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Saturday 24 September 2011

Transformers Short Interview Questions 1

  • Why do we represent leakage flux in a transformer by inductive reactance?

Ans. In a actual transformer, there is primary leakage flux as well as secondary leakage flux. The primary leakage flux is the flux which links with the primary but not with the secondary. Similarly, the secondary leakage flux is the flux which links with the secondary but not with the primary winding. Since leakage flux links with only one winding, it produces an induced. e.m.f. in that winding. It is, therefore, equivalent to a small inductance in series with that winding.

  • Why is the efficiency of a transformer not determined by direct loading ?

Ans, The efficiency of a transformer is not determined by direct loading as this method has the following disadvantages :

(i) It requires a large supply of power and a means of dissipating it.

(ii) It gives no information regarding the proportions of various losses.

In practice, the efficiency of a transformer is determined by two simple tests viz., open-circuit test and short-circuit test.

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  • What are the advantages of open and short-circuit tests on a transformer ?

Ans. The efficiency of a transformer is always determined by open-circuit and short-circuit tests due to the following reasons :

(i) The power required to carry out these tests is very small .

(ii) These tests give the core loss and copper losses separately.

  • What are the advantages of back to back test in determining the efficiency of a transformer ?

Ans. In this method, the efficiency of a transformer is determined by putting it under full-load conditions and yet the power demand is small. This test requires two identical transformers (or, even number of identical transformers). The primary windings are connected in parallel and supplied at normal voltage and frequency and the secondary windings are connected in series opposition and supplied through a variable voltage regulator. The method offers the following advantages :

(i) The transformers are tested under full-load conditions and yet the power demand is small.

(ii) The losses can be determined very accurately.

(iii) Two large transformers can be put under full-load conditions for several hours, so that the temperature rise can be measured, with an expenditure of energy equal to that re­quired for losses only.

  • Why are iron losses constant at all loads in a transformer ?

Ans. Since the induced primary ampere-turns and secondary ampere-turns always neutralize one another, the flux in the core on load is the same as the flux on no load. Hence, the iron losses are constant and are independent of load.

  • What is the difference between power transformers and distribution transformers ?

Ans. Those transformers installed at the sending or receiving end of long high-voltage transmission lines are the power transformers. The distribution transformer (generally pole mounted) are those installed in the localities of the city to provide utilization voltage at the consumer termi­nals.

(i) Power transformers generally operate at nearly full-load. However, distribution trans­formers operate at light loads during major part of the day.

(ii) The performance of a power transformer is generally judged from commercial efficiency whereas the performance of a distribution transformer is judged from all-day efficiency

(iii) The rating of a power transformer is many times greater than that of a distribution trans­former.

  • What would happen if a power transformer designed for operation on 50 Hz is  connected to a 500 Hz source of the same voltage ?

Ans. Power transformers are made to operate on one particular frequency, usually 50 Hz. If the frequency is too high, the inductive reactance of the primary will prevent the primary from drawing sufficient power. The hysteresis and eddy current losses will be excessive.

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  • What would happen if a power transformer designed for operation on 50 Hz were connected to a 5 Hz source of the same voltage ?

Ans. Power transformers are made to operate on one particular frequency, usually 50 Hz. If the frequency is too low, the primary will have insufficient reactance and too much primary current will flow, producing considerable copper losses. The transformer may start to smoke.

  • If part of a primary winding of a transformer were accidentally short-circuited, what would be the immediate effect ?

Ans. If a few turns of the primary of a transformer short out for some reason, a high current will be induced in the turns, producing excessive heat in the transformer. This is not only because of the 'shorted turns' heating but also because of the cancellation of the inductance of the pri­mary by the magnetic field set up by the shorted turns. Cancellation of the inductance de­creases the inductive reactance of the primary and excessive primary current flows. This causes excessive heat in the transformer.

  • Why are autotransformers not safe for supplying a low-voltage from a high-voltage source ?

Ans. Autotransformers are not safe for supplying a low voltage form a high voltage source. It is because if the winding that is common to both primary and secondary accidentally becomes open-circuited, the full primary voltage will appear across the secondary. This may cause severe shock to the operating personnel.

  • What functions are performed by instrument transformers ?

 

Ans. Instrument transformers perform two functions viz.

(i) they act as ratio devices, making possible the use of standard low-voltage and low-current meters and instruments.

(ii) they act as insulating devices to protect the apparatus and operating personnel from high voltages.

There are two types of instrument transformers viz., potential transformers and current transformers:

  • What are the advantages and disadvantages of 3-phase transformer over 3 single-phase bank of transformers ?

Ans. Advantages of 3-phase transformers over 3 single-phase bank of transformers are :

(i) less cost

(ii) less weight

(iii) requires less space

(iv) somewhat higher efficiency

Disadvantages of 3-phase transformers over 3-single-phase bank of transformers are :

(i) greater cost of standby units

(ii) increased cost and inconvenience of repairs

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