Free Tutorials On Electrical Engineering Question and Answer

1.What are taps and when are they used?
Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions and still deliver full rated output voltages at the secondary terminal.  Standard tap arangements are at two and one-half and five percent of the rated primary voltage for both high and low voltage conditions.
2.What is the difference between “Insulating”, “Isolating” and “Shielded Winding” transformers?
Insulating and isolating are identical.  These terms are used to describe the isolation of the primary and secondary windings or isolation between the two.  A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise.
3.Can transformers be operated at voltages other  than nameplate voltages?
In some cases, transformers can be operated at voltages below the nameplate rated voltage.  In NO case should a transformer be operated at a voltage in excess of its nameplate rating unless taps are provided for this purpose.
4.Can 60 Hz transformers be operated at 50 Hz?
Transformers 1 kVA and larger rated at 60 Hz should not be used on 50 Hz service due to the higher losses and resultant heat rise.  However, any 50 Hz transformer will operate on a 60 Hz service.
5.Can transformers be used in parallel?
Single phase transformers can be used in parallel only when their impedances and voltages are equal.  If unequal volatages are used, a circulating current exists in the closed network between the two transformers which will cause heating and result in a shorter life of the transformer.
6.Can transformers develop three phase power from a single phase source?
No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phase.
7.What is meant by regulation in a transformer?
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.
8.What is temperature rise in a transformer?
- Temperature rise in a transformer is the temperature of the windings and insulation above the existing ambient or surrounding temperature.
9.What is “Class” in insulation?
Insulation class was the orioginal method used to distinguish insulating materials operating at different temperature levels.Letters were used for different designations.
10.Why should dry type transformers never be over-loaded?
Overloading of a transformer results in excessive temperature. This excessive temperature causes overheating which will result in rapid deterioration of the insulation and cause complete failure of the transformer coils.
11.What is meant by “impedance” in transformers?
Impedance is the current limiting characteristic of a transformer and is expressed in percentage.
12.Why is impedance important?
It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.
For Example:
1. Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 kVA single phase transformer with 4% impedance to be operated from a 480V, 60 Hz source.
13.What is polarity when associated with a transformer?
Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding.
14.What is exciting current?
It is the current or amperes required  for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 kVA and smaller to approximately 0.5% to 4% on larger sizes of 750 kVA.


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