BASIC QUESTIONS & ANSWERS about Transformers

1. What is a Transformer?

ANSWER: A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to "step up" or "step down" voltages. 

2. Is it Possible to Change Three Phase to Two Phase or Vice-Versa with Standard Transformers?

ANSWER: Yes. This is a very practical application for standard single phase off-the-shelf transformers. Some typical voltage combinations are as follows: 480 volts three phase to 240 volts two phase, or 240 volts three phase to 480 volts two phase, or 240 volts three phase to 240 volts two phase. Please refer to us for an exact schematic.

3. How Does a Transformer Work?

ANSWER: 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 con¬ditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire wound on a lami¬nated 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.

4. What are Taps and When are They Used?

ANSWER: Taps are provided on some transformers on the high voltage winding to correct for high or low voltage con¬ditions, and still deliver full rated output voltages at the sec¬ondary 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 trans¬former 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.

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

ANSWER: 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 pre¬venting accidental contact between windings under faulty con¬ditions. 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.

6. Can Transformers be Operated at Voltages other than Nameplate Voltages?

ANSWER: 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 trans¬former with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts and if the trans¬former were originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage.

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

ANSWER: 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 .

8. Can Transformers be Used in Parallel?

ANSWER: 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.

9. Can 60 Hz Transformers be Used at Higher Frequencies?

ANSWER: Transformers can be used at fre¬quencies above 50 Hz up through 400 Hz with no limitations provided nameplate voltages are not exceeded. However, 50 Hz transformers will have less voltage regulation at 400 Hz than at 50 Hz. Where better regulation and smaller physical size are required, contact us for special 400 Hz designs.

10. What is Meant by Regulation in a Transformer?

ANSWER: 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.

11. What is Meant by Temperature Rise in a Transformer?

ANSWER: Temperature rise in a transformer is the temper¬ature of the windings and insulation above the existing ambi¬ent or surrounding temperature, and is determined by the insulation class used in the transformer coils.

12. What is Meant by Insulation "Class"?

ANSWER: 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".

13. Is One Insulation System Better Than Another?

ANSWER: 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?

ANSWER: 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. Are Temperature Rise and Actual Surface Temperature Related?

ANSWER: No. This can be compared with an ordinary light bulb. The filament temperature of a light bulb can exceed 2000 degrees, yet the surface temperature of the bulb is low enough to permit touching with bare hands.

16. What is Meant by "Impedance" in Transformers?

ANSWER: Impedance is the current limiting characteristic of a transformer and is expressed in percentage.

17. Why is Impedance Important?

ANSWER: It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.

18. Can Single Phase Transformers be Used for Three Phase Applications?

ANSWER: Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can gen¬erally 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 pri¬mary to wye secondary, since is will result in unstable Secondary voltages. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer, or example: Three 10 KVA single phase transformers will accommodate a 30 KVA three phase load.

19. WHAT is ZIG ZAG Grounding Transformer

ANSWER: Three Single Phase Transformers can be connected to have a three phase Zig Zag Transformer. This system can be used for either grounding or developing a fourth WIRE from a three phase neutral. An example would be to change a 480 V — three phase — three wire system to a 480Y/277 V — three phase — four wire system.

20. What Color are generally used for Dry Type Transformers?

ANSWER: Normally ligh gray is used on: Single phase 50 VA through 25 KVA and on three phase 3 KVA through 15 KVA. Light gray is used with a pleasing blue-gray on the side and top covers on — Single phase 37 ½ KVA through 250 KVA and three phase 20 KVA through 750 KVA.

21. How Do You Select a Transformer to Operate in an Ambient Higher Than 40° Centigrade?

ANSWER: If the 24 hour average ambient does not exceed 40° C, standard transformers can be used. When the ambient exceeds 40° C use the following chart for de-rating standard transformers.

Maximum Ambient  Maximum Percentage of Loading  
40° C
50° C
60° C
70° C

Instead of ordering custom built transformers to operate in ambient higher than 40° C, it is more economical to use a standard transformer of a larger KVA rating.

22. Can Transformers be Reconnected as Autotransformers to Increase Their KVA Rating?

ANSWER: Several of standard single phase transformers can be connected as autotransformers. The KVA capacity will be greatly increased when used as an Autotransformer, in comparison to the nameplate KVA as an insulating transformer. Examples of autotransformer applications are changing 600 volts to 480 volts in either single phase or three phase; changing 480 volts to 240 volts single three phase or vice versa; or the developing of a fourth vire (neutral) from a 480 volt three phase three wire system or obtaining 277 volts single phase. This voltage is normally used for operating fluorescent lamps or similar devices requiring 277 volts.

