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 59 of 158
- A. reduce windage losses
- B. reduce eddy currents
- C. reduce accumulation of dirt and dust
- D. reduce magnetic hum
Explanation: Slightly skewed rotor slots reduce slot harmonics and the resulting magnetic pulsations, which minimises magnetic hum.
Correct answer: reduce magnetic hum- A. 0.04%
- B. 0.4%
- C. 4%
- D. 14%
Explanation: Medium-sized induction motors commonly operate with a slip of a few percent, typically about 4%.
Correct answer: 4%- A. Bearings
- B. Shaft
- C. Statorcore
- D. None of the above
Explanation: The stator core is built from laminated silicon-steel sheets because silicon steel has low hysteresis loss and laminations limit…
Correct answer: Statorcore- A. three phase transformers
- B. compound generators
- C. D.C. series motors
- D. induction motors
Explanation: Cogging is the tendency of an induction motor to refuse starting when rotor and stator slot harmonics lock magnetically.
Correct answer: induction motors- A. two
- B. three
- C. four
- D. none
Explanation: A squirrel-cage rotor has conducting bars permanently short-circuited by end rings, so it has no slip rings.
Correct answer: none- A. 60 to 90%
- B. 80 to 90%
- C. 95 to 98%
- D. 99%
Explanation: A practical induction motor commonly achieves about 80% to 90% efficiency, depending on its rating and load.
Correct answer: 80 to 90%- A. 200 Hz
- B. 50 Hz
- C. 2 Hz
- D. 0.2 Hz
Explanation: Rotor emf frequency is f2 = sf1, so f2 = 0.04 × 50 = 2 Hz. Rotor frequency is much lower than the 50 Hz supply frequency during normal…
Correct answer: 2 Hz- A. less than 1%
- B. 1.5%
- C. 2%
- D. 4%
Explanation: At no load, the rotor runs close to synchronous speed, so only a small slip is needed to produce the torque required for losses.
Correct answer: less than 1%- A. high speed steel
- B. stainless steel
- C. carbon steel
- D. cast iron
Explanation: The shaft must withstand torsional stress, so carbon steel is commonly used because it is strong and tough.
Correct answer: carbon steel- A. silicon steel
- B. cast iron
- C. aluminium
- D. bronze
Explanation: Cast iron is commonly used for induction-motor frames because it provides mechanical strength, rigidity and good protection.
Correct answer: cast iron- A. only if the load current is zero
- B. only if the load current is not zero
- C. irrespective of the value of load current
- D. none of the above
Explanation: A series generator needs load current to flow through its series field winding before that winding can build useful flux and voltage.
Correct answer: only if the load current is not zero- A. Lenz's law
- B. Ohm's law
- C. Faraday's law of electromagnetic induction
- D. none of the above
Explanation: A generator produces emf when conductors cut magnetic flux, as described by Faraday's law of electromagnetic induction.
Correct answer: Faraday's law of electromagnetic induction- A. will be less than 250 V
- B. will always be 250 V
- C. may be greater or less than 250 V
- D. none of the above
Explanation: The full-load voltage depends on the compounding: a cumulatively compounded generator may maintain or raise voltage, while a…
Correct answer: may be greater or less than 250 V- A. 150 V
- B. less than 150 V
- C. greater than 150 V
- D. none of the above
Explanation: Increasing load current strengthens the series-field flux, so the generated emf of a series generator generally rises with load current…
Correct answer: greater than 150 V- A. only if the resistance of the field circuit is less than critical value
- B. only if the resistance of the field circuit is greater than critical value
- C. irrespective of the value of the resistance in the field circuit
- D. none
Explanation: Self-excitation requires the shunt-field resistance to be below the critical resistance at the operating speed, allowing residual…
Correct answer: only if the resistance of the field circuit is less than critical value- A. 100% regulation
- B. infinite regulation
- C. 50% regulation
- D. 1% regulation
Explanation: Voltage regulation measures the change from no-load to full-load voltage, so a smaller percentage is preferred for a generator.
Correct answer: 1% regulation- A. same kW rating
- B. the same operation r.p.m.
- C. the same drooping voltage characteristics
- D. same percentage regulation
Explanation: Drooping voltage characteristics make the load division between parallel generators stable: a generator that takes more current develops a…
Correct answer: the same drooping voltage characteristics- A. to increase the series flux
- B. to increase the generated e.m.f.
- C. to reduce the combined effect of ar-mature reaction of both the machines
- D. so that the two identical machines will pass approximately equal cur¬rents to the load
Explanation: The equalizer bar connects the armature circuits so that the series-field effects remain balanced, allowing identical generators to share…
Correct answer: so that the two identical machines will pass approximately equal cur¬rents to the load1179. Which of the following generators will be preferred if they are required to be run in parallel?
- A. Shunt generators
- B. Series generators
- C. Compound generators
- D. None of the above
Explanation: Shunt generators have a naturally drooping external characteristic, which gives stable load sharing when units operate in parallel.
Correct answer: Shunt generators- A. Self-excited generator
- B. Separately excited generator
- C. Level compounded generator
- D. All of the above
Explanation: A level-compounded generator is designed so that the rise in generated emf offsets internal voltage drops as load increases, keeping…
Correct answer: Level compounded generator