Electrical Machines Mcqs


Two tranformer having same voltage but different KVA are operating in parallel. For a good load sharing __________

A) impedance must be equal B) pu impedances must be equal
C) pu impedance and X/R ratios must be equal D) impedance and X/R ratios must be equal
Answer & Explanation Answer: C) pu impedance and X/R ratios must be equal

Explanation: Both the conditions are to be met in order to have an efficient operation while in parallel condition.

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When a transformer winding suffers a short circuit, then inter turns of the same winding experience ______ forces.

A) an attractive B) a repulsive
C) no force D) may be attractive or repulsive type
Answer & Explanation Answer: A) an attractive

Explanation: As the current will flow in one direction, attractive force will appear across the inter turns as per Biot-savartís law.

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In a transformer, we place the tapping windings at the middle because __________

A) of radial forces on the windings B) axial forces on the windings
C) insulation level of the winding D) to provide a mechanical balance to the windings
Answer & Explanation Answer: B) axial forces on the windings

Explanation: Due to two different voltages, different currents will flow in the windings and it will cause the axial forces due to currents interaction appear causing damage.

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The tappings on the transformers is provided on __________

A) h.v side at one end of the winding B) l.v. side at one end of the winding
C) h.v side at middle D) l.v side at middle
Answer & Explanation Answer: C) h.v side at middle

Explanation: High voltage side winding has more voltage so the variation of the voltage will be more and better applicable. And it is better to place at the middle because it gives uniform voltage distribution by eliminating axial forces.

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If a transformer is at no load, then it will act like __________

A) a resistor at p.f = 0 B) an inductive reactor at 0.2 lagging
C) a capacitive reactor at0.2 leading D) an inductive reactor at 0.8 lagging
Answer & Explanation Answer: B) an inductive reactor at 0.2 lagging

Explanation: Transformer is nothing but the arranged windings which are magnetically coupled. The windings will inductive predominantly with very low resistance.

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A 1-phase tranformer has a leakage impedance of 1+ j4 ? for primary and 3+ j11 ? for secondary windings. This transformer has __________

A) H.V primary B) Medium voltage primary
C) L.V primary D) L.V secondary
Answer & Explanation Answer: C) L.V primary

Explanation: The side which has lower impedance will have lower number of turns and so the low voltage side.

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Maximum efficiency of a transformer for a constant load current, occurs at __________

A) at any p.f B) zero p.f leading
C) zero p.f lagging D) unity p.f
Answer & Explanation Answer: D) unity p.f

Explanation: Efficiency = KVA*p.f/(KVA*p.f + Losses); So the efficiency is maximum at unity power factor.

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Three transformers having identical dimensions but with core of iron, aluminium and wood are wound with same number of turns and have same supply. Then choose the order for eddy current losses.

A) wood > aluminium > iron B) aluminium > iron > wood
C) iron > wood > aluminium D) iron > aluminium > wood
Answer & Explanation Answer: A) wood > aluminium > iron

Explanation: The eddy current losses are dependent on resistance offered to the currents. Wood is an insulator so it will get heated up most.

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