10 Interview Questions and Answers on Transformers - Part-1

Figure: Power transformer over-excitation condition caused by decreased frequency; flux (green), iron core's magnetic characteristics (red) and magnetizing current (blue).

1. How is magnetic leakage reduced to a minimum in commercial transformers?

By interleaving the primary and secondary windings.

2. Mention the factors on which hysteresis loss depends?

(i) Quality and amount of iron in the core

(ii) Flux density and

(iii) Frequency.

3. How can eddy current loss be minimized?

By laminating the core.

4. In practice, what determines the thickness of the laminate or stampings?

Frequency.

5. Does the transformer draw any current when its secondary is open?

Yes, no-load primary current.

6. Why?

For supplying no-load iron and copper losses in primary.

7. Is Cu loss affected by power factor?

Yes, Cu loss varies inversely with power factor.

8. Why?

Cu loss depends on current in the primary and secondary windings. It is well-known that current required is higher when power factor is lower.

Figure: Ideal transformer and induction law.

9. What effects are produced by change in voltage?

1. Iron loss.........varies approximately as V2.
2. Cu loss.........it also varies as V2 but decreases with an increase in voltage if constant kVA output is assumed.
3. Efficiency.........for distribution transformers, efficiency at fractional loads decreases with increase in voltage while at full load or overload it increases with increase in voltage and vice versa.
4. Regulation.........it varies as V2 but decreases with increase in voltage if constant kVA output is assumed.
5. Heating.........for constant kVA output, iron temperatures increase whereas Cu temperatures decrease with increase in voltages and vice-versa.

10. How does change in frequency affect the operation of a given transformer?

1. Iron loss .........increases with a decrease in frequency. A 60-Hz transformer will have nearly 11% higher losses when worked on 50Hz instead of 60 Hz. However, when a 25-Hz transformer is worked on 60 Hz, iron losses are reduced by 25%.
2. Cu loss.........in distribution transformers, it is independent of frequency.
3. Efficiency.........since Cu loss is unaffected by change in frequency, a given transformer efficiency is less at a lower frequency than at a higher one.
4. Regulation.........regulation at unity power factor is not affected because IR drop is independent of frequency. Since reactive drop is affected, regulation at low power factors decreases with a decrease in frequency and vice-versa. For example, the regulation of a 25-Hz transformer when operated at 50-Hz and low power factor is much poorer.
5. Heating.........since total loss is greater at a lower frequency, the temperature is increased with decrease in frequency.

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Read More: 12 Most Common Interview Questions and Answers on Transformers - Part-2

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