Transformer Questions & Answers-2

Ques-5: Why efficiency of transformer is high?

Ans: Transformer is a static device which transfers the power from one circuit to other through electromagnetic induction. In electrical transformer as there are no moving parts, no friction and windage losses occur and other losses in transformer are less as magnetic path of a transformer is a closed loop (no air gap exist like induction motor between two circuits). Losses in the transformer are very less compared to any other rotating machine, hence efficiency of transformers will be very high. Efficiency will be of the order of 95% to 98%.

Ques-6: Can transformers be used in parallel? 
Ans: Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5% of each other.
For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have an impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers,the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical.


Ques-7: Can a Single Phase Transformer be used on a Three Phase source? 
Ans: Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase.


Ques-8: Can Single Phase Transformers be used for Three Phase applications? 
Ans: Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer. For example: Three 10 KVA single phase transformers will accommodate a 30 KVA three phase load.

Ques-9: Can Transformers develop Three Phase power from a Single Phase source? 
Ans: No. Phase converters or phase shifting devices such as reactors and capacitors are
required to convert single phase power to three phase.


Ques-10: What is meant by regulation in a transformer?
Ans: Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usually expressed in terms of percentage. For example: A transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the regulation would be 5%.