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Transformer Questions and Answers

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Transformers Questions and Answers for Interview

These questions are related to the following topics:

Transformer Working Principle

Voltage Regulation of Transformer

Efficiency and Losses in Transformer

1. What are the following relationships in an ideal transformer?
a. Turns ratio and voltage ratio?
The turn’s ratio is the same as the voltage ratio.
\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{{{V}_{1}}}{{{V}_{2}}}\]
b. Voltage ratio and the current ratio?
The current ratio is the inverse of the voltage ratio.
\[\frac{{{I}_{2}}}{{{I}_{1}}}=\frac{{{V}_{1}}}{{{V}_{2}}}\]
c. Primary and secondary power?
The primary power is equal to the secondary power according to the law of conservation of energy.
2. A transformer rated at a primary voltage 4,800 volts and a secondary voltage of 240 volts what is the turn’s ratio?
\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{4800}{240}=20:1\]
3. A transformer that with a 120 volt primary and 12 volts in the secondary, and a primary with 800 turns of wire, how many turns would be required in the secondary?
\[\frac{800}{{{N}_{2}}}=\frac{120}{12}\Rightarrow {{N}_{2}}=80\]
4. With a turn’s ratio of 1:2 and a secondary voltage of 960 volts, what would be the primary voltage?

\[\begin{align}  & \frac{{{N}_{1}}}{{{N}_{2}}}=\frac{{{V}_{1}}}{{{V}_{2}}}\Rightarrow \frac{1}{2}=\frac{{{V}_{1}}}{960} \\ & {{V}_{1}}=480V \\\end{align}\]
5. The power rating of a transformer is in Volt-amps rather watts, why is that?
Both reactive and resistive power contribute to temperature rise in a transformer. For this reason, the total apparent power or VA is used to rate transformers, rather than watts, as this measurement is independent of power factor so it takes into account both the resistive and reactive factors.
6. A 60-KVA single phase transformer with a primary voltage of 2,400 volts and a secondary voltage of 240 volts.
a. List the rated current in the primary
\[S={{V}_{1}}{{I}_{1}}\Rightarrow {{I}_{1}}=\frac{60*{{10}^{3}}}{2400}=25A\]
b. list the rated current in the secondary.
\[S={{V}_{2}}{{I}_{2}}\Rightarrow {{I}_{2}}=\frac{60*{{10}^{3}}}{240}=250A\]
c. list the turns ratio.
\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{2400}{240}=10:1\]
7. Winding taps are used for which applications?
When the applied primary voltage is slightly higher or lower than its rated value, to compensate for varying line drops where occasional heavy loads are used.
8. With a secondary transformer output of 1,320 watts and a primary input of 1,800 watts, calculate the efficiency of the transformer.
\[\eta =\frac{Output}{Input}*100=\frac{1320}{1800}*100=73.33%\] 
9. A transformers secondary no-load voltage is 480 volts and has a full load voltage measuring 465 volts, the transformers regulation percentage would be what?
\[VR=\frac{{{V}_{NL}}-{{V}_{FL}}}{{{V}_{NL}}}*100=\frac{480-465}{480}*100=3.125%\] 
10. A 37.5 KVA transformer rated at 480 volts in the primary and 208 volts in the secondary, what would be the primary and secondary line current?
\[S=\sqrt{3}{{V}_{1}}{{I}_{1}}\Rightarrow {{I}_{1}}=\frac{37.5*{{10}^{3}}}{\sqrt{3}*480}=45.105A\]
\[S=\sqrt{3}{{V}_{2}}{{I}_{2}}\Rightarrow {{I}_{2}}=\frac{37.5*{{10}^{3}}}{\sqrt{3}*208}=104.08A\]
11. A transformer nameplate has the following data:
a. 35 KVA
b. 60 Hz
c. Single phase
HV 480 V
LV 240 V
Impedance 2.6%
Temperature rise 80 oC
What is the determination of the following information?
a. What would be the primary terminal markings?
H1 and H2
b. What would be the secondary terminal markings?
X1 and X2
c. What is the rated frequency?
60 Hz
d. List the values of the turn’s ratio.
\[\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{480}{240}=2:1\]
e. List the maximum permit-able current the can be delivered to the load.
\[S={{V}_{2}}{{I}_{2}}\Rightarrow {{I}_{2}}=\frac{35*{{10}^{3}}}{240}=145.8A\]
f. The current at the output terminal under a short condition would be how much? 

\[{{\text{I}}_{\text{sc}}}\text{=}\frac{\text{Seconday Current}}{\text{Percentage Impedance}}\text{=}\frac{{{\text{I}}_{\text{secondary}}}}{\text{Z }\!\!%\!\!\text{ }}\text{=}\frac{\text{145}\text{.8}}{\text{2}\text{.6 }\!\!%\!\!\text{ }}\text{=5607}\text{.69A}\]
g. When operating at full load in a 40 oC ambient temperature, the winding temperature should be what amount?
$\text{Winding Temperature=Ambient Temperatre+Temperature Rise=4}{{\text{0}}^{\text{o}}}\text{+8}{{\text{0}}^{\text{o}}}={{120}^{o}}$
12. Which winding of the transformer has the larger conductor size if a 10-KVA transformer with a primary rating of 480 V and a secondary rating of 24 V? Why?
The 24 V secondary windings because it would be rated for a higher current than the primary winding.

13. What are buck-boost transformers used for?
A buck-boost transformer can be used when the available supply voltage does not match the voltage required by the load.
14. What are three advantages of the three-phase transformer over three single-phase transformers when transferring three-phase voltage?
A single three-phase transformer is cheaper, easier to install, and will operate more efficiently than three single phase units.
15. What would safety reasons explain why the secondary circuit of an instrument transformer should be closed whenever there is primary current? Why?
If the secondary is not loaded, this transformer acts to step up the primary voltage to a dangerously high level.
16. Why is a megger preferred to an ohmmeter when measuring the insulation breakdown of a distribution transformer?
To make this resistance test for insulation breakdown, a very high voltage is necessary beyond the range of a standard ohmmeter.
17. A transformer vault serves what main functions?
Transformer vaults serve the following main purposes: Provide a means to isolate a potentially hazardous electrical component from unqualified personnel and contain any fire or combustion that may occur as a result of a transformer malfunction.
18. Give a compared analysis of the extent of overcurrent for a transformer overload condition as opposed to a short circuit condition.
A typical transformer overload condition may have a current flow that is from one to six times that of normal full load current. Short circuits currents can reach levels that are hundreds of times greater than the full load current.
19. Harmonics are created by what characteristics of non-linear loads?
The characteristics of non-linear loads that they demand current only during part of the cycle creates harmonic effects.
20. List some ways that harmonics shortens the service life of a transformer.
Harmonics shorten the service life of a transformer by causing additional heat in the transformer windings.

 

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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.