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. Primary winding
- B. Secondary winding
- C. Low voltage winding
- D. High voltage winding
Explanation: For approximately equal power, the high-voltage winding carries lower current than the low-voltage winding.
Correct answer: High voltage winding- A. hysteresis loss
- B. eddy current losses
- C. copper losses
- D. all of the above
Explanation: Adding silicon to steel increases its electrical resistivity and reduces hysteresis loss, while the use of thin laminations reduces…
Correct answer: hysteresis loss- A. load is balanced only
- B. load is unbalanced only
- C. on balanced as well as unbalanced loads
- D. none of the above
Explanation: A delta winding provides a closed path for triplen-harmonic and zero-sequence currents, while the star winding can supply an unbalanced…
Correct answer: on balanced as well as unbalanced loads- A. load is unbalanced only
- B. load is balanced only
- C. on balanced as well as unbalanced loads
- D. none of the above
Explanation: A star/star transformer is normally suitable for balanced loads, but unbalanced loading and third-harmonic currents can cause neutral…
Correct answer: load is balanced only1005. Which of the following property is not necessarily desirable in the material for transformer core ?
- A. Mechanical strength
- B. Low hysteresis loss
- C. High thermal conductivity
- D. High permeability
Explanation: Transformer-core material should have high permeability, low hysteresis loss, and adequate mechanical strength.
Correct answer: High thermal conductivity1006. Distribution transformers are generally designed for maximum efficiency around________________?
- A. 90% load
- B. zero load
- C. 25% load
- D. 50% load
Explanation: Distribution transformers remain energized continuously and are designed with core loss in mind, so their maximum efficiency is commonly…
Correct answer: 50% load- A. average value
- B. r.m.s. value
- C. maximum value
- D. instantaneous value
Explanation: The transformer emf equation is written as E = 4.44 f N Φm, where Φm is the maximum value of sinusoidal flux.
Correct answer: maximum value- A. fluctuating load
- B. poor insulation
- C. mechanical vibrations
- D. saturation of core
Explanation: When the transformer core approaches saturation, the magnetizing current becomes non-sinusoidal and contains harmonic components.
Correct answer: saturation of core- A. 50°C
- B. 80°C
- C. 100°C
- D. 150°C
Explanation: Transformer oil generally begins significant thermal dissociation and gas formation only at temperatures above about 150°C.
Correct answer: 150°C- A. 6400 W
- B. 1600 W
- C. 800 W
- D. 400 W
Explanation: Copper loss varies as the square of load current: Pcu ∝ I². At half-load it is (1/2)² × 1600 = 400 W, so the loss becomes one-fourth of…
Correct answer: 400 W- A. A Buchholz relay can be installed on
- B. auto-transformers
- C. air-cooled transformers
- D. welding transformers
- E. oil cooled transformers
Explanation: A Buchholz relay is fitted in the oil pipe between the main tank and conservator of an oil-immersed transformer, where it detects gas…
Correct answer: oil cooled transformers- A. sludge
- B. odour
- C. gases
- D. moisture
Explanation: Moisture sharply reduces transformer oil's dielectric strength and can damage the insulation system, so oil must be kept dry.
Correct answer: moisture- A. copper core
- B. cost iron core
- C. air core
- D. mild steel core
Explanation: At high frequencies, an air core avoids the large hysteresis and eddy-current losses associated with iron cores.
Correct answer: air core- A. zero
- B. 1 ohm
- C. 1000 ohms
- D. infinite
Explanation: The primary and secondary windings are electrically insulated from each other, so their insulation resistance is ideally infinite.
Correct answer: infinite- A. to project against'internal fault
- B. to reduce copper as well as core losses
- C. to cool the transformer oil
- D. to take care of the expansion and contraction of transformer oil due to variation of temperature of sur-roundings
Explanation: Transformer oil expands when heated and contracts when cooled. The conservator provides an oil reservoir to accommodate these volume…
Correct answer: to take care of the expansion and contraction of transformer oil due to variation of temperature of sur-roundings- A. 0.4 mm to 0.5 mm
- B. 4 mm to 5 mm
- C. 14 mm to 15 mm
- D. 25 mm to 40 mm
Explanation: Transformer core laminations are typically about 0.35 to 0.5 mm thick, matching the 0.4 to 0.5 mm range given.
Correct answer: 0.4 mm to 0.5 mm- A. wood
- B. copper
- C. aluminium
- D. silicon steel
Explanation: Silicon steel is used for transformer cores because its high electrical resistivity reduces eddy-current loss and its magnetic properties…
Correct answer: silicon steel- A. Bmax
- B. Bmax1-6
- C. Bmax1-83
- D. B max
Explanation: By the Steinmetz relation, hysteresis loss is approximately proportional to Bmax^1.6, where Bmax is the maximum flux density.
Correct answer: Bmax1-6- A. magnetostrication
- B. boo
- C. hum
- D. zoom
Explanation: The audible noise produced by magnetically induced lamination vibrations is called transformer hum.
Correct answer: hum- A. volts
- B. amperes
- C. kW
- D. kVA
Explanation: Transformer capacity is rated in apparent power, S = VI, measured in VA or kVA.
Correct answer: kVA