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. Total m.m.f. required
- B. The number of turns in the exciting winding
- C. The way in which the winding is distributed
- D. All of the above
Explanation: Magnetizing current follows approximately from I = MMF/N, so it depends on the required total MMF and the number and distribution of…
Correct answer: All of the above- A. One sided axial ventilation
- B. Two sided axial ventilation
- C. Multiple inlet system
- D. All of the above
Explanation: Air-cooled turbo-alternators may use one-sided axial ventilation, two-sided axial ventilation, or multiple-inlet ventilation depending on…
Correct answer: All of the above- A. The current in the slot conductors is uniformly distributed over their cross-section
- B. The leakage path is straight across the slot and around the iron at the bottom
- C. The permanence of air paths is only considered. The reluctance of iron paths is assumed as zero
- D. All of the above
Explanation: Slot-leakage calculations assume uniformly distributed slot current, a simplified straight leakage path, and negligible iron reluctance so…
Correct answer: All of the above- A. Equivalent power method
- B. Equivalent current method
- C. Method of average losses
- D. Equivalent torque method
Explanation: The method of average losses uses the mean loss over the duty cycle and does not directly account for the peak temperature rise…
Correct answer: Method of average losses- A. heating time constant
- B. cooling time constant
- C. either A. or B.
- D. none of the above
Explanation: For exponential heating, the temperature reaches 1 − e⁻¹ = 0.632 of its final value after one heating time constant.
Correct answer: heating time constant- A. 200 MW
- B. 300 MW
- C. 400 MW
- D. 600 MW
Explanation: Direct water cooling removes heat very effectively from the conductors, allowing large turbo-generators to reach ratings of roughly 600…
Correct answer: 600 MW- A. 2 to 3 times greater
- B. 3 to 4 times greater
- C. 4 to 5 times greater
- D. none of the above
Explanation: In a self-cooled motor, the fan or rotating parts provide much less airflow when the motor is stopped, so cooling is slower at standstill.
Correct answer: 2 to 3 times greater- A. no mechanical difficulties
- B. lesser mechanical difficulties
- C. greater mechanical difficulties
- D. none of the above
Explanation: Water passing through a rotating rotor winding requires reliable insulation, seals, joints and rotating connections, creating greater…
Correct answer: greater mechanical difficulties- A. solid
- B. hollow
- C. perforated
- D. any ofthe above
Explanation: In direct hydrogen cooling, hydrogen must flow through the conductors to remove heat from the winding interior, so both stator and rotor…
Correct answer: hollow- A. "supercharged"
- B. "inner cooled"
- C. "conductor cooled"
- D. any of the above
Explanation: Direct cooling carries the coolant directly through or along the conductors, so such machines are described as supercharged, inner-cooled…
Correct answer: any of the above- A. 0.07
- B. 1.5
- C. 2.5
- D. 3.5
Explanation: Hydrogen has a relative density of approximately 0.07 compared with air, making it much lighter.
Correct answer: 0.07- A. Increase in efficiency
- B. Increase in ratings
- C. Increase in life
- D. Smaller size of coolers
- E. All of the above
Explanation: Hydrogen cooling reduces windage loss, improves heat transfer and permits a more compact cooler, thereby increasing efficiency and rating…
Correct answer: All of the above- A. Conduction
- B. Convection
- C. Radiation
- D. All of the above
Explanation: Heat can leave an electrical machine by conduction through solid parts, convection to the surrounding fluid and radiation from exposed…
Correct answer: All of the above- A. air
- B. hydrogen
- C. either A. or B.
- D. none of the above
Explanation: Hydrogen is much less dense than air, so a rotor experiences less windage and produces less aerodynamic noise while rotating in it.
Correct answer: hydrogen- A. 2
- B. 3
- C. 5
- D. 7
Explanation: Hydrogen conducts heat about seven times better than air, which makes it effective for removing heat from large turbo-alternators.
Correct answer: 7- A. Direct
- B. Indirect
- C. Conventional
- D. Any of the above
Explanation: Direct cooling circulates the cooling medium within or in direct contact with the winding conductors, so heat does not have to cross the…
Correct answer: Direct- A. 50 MW
- B. 100 MW
- C. 200 MW
- D. none of the above
Explanation: Conventional hydrogen cooling is suitable for turbo-alternators up to roughly 200 MW per unit.
Correct answer: 200 MW- A. 1.5
- B. 2.5
- C. 3.5
- D. 4.5
Explanation: Hydrogen has a heat-transfer coefficient approximately 1.5 times that of air under comparable conditions, allowing it to remove heat more…
Correct answer: 1.5- A. 10 MW
- B. 30 MW
- C. 60 MW
- D. 150 MW
Explanation: A multiple-inlet air-cooling arrangement improves the distribution of cooling air and is commonly used for turbo-alternators rated up to…
Correct answer: 60 MW- A. conduction
- B. convection
- C. radiation
- D. any ofthe above
Explanation: Air blasts increase the movement of air over the heated surface, thereby increasing forced convection.
Correct answer: convection