Machine Parts

Temperature rise in a transformer is the temperature of the windings and insulation above the existing ambient or surrounding temperature.

Primary and Secondary Terminations are provided on ACME Dry Type Transformers as follows: No lugs-lead type connection on:  •0-25 KVA single phase •0-15 KVA three phase •Bus-bar terminations (drilled to NEMA standards) •37 1/2-250 KVA single phase •25-500 KVA three phase

No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phase. 

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. 

Dry type Distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 15 KVA and larger three phase can be reverse connected without any adverse affects or loss in KVA capacity. The reason for this limitation in KVA size is, the turns ratio is the same as the voltage ratio.  Example: A transformer with a 480 volt input, 240 volt output - can have the output connected to a 240 volt source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will become the output or 480 volt secondary. On transformers rated below 1 KVA single phase there is a turns ratio compensation on the low voltage winding. This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load.  For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts se

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. 

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. 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. If the transformer was originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage. http://newmachineparts.blogspot.com/

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 fine 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. The standard ASA and NEMA designation for taps are " ANFC " means above normal full capacity) and " BNFC" means below normal full capacity. 

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 completely static solid state device, which insures, 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. Ideal transformer equations  By Faraday's law of induction     Combining ratio of  Turns ratio for step-do

If a 60 Hz transformer is operated in 50 Hz, but at rated voltage, will it be hotter or cooler at full load?   We know , If a transformer rated in KW, it will be meaning less. Because   will change according to load. So transformer is rated in KVA. As seen, Cu loss of a transformer depends on current and   iron loss on voltage. Hence total transformer loss depends on volt-amp (VA) and not on phase angle between voltage and current i.e. it is independent of load power factor. That is why rating transformer is in KVA not in KW.             If rated voltage is applied but a frequency other than rated is used in a transformer, it is seen from equation, , that the flux must change inversely with the frequency change in order to maintain the same induced voltage. The core loss varies almost directly as the square of the flux and only directly with the frequency. A decrease in the supply Frequency will therefore require an increase in flux and consequently

                                               If we connect a load to the secondary of the transformer as shown in fig-1, the induced emf in the secondary, causes the current   to flow in the secondary winding. At the instant shown in the diagram, the source of power causes the upper terminal of the primary to be positive, and the instantaneous direction of current is given by . In accordance with Lenz’s law, the secondary induced emf and the current it causes must be in such a direction as to oppose the setting up of the flux . Application of the right hand rule will show that this secondary current direction is given by. We now have a magnetomotive force in   the secondary. , opposing that of the primary . It must be remembered, however, that the primary induced voltage   is always directly proportional to the flux , and is also equal to the impressed voltage , with all values now taken as the effective, or rms ones. Since the impressed voltage does not c