Transformers Questions and Answers – Excitation Phenomenon

This set of Transformers online quiz focuses on “Excitation Phenomenon”.

1. No-load current in the transformer is _______
a) Straight DC
b) Steps
c) Sinusoidal
d) Sinusoidal distorted
View Answer

Answer: d
Explanation: The no-load current in a transformer is non-sinusoidal. The basic cause for this phenomenon, which lies in hysteresis and saturation non-linearities of the core material, will now be investigated; this can only be accomplished graphically.

2. The main reason for generation of harmonics in a transformer is ____________
a) Fluctuating load
b) Poor insulation
c) Mechanical vibrations
d) Saturation of core
View Answer

Answer: d
Explanation: The iron core which is used in transformer is subjected to saturation effect. Thus, according to the hysteresis loop, the generation of harmonics at particular saturation level can be identified.

3. The magnetising current of a transformer is usually small because it has _______
a) Small air gap
b) Large leakage flux
c) Laminated silicon steel core
d) Fewer rotating parts
View Answer

Answer: a
Explanation: Air Gap increases the reluctance in the magnetic path of magnetic lines of force. When there is an air gap in the magnetic circuit the reluctance is high owing to the permeability of air which is much lower as compared to ferromagnetic materials. The mmf required to overcome that (maintaining flux density in the air gap) is more.

4. Harmonics in transformer result in ___________
a) Increased core losses
b) Increased I2R losses
c) Magnetic interference with communication circuits
d) Increased core lossed, Increased I2R losses and magnetic interference
View Answer

Answer: d
Explanation: harmonics are produced in an particular circuit as a result of magnetization of core material. Thus, magnetic losses are increased that is, iron losses are more so to maintain constant more current will be drawn giving rise to I2 losses.

5. For given applied voltage, with the increase in frequency of the applied voltage ___________
a) Eddy current loss will decrease
b) Eddy current loss will increase
c) Eddy current loss will remain unchanged
d) Cannot be determined
View Answer

Answer: b
Explanation: Eddy current losses are directly proportional to the square of the frequency of input applied rated voltage. This loss is independent of voltage applied to the transformer. Thus, eddy current loss will increase with frequency.
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6. Which of the following is the most likely source of harmonics in a transformer?
a) Poor insulation
b) Overload
c) Loose connections
d) Core saturation
View Answer

Answer: d
Explanation: Core material used in a transformer is iron, which undergoes saturation when it is excited by some external supply, which causes some unlikely effects like generation of harmonics which leads in losses for a particular transformer.

7. Third harmonic currents can be as large as ______________
a) 100% of fundamental
b) 10% of fundamental
c) 70% of fundamental
d) 40% of fundamental
View Answer

Answer: d
Explanation: While odd symmetry is preserved and the current and flux maximas occur simultaneously, the current zeros are advanced in time with respect to the flux wave shape. As a consequence, the current has fundamental and odd harmonics, the strongest being the third harmonic which can be as large as 40% of the fundamental.

8. Flux transient goes till the maximum value due to ___________
a) Doubling effect
b) Saturation effect
c) Hysteresis effect
d) Modulation effect
View Answer

Answer: a
Explanation: A transient flux component (off-set flux) ft = fm originates so that the resultant flux is (ft + fss) which has zero value at the instant of switching. The transient component ft will decay according to the circuit time constant (L/R) which is generally low in a transformer. If the circuit dissipation (core-loss) is assumed negligible, the flux transient will go through a maximum value of 2fm.

9. The transformer switching transient is referred as _____________
a) Inrush current
b) Outrush current
c) Middlerush current
d) Harmonic current
View Answer

Answer: a
Explanation: In subsequent half-periods ft gradually decays till it vanishes and the core flux acquires the steady-state value, Because of the low time constant of the transformer circuit, distortion effects of the transient may last several seconds. The transformer switching transient is referred to as the inrush current.

10. Which of the following is not the method of reducing harmonics?
a) Adding filters
b) Adding capacitors
c) Adding inductors
d) Adding resistance
View Answer

Answer: d
Explanation: Reduce the harmonic currents produced by the load. Add filters to siphon the harmonic currents off the system, or block the currents from entering the system, or supply the harmonic currents locally. Modify the frequency response of the system by filters, inductors, or capacitor.

11. It is possible to reduce harmonic currents completely.
a) True
b) False
View Answer

Answer: b
Explanation: It is not possible to prevent harmonic currents completely. But they can be prevented from flowing through the main system by providing a separate low-impedance path for them. This is done by the use of rated series tuned circuits consisting of a reactor and capacitor, which have equal impedance at a specific harmonic frequency.

Sanfoundry Global Education & Learning Series – Transformers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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