Transformer Interview Questions and Answers

1. What is the principle of operation of a transformer ?

Answer:- A simple transformer consists of two windings placed on an iron core. When an alternating voltage is applied to one winding (called the primary), it sets up alternating flux in the core. The alternating flux links the other winding (called the secondary) and induces a voltage in it. When the load is connected to the secondary, current flows through the load, thus delivering power to the load. In this way, power is transformed from primary to secondary through the agency of magnetic flux.


2. Why do we use iron-core in a transformer ?

Answer:- Transformer action demands only the existence of alternating flux linking the two windings. No doubt such action will be obtained if an air core is used but it will be obtained much more effectively if an iron core is used. It is because the flux is then substantially confined to a definite path (i.e., iron path) having a much higher permeability than air.


3. What will happen if the primary of a transformer is connected to d.c. supply ?

Answer:- If the primary of a transformer is connected to a d.c. supply, the primary will draw a steady current and hence produce constant flux. Consequently, no back e.m.f. will be produced. The primary winding will draw excessive current due to the low resistance of the primary. The result is that the primary will overheat and burn out or the fuses will blow. Care must be taken not to connect the primary of a transformer across the d.c. supply.


4. Under what condition is d.c. supply applied safely to the primary of a transformer?

Answer:- When the primary winding of a transformer is to be connected to a d.c. supply, a high resistance is connected in series with the primary. This series resistance limits the primary current to a safe d.c. value and thus prevents the primary from burning out.


5. Why is iron-cored transformer not used for high frequency applications?

Answer:- At high frequencies, an iron-core transformer has the following disadvantages :

(i) High hysteresis and eddy current loss

(ii) Iron becomes less effective as a magnetic circuit because, at high frequencies, the flux penetrates to a lesser extent in the core.

Since air does not exhibit eddy current or hysteresis effects, the use of air-core transformers is common at high frequencies found in electronics and communications.


6. What conditions are required for a transformer to operate satisfactorily at very low frequencies?

Answer:-

A transformer intended to operate satisfactorily at low frequencies should meet the following requirements

(i) A magnetic core of large-section.

(ii) A large number of turns. Clearly, this makes the transformer large in size.


7. Where is a core-type construction suitable for a transformer ?

Answer:- In the core-type transformer, a core shaped as shown has primary and secondary windings wound on it. It is a usual practice to wind one half of each winding on one limb, low-voltage winding being innermost for mechanical strength. The core-type transformer affords a better cooling surface for the winding than the shell-type core and is, therefore, more suitable for transformers that remain fully loaded.


8. Where is shell-type construction suitable for a transformer?

Answer:- In shell-type core, both the windings are wound on the central limb. This type of core is used for those transformers which work on poor power factors.

Another advantage of the shell type is that it offers a separate path for the zero-sequence currents through the core, as compared to the core type in which the zero-sequence path exists only through the transformer tank and end connections.

The shell type has better provision for mechanically supporting and bracing the coils. This allows better resistance to the very high mechanical forces that develop during a high-current short circuit. The core type is generally more suitable for high voltage and small output while the shell type is generally more suitable for low voltage and high output.


9. Why are the cores of large transformers built up of circular cross-section ?

Answer:- The cross-section of the cores of small transformers are rectangular but for large transformers, it is a common practice to use approximately circular cross-section. It is because a circular section has the smallest perimeter for a given area and, therefore, requires less copper than a rectangular section.


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

Answer:- In an actual transformer, there is primary leakage flux as well as secondary leakage flux. The
primary leakage flux is the flux that links with the primary but not with the secondary
Similarly, the secondary leakage flux is the flux that 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.


11. What are the advantages of back to back test in determining the eficiency ofa transformer?

Answer:- 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 a 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.


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

Answer:- 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.


13. What is the diference between power transformers and distribution transformers?

Answer:- Those transformers installed at the sending or receiving end of long high-voltage transmission lines are the power transformers.

The distribution transformers (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 the major part of the day

(ii) The performance of a power transformer is generally judged from commercial efficiency 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.


14. What would happen if a power transformer designedfor operation on 50 Hz were connected to a 500 Hz source of the same voltage?

Answer:- 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.


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

Answer:- 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.


16. If a part of aprimary winding of a transformer were accidentally short-circuited, what would be the immediate effect?

Answer:- 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.


17. Why are autotransfornmers not safe for supplying a low-voltage from a high-voltage source?

Answer:- Autotransformers are not safe for supplying a low voltage from 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.


18. What functions are performed by instrument transformers?

Answer:- 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.


19. What are the advantages and disadvantages oj 3-phase transformers over 3 single-phase bank of transformers?

Answer:- Advantages of 3-phase transformers over 3 single-phase banks 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 banks of transformers are:

(i) Greater cost of standby units

(iii) Increased cost and inconvenience of repairs


20. List four applications of transformers.

Answer:- Four important applications of transformers are

(i) It can raise or lower the voltage or current in an a.c. circuit.
(ii) It can increase or decrease the value of a capacitor, an inductor, or resistance in an a.c. circuit. It can thus act as an impedance transferring device
(iii) It can isolate two circuits electrically.
(iv) It can be used to prevent d.c. from passing from one circuit to the other.

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