Transformers - Question and Answer

1. Define a transformer.

A transformer is a static device which change the alternating voltage form one level to another.

2. What is staggering in the construction of transformers?

In transformer, the joints in the alternate layers are staggered in order to avoid the presence of narrow gaps right through the cross section of the core.

3. What determines the thickness of the lamination or stampings?

Frequency
Iron loss

4. Classify the transformer according to the construction.

Core type transformer
Shell type transformer
Berry type transformer

5. What are the two components in transformer’s no load current?

Active or working component (I_w)
Reactive or magnetizing component (I_μ)

6. Why is the core transformer laminated?

The core transformer is laminated in order to minimize eddy current loss.

7. Why transformer rating is expressed interms of kVA?

Copper loss depends on current and iron loss depends upon voltage. Hence the total loss in a transformer depends upon volt-ampere (VA) only and not on the phase angle between voltage and current. It is independent of load power factor. That is why the rating of a transformer is given in kVA and not in kW.

8. What is an ideal transformer?

100% efficiency of the transformer is called ideal transformer.

9. Why the open circuit test on a transformer is conducted at rated voltage?

The open circuit test on a transformer is conducted at rated voltage because core loss depends upon the voltage. This open circuit test gives only core loss or iron loss of the transformer.

10. Why is the range of efficiency in transformer higher than those of other electrical machines?

The range of efficiency in transformer is higher than those of other electrical machines because there are no rotating parts in the transformer. rotational loss is zero.

11. Why polarity test has to be done in a transformer?

A polarity test is carried out find out the terminal having the same instantaneous polarity assuming that the terminals are not marked.

12. What are the advantages of Sumpner’s test?

The power required to carry out the test is small.
The transformers are tested under full load conditions.
The iron losses and full load copper losses are measured simultaneously.
The temperature rise of the transformers can be noted.

13. How can the iron loss be minimised in a transformer?

The iron loss in a transformer is made up of hysteresis loss and eddy current loss. Hysteresis loss can be minimized by using steel of high silicon content for the transformer core. The eddy current loss can be minimized by using very thin laminations of transformer core.