Top Electrical Interview Questions On Transformer for Steel Plant/Power plant/Cement Plant/Construction
TRANSFORMER
1. What is the function of a transformer?
Ans: Transformers are energy converting devices, converting AC electrical energy with one level of voltage and current, to AC electrical energy with another level of voltage and current.
2. Mention the difference between core and shell type transformers.
Ans: In core type, the windings surround the core considerably and in shell type the core surround the winding.
3. What is the purpose of laminating the core in transformers?
Ans: To reduce eddy current loss.
4. Give the emf equation of a transformer and define each term
Ans: Emf induced in primary coil E1 = 4.44 fFmN1 volt
Emf induced in secondary coil E2 = 4.44fFmN2 volt
Where,
f is the frequency of AC input
Fm is the maximum value of flux in the core
N1, N2 are the number of primary and secondary turns.
5. Does the transformer draw any current when secondary is open? Why?
Ans: Yes, it (primary) will draw the current from the main supply in order to magnetize the core and to supply iron and copper losses on no load. There will not be any current in the secondary since secondary is open.
6. Define all day efficiency of a transformer.
Ans: It is the computed on the basis of energy consumed during a certain period, usually a day of 24 hrs.
All day efficiency = output in kWh for 24 hrs /input in kWh for 24 hrs.
7. Define voltage regulation of a transformer
Ans: The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full load voltage is termed as regulation.
% regulation = (0V2-V2) x 100/0V2
8. Full load copper loss in a transformer is 1600 watts. What will be the loss at half load?
Ans: If x is the ratio of actual load to full load then copper loss = x2(full load copper loss).Here Wc = (0.5)2 x 1600 = 400 watts
9. Why transformers are rated in kVA ?
Ans: Copper loss of a transformer depends on current and iron loss on voltage. Hence total losses depend on Volt- Ampere and not on the power factor. That is why the rating of transformers is in KVA and not in KW.
10. Why are breathers used in transformers?
Ans: Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil. Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases. Also to avoid sledging of oil i.e. decomposition of oil. Addition of 8 parts of water in 1000000 reduces the insulations quantity of oil. Normally silica gel is filled in the breather having pink color. This color will be changed to white due to continuous use, which is an indication of bad silica gel, it is normally heated and reused.
11. A 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary winding. Calculate the number of turns on its primary.
Ans: We know V1 / V2 = k = N2 / N1
Substituting 400/1100 = 100/N1
N1 = 100/400 x 1100 = 275 turns.
12. What is meant by turns ratio in transformer?
Ans: Turns ratio in transformers, K is the ratio of number of turns in the secondary
winding T2 to number of turns in the primary winding T1
K = T2/T1
13. What are the functions of no-load current in a transformer?
Ans: No-load current produces flux and supplies iron loss and copper loss on no-load.
14. Can the voltage regulation of a transformer go to negative? If so under what condition?
Ans: Yes. If the load has leading power factor.
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15. Why are cooling tubes provided in transformer tanks?
Ans: By providing cooling tubes, oil circulation and hence heat dissipation can further be improved by providing cooling tubes in two or all four walls of the transformer tanks.
16. What is the purpose of conducting open circuit and short circuit tests in transformers?
Ans: Open circuit Test : (i) To find out the equivalent circuit parameters R0 & X0 or no load resistance and reactance
ii) To find out the Iron loss of the transformer
Short circuit Test : (i) To find out the equivalent circuit parameters R01 & X01 or resistance and reactance of the transformer referred to primary or secondary
ii) To find out the copper loss of the transformer
By using these two tests we can find out the efficiency and regulation of the transformer.
17.When will a Buchholz relay operate in a transformer?
Ans: Buchholz rely is a protective device in a transformer. If the temperature of the coil exceeds its limit, Buchholz relay operates and gives an alarm.
18. List out general application of transformers.
Ans: 1) Stepping-up of voltage
2) Stepping-down of voltage
3) Instrument extension
4) Electrical isolation
5) Impedance matching
6) Link between AC and DC systems
