Electrical Machines Multiple Choice Questions on “Transformer Phasor Diagram”.
1. The output voltage seen at the CRO connected at the secondary terminals is square wave. Then the flux density used for energizing the primary is?
A. triangular
B. sinusoidal
C. saw tooth wave
D. square wave
Answer: A
Clarification: E = -N*dϕ/dt. Integration of square wave will be triangular in nature.
2. The non linear magnetization curve of a practical transformer will introduce _____________
A. heating
B. higher order harmonics
C. magnetostriction
D. all of the mentioned
Answer: D
Clarification: If the magnetization is non linear in nature then it will cause a saturation in the core and harmonics will be introduced to cause humming sounds.
3. Transformer action requires a ___________
A. constant magnetic flux
B. increasing magnetic flux
C. alternating magnetic flux
D. alternating electric flux
Answer: C
Clarification: As per the Faraday’s laws, the emf will be induced when flux is time varying as in transformer, there is no moving part.
4. If a transformer is fed from a 220V and dc supply rather than a 1-phase ac supply, then the transformer will ___________
A. burn its windings
B. operate normal
C. will not operate
D. will give very small leakage flux
Answer: A
Clarification: Dc is nothing but ac at zero frequency. So the reactance offered will be zero at dc and the current will be limited only by the small resistance of the winding which will produce very high amount of current to flow through the windings, so burning them up.
5. In an ideal transformer, the impedance can be transformed from one side to the other ___________
A. in direct proportion to square of turns-ratio
B. in direct proportion to turns-ratio
C. in inverse proportion to square turns-ratio
D. in inverse proportion to turns-ratio
Answer: A
Clarification: Impedance is transformed in square of the turns-ratio.
6. A transformer has sometimes more than two ratings depending upon the use of ___________
A. the cooling application
B. type of windings
C. type of core
D. type of insulation to be given
Answer: A
Clarification: Yes, with different types of the cooling methods, the losses can be varied and so the current and the voltages.
7. Considering a transformer at no load is excited at rated voltage. A small recognizable gap is made in the yoke of the limbs. With this altercation, the transformer core flux ___________
A. will decrease and magnetizing current will increase
B. will remain constant and magnetizing current will increase
C. as well as magnetizing current will increase
D. as well as magnetizing current both will decrease
Answer: B
Clarification: The reluctance of the path increases here after making the cut. so the magnetizing current will increase but the core flux will be same as it depends on the applied voltage not the reluctance.
8. In an oil filled transformer, the application of oil is for ___________
A. cooling
B. insulation
C. both cooling and insulation
D. preventing the accumulation of dust
Answer: C
Clarification: Oil can be used not only for cooling but also for insulation from the metallic parts.
9. We laminate transformer core to reduce ___________
A. eddy current loss
B. hysteresis loss
C. both eddy current and hysteresis loss
D. ohmic loss
Answer: A
Clarification: Laminations provide larger area so that the current path increases and current reduces.
10. Which of the following two are matched correctly?
I. Core flux - Depends on applied voltage II. Leakage flux - Depends on winding current
A. I
B. II
C. I, II
D. none of the mentioned
Answer: A
Clarification: Both the statements are correct, as the leakage flux depends on the current flowing in the respective winding and the core flux depends on the voltage.
11. In case of a power transformer, the no load current in terms of rated correct is?
A. 10-20%
B. 2-6%
C. 15-20%
D. 30-50%
Answer: B
Clarification: The air gap is very less in the transformer, so the magnetizing current needed is around 2-6% only.
12. Energizing the transformer primary from a triangular wave voltage source makes the output voltage as __________
A. zero
B. a sine wave
C. a triangular wave
D. a pulsed wave
Answer: C
Clarification: Output will follow the shape of input wave. As the primary input is triangular in shape, its secondary will also be same.
13. In a single phase transformer,the no-load current lags the applied voltage by __________
A. 90°
B. about 75°
C. 0
D. about 110°
Clarification: Check the phasor diagram.
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14. A 400/200V transformer has total resistance of 0.04 p.u on its L.V side. This resistance when referred to H.V side would be?
A. 0.04
B. 0.02
C. 0.01
D. 0.08
Answer: A
Clarification: The per unit value does not change from the winding side of a transformer.
15. Energising the transformer primary from a triangular wave flux makes the output voltage as __________
A. square wave shifted by 90°
B. a sine wave
C. a triangular wave
D. a square wave
Answer: D
Clarification: E = -N*dϕ/dt.
Differentiation of triangular wave results in square wave.