23. What is Corona and How Does it Affect Dry Type Transformers?

ANSWER: Corona — a type of localized discharge resulting from transient gaseous ionization in insulation sys¬tems when the voltage stress exceeds a critical value." In a dry type transformer, part of the insulation system is air, which is referred to in the definition as gas. If a transformer is not designed properly and the insulation is overworked, it will result in the development of ionization of the air which in turn causes deteriorating affects on the insulation of the trans¬former. This will result in an extremely short life of the trans¬former. Our transformers are designed so that they are corona-free even at voltages considerably higher than recommended operating voltages. The reason is, if a momen¬tary transient voltage occurs and causes corona inception, the design of the transformer is such that the corona extinction voltage level is appreciably higher than the operating voltage level; therefore, corona will not be present during operation.

24. What is BIL and How Does it Apply to our Transformers ?

ANSWER: BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the applica¬tion of a high frequency steep wave front voltage between windings, and between windings and ground. The Basic Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that a trans¬former will tolerate without breakdown. All transformers manufactured for 600 volts and below will with¬stand BIL rating, which is 10 KV. This assures the user that he will not experience breakdowns when his system is properly protected with lightning arrestors or similar surge protection devices.

25. What is Polarity, When Associated With a Transformer?

ANSWER: Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity — that is, the terminals of the high voltage and low voltage windings on the left hand side are connected together, This leaves one high voltage and one low voltage terminal unconnected. When the trans¬former is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage wind¬ings. This is useful when connecting single phase transform¬ers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

26. What is Meant by Indoor or Outdoor Transformers?

ANSWER: Indoor transformers are ordinarily defined as trans¬formers suitable for indoor operation only. Outdoor trans¬formers 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.

27. Will a Transformer Change Three Phase to Single Phase?

ANSWER: A transformer will not act as a phase changing device when attempting to change three phase to single phase. There is no way that a transformer will take three phase in and deliver single phase out while at the same time presenting a balanced load to the three phase supply system. There are, however, circuits available to change three phase to two phase or vice versa using standard dual wound trans¬formers.

28. Range of our Dry Type Transformers?

ANSWER: We manufactures the most complete line available of boost-buck transformers for applications ranging from 80 to 520 volts single or three phase.

Further complete engineering and manufacturing facili¬ties are available for custom designed transformers — frac¬tional through 5000 KVA ratings, low voltage through 15 KV. Some of the applications in which these custom built trans¬formers are used are — instrumentation, telecommunications, computer peripherals, rectifiers, reactors, oven and heating applications, and various others to match the customer's exact specifications. These transformers are available in vari¬ous temperature rise and insulation systems, along with pos¬sible convection, forced air, water, or oil cooling as required by the customer.

29. How Do You Size a Transformer for Operating a DC Motor?

ANSWER: There are no straight forward simple formulas which can be used in sizing transformers to operate DC motors. However, the transformer size can be calculated accurately when the circuit is known for rectifying the AC to DC for operating the motor. There are a great variety of circuits now in common use for changing AC to DC. One of the more common circuits is the three phase full wave bridge circuit employing three SCR's and three diodes.

30. What is Meant by "Balanced Loading" on Single Phase Transformer Applications?

ANSWER: Since most single phase transformers have a sec¬ondary voltage of 120/240, they will be operated as a three wire system. Care must be taken in properly distributing the load as the transformer secondary consists of 2 separate 120 volt windings. Each 120 volt winding is rated at one-half the nameplate KVA rating. For example: A 10 KVA transformer, 120/240 volt secondary is to service an 8 KVA load at 240 volts and two 1 KVA loads at 120 volts each.

If the incorrect method is used, winding A will be loaded at 6 KVA, and winding B will be loaded at 4 KVA. These do total 10 KVA but, since each winding is only rated at 5 KVA (V2 of nameplate rating), we have an overloaded transformer and a certain failure.

31. What About Balanced Loading on Three Phase?

ANSWER: Each phase of a three phase transformer must be considered as a single phase transformer when determining loading. For example: A 45 KVA three phase transformer with a 208Y/120 volt secondary is to service 4 loads at 120 volts single phase each. These loads are 10 KVA, 5 KVA, 8 KVA, and 4 KVA.

Note that maximum loading on any phase does not exceed 10 KVA. Each phase has a 15 KVA capacity.

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