|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
3. How can eddy current loss be minimized?
By laminating the core.
4. In practice, what determines the thickness of the laminate or
5. Does the transformer draw any current when its secondary is open?
Yes, no-load primary current.
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.
Cu loss depends on current in the primary and secondary windings. It is well-known that
required is higher when power factor is lower. current
9. What effects are produced by
in voltage? change
- Iron loss........
varies approximately as V2. .
- Cu loss........
it also varies as V2 but decreases with an increase in voltage if constant . output is assumed. kVA
for distribution transformers, efficiency at fractional loads decreases with . in voltage while at full load or overload it increases with increase in voltage and vice versa. increase
it varies as V2 but decreases with . in voltage if constant increase output is assumed. kVA
for constant . output, iron temperatures kVA Cu temperatures decrease with increase in voltages and vice-versa. increase whereas
10. How does
in frequency affect the operation of a given transformer? change
- Iron los
s ...... . . 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 . on 60 Hz, iron losses are reduced by 25%. is worked
- Cu loss........
in distribution transformers, it is independent of frequency. .
since Cu loss is unaffected by change in frequency, a given transformer efficiency is less at a lower frequency than at a higher one. .
regulation at unity power factor is not affected because . 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. IR drop
since total loss is greater at a lower frequency, the temperature . with decrease in frequency. is increased
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