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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821. If the load on a DC shunt motor is increases, its speed is slightly reduced due to______________?
- A. increase in armature current
- B. increase in flux
- C. decrease in reactance
- D. decrease in back emf
Explanation: An increased load raises armature current, increasing the armature resistance drop.
Correct answer: decrease in back emf- A. runs as a generator
- B. does not run as a generator
- C. also runs as a motor comes to stop after sometime
Explanation: When the second DC motor is excited and mechanically driven by the first motor, its armature conductors cut magnetic flux and generate…
Correct answer: runs as a generator- A. reducing the field current
- B. decreasing the armature current
- C. increasing the armature current
- D. increasing the excitation current
- E. none of the above methods
Explanation: Speed of a shunt motor can be increased above its normal value by field weakening.
Correct answer: reducing the field current- A. the field
- B. the armature
- C. the brushes
- D. the commutator
Explanation: Iron losses are caused by alternating magnetisation of the iron core, producing hysteresis and eddy-current losses.
Correct answer: the armature- A. Series motor
- B. Shunt motor
- C. Cumulatively compounded motor
- D. Differentially compounded motor
Explanation: A series motor has the strongest automatic speed response to load changes: increased load current strengthens its flux and reduces speed…
Correct answer: Series motor- A. series motor
- B. shunt motor
- C. compound motor
- D. any of the above
Explanation: A compound motor is suitable for a load that varies from nearly zero to a short-duration peak because it combines good no-load speed…
Correct answer: compound motor- A. series motor
- B. shunt motor
- C. compound motor
- D. any of the above
Explanation: A series motor is suitable when the load remains present but periodically reaches a high peak, because its series field provides high…
Correct answer: series motor- A. drum type controller is used
- B. three point starter is used
- C. four point starter is used
- D. all above can be used
Explanation: A drum controller is designed for frequent starting, stopping, reversing and speed control of comparatively large motors.
Correct answer: drum type controller is used- A. self-induced e.m.f.
- B. mutually induced e.m.f.
- C. back e.m.f
- D. none of the above
Explanation: As a motor armature rotates, it cuts the magnetic flux and generates an e.m.f. opposing the applied voltage, so it is called back e.m.f.
Correct answer: back e.m.f- A. a stronger commutating field is needed at low speed than at high speed
- B. a weaker commutating field is needed at low speed than at high speed
- C. same commutating field is needed at low speed than at high speed
- D. none of the above is correct
Explanation: At higher speed, commutation occurs in less time, so a stronger interpole or commutating field is needed to reverse armature current…
Correct answer: a weaker commutating field is needed at low speed than at high speed- A. Plugging
- B. Regenerative braking
- C. Rheostatic braking
- D. None of the above
Explanation: Plugging applies a reverse torque and brings an elevator to rest quickly, which is important for accurate stopping at floors.
Correct answer: Plugging- A. Shunt motors
- B. Series motors
- C. Compound motors
- D. All of the above
Explanation: Dynamic braking converts the motor into a generator and dissipates the generated energy in a resistance.
Correct answer: All of the above- A. motor is run as a generator
- B. motor is reversed in direction
- C. motor is run at reduced speed
Explanation: In rheostatic or dynamic braking, the supply is disconnected and the running motor is connected as a generator feeding a braking resistor.
Correct answer: motor is run as a generator- A. no-load
- B. part load
- C. full-load
- D. overload
Explanation: Hopkinson’s test is a regenerative back-to-back test in which one machine operates as a motor and the other as a generator, normally at…
Correct answer: full-load- A. Swinburne's test
- B. Hopkinson's test
- C. Field test
- D. Brake test
Explanation: Field’s test is suitable for two similar large D.C. series motors because the machines can be mechanically coupled and tested under load…
Correct answer: Field test- A. iron losses
- B. line losses
- C. sparking
- D. corona losses
Explanation: Locating generators near load centres shortens the transmission and distribution lines, thereby reducing I²R losses in the line…
Correct answer: line losses- A. yoke
- B. commutator
- C. armature conductors
- D. armature rotor
Explanation: Iron losses consist of hysteresis and eddy-current losses, and they occur mainly in the laminated armature core as it rotates through the…
Correct answer: armature rotor- A. full-load
- B. rated r.p.m.
- C. rated voltage
- D. all of the above
Explanation: A DC generator is normally designed so that its highest efficiency occurs close to full-load operation, where its fixed losses and…
Correct answer: full-load- A. eddy current losses = stray losses
- B. hysteresis losses = eddy current losses
- C. copper losses = 0
- D. variable losses = constant losses
Explanation: Maximum efficiency occurs when the variable losses, chiefly armature copper loss, equal the constant losses, such as iron, friction and…
Correct answer: variable losses = constant losses- A. 100 W
- B. 500 W
- C. 1000 W
- D. 1500 W
Explanation: A well-designed 10 kW generator commonly has total losses of roughly 5% of its rating, giving about 0.05 × 10,000 = 500 W.
Correct answer: 500 W