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  FAQ's

What is a transformer and how does it work? 
What are taps and when are they used? 
What is the difference between "Insulating", "Isolating", and 
"Shielded Winding" transformers ?

Can transformers be operated at voltages other than nameplate voltages? 
Can 50 Hz transformers be operated at 60 Hz? 
Can transformers be used in parallel? 
What is meant by regulation in a transformer? 
What is temperature rise in a transformer? 
What is "Class" of insulation? 
Is one insulation system better than another? 
What is meant by "Impedance" in transformers? 
Why is impedance important? 
What is exciting current? 

What is a transformer and how does it work? 

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 and works on the magnetic induction principle. A transformer has no moving parts and is a static device, which ensures, under normal operating conditions, 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 then 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. 

What are taps and when are they used? 

Taps are provided on transformers generally 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 variations. For example, if the transformer has a 11000 volt primary and the available voltage is running at 550 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper 
rating. 

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 transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise. This is especially important in critical applications such as medium frequency induction furnace and rectifier loads. 
All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety. 

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 beyond 10% of its nameplate rating. When operating below the rated voltage the KVA capacity is reduced correspondingly. For example, if a 22000 volt primary transformer with a 440 volt secondary is operated at 11000 volts, the secondary voltage is reduced to 220 volts. If the transformer was originally rated 1000 KVA, the reduced rating would be 500 KVA, or in direct proportion to the applied voltage. 

Can 50 Hz transformers be operated at 60 Hz? 

Any 50 Hz transformer will operate on a 60 Hz service. However, Transformers rated at 60 Hz, should not be used on 50 Hz service due to the higher losses and resultant heat rise. Special designs are required for this service. 

Can transformers be used in parallel? 

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 10% 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 3.6% and 4.4%. When paralleling three phase transformers the same precautions must be observed as listed above, plus the angular displacement and phase sequence between the two transformers must also be identical. 

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%. 

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. 

What is "Class" of insulation? 

Insulation class was the original method used to distinguish insulating materials operating at different temperature levels. Letters were used for different designations. Letter classifications have been replaced by insulation system temperatures in degrees Celsius. The system temperature is the maximum temperature at the hottest spot in the winding (coil). 

Is one insulation system better than another?

Not necessarily. It depends on the application and the cost benefit to be realized. Higher temperature class insulation systems cost more. All of these insulation systems will normally have the same number of years operating life. A well designed transformer, observing these temperature limits, will have a life expectancy of 20-25 years. 

What is meant by "Impedance" in transformers? 

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

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. Voltage regulation is also a function of impedance.

What is exciting current? 

Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The exciting current on most fighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA. The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and is referred to as KVAR.

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