FAQ's
P.Balaji Trafo Limited

5. How Does a Transformer Work ?

A transformer works on the magnetic induction principle. It has no moving parts and is a completely static solid state device, which insures, under normal operating coniditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

6. 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. 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%. Our dry type distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for which they are used.

7. What is Meant by 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, and is determined by the insulation class used in the transformer coils.

8. What is Meant by Insulation "Class" ?

Insulation class was a popular way of referencing insulating materials in their ability to sustain long life while operating at different temperatures. Since it Is difficult and at times confusing to describe different insulations by letter designations, such as A, E, B, F & H; it is better to describe insulation as "insulation systems".

9. What is Meant by Indoor or Outdoor Transformers ?

Indoor transformers are ordinarily defined as transformers suitable for indoor operation only. Outdoor transformers are intended for indoor or outdoor operation. The transformers illustrated in this catalog may be used for either indoor or outdoor operation. However, it is desirable to supply some protection for the transformer such that it will be shielded from direct exposure to rain, snow, or submersion in water.

10. 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 terminals. Standard tap arrangements are at two and one-half and five percent of the rated primary voltage for both high and low voltage conditions. For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating.

11. What is the Difference Between "Insulating", "Isolating", and "Shielded Winding" Transformers ?

Insulating and Isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two. A shielded winding transformer, on the other hand, is designed with a metallic shield between the primary and secondary windings, providing a safety factor by grounding, thus preventing accidental contact between windings under faulty conditions. All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, which are a type whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.

12. Can Transformers be Used in Parallel ?

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 4%, transformer B which is to be parallel to A must have an impedance between the limits of 3.7 % & 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. The resultant KVA when paralleling two or more transformers is 2 times the nameplate KVA of the smallest transformer in the parallel system.

13. Is One Insulation System Better Than Another ?

Not necessarily. For example: Small fractional KVA transformers use the class 105°C insulation system, which is 55°C rise. The class 150°C insulation system, which is 80° C rise, has generally been superseded by a class 185° C insulation system, which is 115° C rise. Medium KVA size transformers, approximately 371/2 KVA and larger, are generally manufactured using a 220° C insulation system, which is 150° C rise. All of these insulation systems from 105° C through 220° C will normally have approximately the same number of years operating life. A well designed transformer, observing these temperature limits, should have a life expectancy of approximately 20-25 years.

14. Why Should Dry Type Transformers Never be Overloaded ?

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.

15. Can 50-Hz Transformers be Operated at 60 Hz & Vice versa ?

50 Hz Transformers rated below 1 KVA can be utilized on 60 Hz service. However transformers of 1 KVA & Larger rated at 60 HZ should not be used on 50 Hz service due to higher losses and resultant heat rise .

16. 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 name plate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly. For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts and if the transformer were originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage.