All Free Electrical Engineering MCQs with Answers
Every Electrical Engineering question in the bank, across all chapters, each with the correct answer and a written explanation. Free and unlimited, with no account needed.
3,143 questions · page 69 of 158
- A. the torque required
- B. the speed of the motor
- C. the voltage across the terminals
- D. none of the above
Explanation: For a motor with constant field flux, electromagnetic torque is proportional to armature current, as given by T = kφIa.
Correct answer: the torque required- A. 100% of the previous value
- B. 50% of the previous value
- C. 25% of the previous value
- D. 10% of the previous value
- E. none of the above
Explanation: Before magnetic saturation, a series motor has flux proportional to armature current, so T ∝ φIa ∝ Ia².
Correct answer: 25% of the previous value- A. proportional to the armature current
- B. proportional to the square of the armature current
- C. proportional to field current
- D. inversely proportional to the armature current
Explanation: In the unsaturated region of a series motor, flux is approximately proportional to armature current, while speed varies approximately as…
Correct answer: inversely proportional to the armature current- A. low
- B. around 500 Q
- C. 1000 Q
- D. infinitely large
Explanation: The motor's armature resistance is normally very low, so a starting resistor is inserted only to limit the large current produced when…
Correct answer: low- A. Blowers
- B. Conveyors
- C. Air compressors
- D. Centrifugal pumps
Explanation: Conveyors often require high starting torque because the motor must overcome static friction and start a loaded belt.
Correct answer: Conveyors1366. Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?
- A. Lenz's law
- B. Faraday's law
- C. Coloumb's law
- D. Fleming's left-hand rule
Explanation: Fleming's left-hand rule gives the direction of force on a current-carrying conductor in a magnetic field, and therefore determines the…
Correct answer: Fleming's left-hand rule- A. Slip rings
- B. Commutator
- C. Field winding
- D. Armature winding
Explanation: The field winding is stationary and generally has better heat dissipation than the rotating armature and commutator assembly.
Correct answer: Field winding- A. series motors
- B. shunt motors
- C. compound motors
- D. none of the above
Explanation: A shunt motor has nearly constant flux and comparatively low starting torque, while series and compound motors develop higher starting…
Correct answer: shunt motors- A. 100%
- B. around 90%
- C. anywhere between 75% and 90%
- D. less than 50%
Explanation: Maximum mechanical power occurs when the back e.m.f. is approximately half the supply voltage, making the developed power about half the…
Correct answer: less than 50%- A. shunt motors
- B. shunt as well as compound motors
- C. shunt, compound and series motors
- D. all D.C. motors
Explanation: A three-point starter has terminals for the line, armature, and shunt field, so it is suitable for shunt motors and compound motors.
Correct answer: shunt as well as compound motors- A. to improve cooling
- B. to reduce copper losses
- C. to increase the generated e.m.f.
- D. to reduce the sparking
Explanation: Fractional-pitch coils reduce the e.m.f. induced in a coil during commutation, making current reversal easier and reducing sparking at the…
Correct answer: to reduce the sparking- A. Starter for a car
- B. Drive for a water pump
- C. Fan motor
- D. Motor operation in A.C. or D.C
Explanation: A D.C. series motor develops very high starting torque, which is required to crank a car engine through the starter mechanism.
Correct answer: Starter for a car- A. Frame
- B. Shaft
- C. Commutator
- D. Stator
Explanation: The commutator is the distinctive mechanical rectifier of a conventional D.C.
Correct answer: Commutator- A. pumping sets
- B. air compressors
- C. electric traction
- D. machine shops
Explanation: D.C. motors are especially valuable in electric traction because their high starting torque and easy speed control suit frequent starting…
Correct answer: electric traction- A. almost negligible
- B. rated full-load current
- C. less than full-load current
- D. more than full-load current
Explanation: For maximum mechanical power in a shunt motor, the armature current is approximately half the short-circuit current, which is normally far…
Correct answer: more than full-load current- A. back e.m.f. is equal to armature drop
- B. armature drop is negligible
- C. flux is proportional to armature current
- D. flux is practically constant in D:C. shunt motors
Explanation: Motor speed is approximately proportional to back e.m.f. divided by flux, and the shunt field maintains nearly constant flux.
Correct answer: flux is practically constant in D:C. shunt motors- A. The motor will stop
- B. The motor will continue to run
- C. The armature may burn
- D. The motor will run noisy
Explanation: If back e.m.f. disappears while the supply remains connected, the opposing voltage is lost and the armature current can rise to a…
Correct answer: The armature may burn- A. up to 5 H.P
- B. up to 10 H.P
- C. up to 15 H.P
- D. up to 20 H.P
Explanation: A direct-on-line starter applies the full supply voltage at starting, causing a high inrush current, so it is generally used for small…
Correct answer: up to 5 H.P- A. independent of armature current
- B. directly proportional to the armature current
- C. proportional to the square of the current
- D. inversely proportional to the armature current
Explanation: In a shunt motor, the field winding is connected across the supply, so its flux remains nearly constant.
Correct answer: independent of armature current1380. The speed of a D.C. shunt motor more than its full-load speed can be obtained by_____________?
- A. decreasing the field current
- B. increasing the field current
- C. decreasing the armature current
- D. increasing the armature current
Explanation: Motor speed is approximately proportional to the back-emf divided by field flux.
Correct answer: decreasing the field current