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. fluctuating loads
- B. variable speed loads
- C. low torque loads
- D. power factor corrections
Explanation: An overexcited synchronous motor draws leading current, which supplies leading reactive power and improves a lagging system power factor.
Correct answer: power factor corrections- A. back e.m.f. is less than the applied voltage
- B. back e.m.f. is equal to the applied voltage
- C. back e.m.f. of rotor is more than the applied voltage
- D. none of the above
Explanation: Regenerative braking occurs when the motor operates as a generator and returns power to the supply, requiring its generated back EMF to…
Correct answer: back e.m.f. of rotor is more than the applied voltage- A. inlet pressure increase
- B. speed shoots up
- C. exhaust gets choked
- D. steam flow gets clogged
Explanation: A turbine is tripped by an overspeed protection system when its speed rises dangerously, preventing mechanical damage.
Correct answer: speed shoots up- A. carbon brushes
- B. coil
- C. commutator
- D. magnetic field
Explanation: A DC generator uses a split-ring commutator to rectify the internally generated AC, whereas an AC generator generally uses slip rings to…
Correct answer: commutator- A. Retaintivity
- B. Permeability
- C. Flux
- D. Hysteresis
Explanation: Hysteresis is the lag of magnetization or flux density behind the changing magnetizing field.
Correct answer: Hysteresis- A. Maximum
- B. Zero
- C. Intermediate values
- D. No back emf
Explanation: For a DC motor, back emf is proportional to speed, E_b = kΦω. Therefore, when speed is maximum under normal operating conditions, the back…
Correct answer: Maximum- A. A torque about horizontal axis
- B. A torque about vertical axis
- C. An upward force along the axis
- D. No resultant force
Explanation: Opposite currents make the coaxial coils repel, but with the small coil exactly concentric and centered, the forces on opposite sides…
Correct answer: No resultant force- A. Magnetic flux density
- B. Torque
- C. Mutual induction
- D. Motional emf
Explanation: The motional emf is e = Blv when the conductor, velocity, and magnetic field are mutually perpendicular.
Correct answer: Magnetic flux density- A. Small output
- B. Eddy currents and magnetic hysterias
- C. Soft iron core
- D. Back emf
Explanation: Transformer losses include core losses from hysteresis and eddy currents, as well as copper loss in the windings.
Correct answer: Eddy currents and magnetic hysterias- A. number of turns in the coil
- B. magnetic field strength
- C. frequency of rotation
- D. all of above
Explanation: For an AC dynamo, generated emf is proportional to the number of turns, magnetic flux or field strength, and rotational speed or…
Correct answer: all of above- A. water pumps
- B. jaw crushers
- C. centrifugal blowers
- D. none of the aboveGet Study Guides
Explanation: A belted slip-ring induction motor provides high starting torque with controlled starting current, making it suitable for heavy-load…
Correct answer: jaw crushers- A. 20 to 30 m/s
- B. 10 to 15 m/s
- C. 5 to 10 m/s
- D. 1 to 2.5 m/sCompare Power Rates
Explanation: Crane travelling mechanisms normally move at about 1 to 2.5 m/s, allowing controlled positioning of suspended loads.
Correct answer: 1 to 2.5 m/sCompare Power Rates- A. horsepower rating increases but r.p.m. decreases
- B. horsepower rating decreases but r.p.m. increases
- C. horsepower rating and operating speed increases
- D. horsepower rating and operating speed decreases
Explanation: An induction motor becomes more expensive when its horsepower increases and its speed decreases, because the lower-speed motor requires a…
Correct answer: horsepower rating increases but r.p.m. decreases- A. Type of bearings
- B. Type of enclosure
- C. Noise
- D. Arrangement for power transmis¬sion
- E. None of the above
Explanation: Low noise is especially important in air conditioners because the motor operates near occupied spaces and often runs for long periods.
Correct answer: Noise- A. A 100 H.P. AC. three phase motor
- B. Four motors of 25 H.P. each
- C. Five motors of 20 H.P. each
- D. Ten motors of 10 H.P. each
Explanation: A single 100 HP three-phase motor is usually cheaper than several motors with the same total rating because it needs fewer frames…
Correct answer: A 100 H.P. AC. three phase motor- A. Shaded pole motor
- B. Repulsion motor
- C. Capacitor start motor
- D. None of the above
Explanation: A repulsion motor has a torque-speed characteristic resembling that of a series motor, including high starting torque and a speed that…
Correct answer: Repulsion motor- A. Totally enclosed
- B. Totally enclosed fan cooled
- C. Open type
- D. Semi closed
Explanation: A totally enclosed fan-cooled enclosure prevents contact with live or rotating parts and protects against dust and moisture while the…
Correct answer: Totally enclosed fan cooled- A. r.p.m. only
- B. Horse power only
- C. Horse power and r.p.m.
- D. Horse power, r.p.m. and power factor
Explanation: Shaft size is governed mainly by the torque that must be transmitted, and torque is proportional to horsepower divided by speed.
Correct answer: Horse power and r.p.m.- A. Tapped field control
- B. Diverter field control
- C. Series-parallel control
- D. Any of the above
Explanation: A series motor's flux can be controlled by tapping the field winding, diverting part of the current around it, or using series-parallel…
Correct answer: Any of the above- A. Oil expeller
- B. Floor mill
- C. Lifts and hoists
- D. Centrifugal pump
Explanation: Lifts and hoists must accelerate suspended loads quickly and safely, so they require high starting torque and high acceleration.
Correct answer: Lifts and hoistsElectrical Machines MCQs: common questions
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