- C. 1.1 to 1.35 Wb/m2
- D. 1.4 to 1.8 Wb/m2
Explanation: Hot-rolled silicon steel is generally operated at a maximum flux density of about 1.1 to 1.35 Wb/m² in distribution transformers.
Correct answer: 1.1 to 1.35 Wb/m2- A. 1.0 to 1.2 A/mm2
- B. 1.5 to 2.0 A/mm2
- C. 2.2 to 3.2 A/mm2
- D. 3.2 to 4.2 A/mm2
Explanation: Large transformers with oil cooling or air-blast cooling commonly use a conductor current density of about 2.2 to 3.2 A/mm².
Correct answer: 2.2 to 3.2 A/mm2- A. 1 + 0.2 kVmm
- B. 2 + 0.5 kVmm
- C. 4 + 0.7 kV mm
- D. 5 + 0.9 kV mm
Explanation: The commonly used empirical expression for insulation thickness between LV and HV windings is approximately 5 + 0.9kV mm, with voltage…
Correct answer: 5 + 0.9 kV mm- A. low voltage transformers
- B. medium voltage transformers
- C. high voltage transformers
- D. any of the above
Explanation: Voltage surges become especially dangerous in high-voltage transformers because insulation stress rises sharply with transient…
Correct answer: high voltage transformers- A. minor insulation
- B. major insulation
- C. either of the above
- D. none of the above
Explanation: Major insulation separates the windings from the grounded core or tank and also separates windings of different voltage levels.
Correct answer: major insulation- A. 0.1 to 0.2 mm
- B. 0.2 to 0.3 mm
- C. 0.3 to 0.4 mm
- D. 0.4 to 0.5 mm
Explanation: Armature laminations in ordinary DC machines are commonly about 0.4 to 0.5 mm thick.
Correct answer: 0.4 to 0.5 mm- A. purely D.C. control systems
- B. purely AC. control systems
- C. both D.C. and AC. control systems
- D. none of the above
Explanation: A DC servomotor is designed for systems in which the actuating and control signals are DC, whereas AC servomotors are used for AC control…
Correct answer: purely D.C. control systems- A. upto 20 A
- B. upto 40 A
- C. upto 60 A
- D. upto 80 A
Explanation: Cylindrical windings made with circular conductors are suitable for relatively high currents, commonly rated up to about 80 A.
Correct answer: upto 80 A- A. Traction
- B. Drives for process industry
- C. Battery driven vehicles
- D. Automatic control
- E. All of the above
Explanation: D.C. motors are used in traction, industrial process drives, battery-powered vehicles and automatic-control systems because they provide…
Correct answer: All of the above- A. main poles
- B. interpoles
- C. frame
- D. all of the above
Explanation: The stator of a D.C. machine includes the frame, main poles and interpoles.
Correct answer: all of the above- A. Mild steel
- B. Copper
- C. Aluminium
- D. Cast-iron
Explanation: Brush rockers are commonly made of cast iron because it provides adequate mechanical strength and rigidity while supporting the brush…
Correct answer: Cast-iron- A. Metal graphite brushes
- B. Hard carbon brushes
- C. Electro-graphitic brushes
- D. Natural graphite brushes
Explanation: Metal-graphite brushes can carry high current density because the metallic content gives them low electrical resistance, while graphite…
Correct answer: Metal graphite brushes155. Which of the following problem arises in D.C. motors which are fed from thyristor bridge circuits ?
- A. Increased I R losses
- B. Increased core losses
- C. Poor commutation
- D. Change in motor parameters
- E. All of the above
Explanation: Thyristor bridge supplies produce ripple and harmonics in the motor current, causing higher copper and core losses and often poorer…
Correct answer: All of the above- A. less than
- B. equal to
- C. greater than
- D. none of the above
Explanation: In the usual simplex lap-wound D.C. machine, one brush arm is provided for each pole, so the number of brush arms equals the number of…
Correct answer: equal to- A. worst, ventilation
- B. poor ventilation
- C. better ventilation
- D. none of the above
Explanation: A large air gap leaves more space around the armature, allowing air to circulate more freely and improving ventilation.
Correct answer: better ventilation- A. between 5 to 10 mm
- B. between 10 to 15 mm
- C. between 15 to 20 mm
- D. between 25 to 35 mm
Explanation: The usual D.C. armature slot pitch is kept around 25 to 35 mm. A very small pitch makes slots and teeth difficult to construct, while an…
Correct answer: between 25 to 35 mm- A. quiet
- B. noisy
- C. either of the above
- D. none of the above
Explanation: A larger air gap reduces the magnetic pull between the stator and armature, which lowers vibration and magnetic noise. Thus, D.C.
Correct answer: quiet- A. equal to that of armature mmf
- B. less in comparison with the armature mmf
- C. large in comparison with the armature mmf
- D. none of the above
Explanation: A strong field mmf dominates the armature mmf, so the armature reaction produces less relative distortion of the main field.
Correct answer: large in comparison with the armature mmf