Machine Parts

Several standard single phase transformers listed in this catalog can be connected as auto transformers. The KVA capacity will be greatly increased when used as an auto transformer, in comparison to the nameplate KVA as an insulating transformer.  E xamples of auto transformer applications are changing 600 volts to 480 volts in either single phase or three phase; changing 480 volts to 240 volts single or three phase or vice versa; or the developing of a fourth wire (neutral) from a 480 volt three phase three wire system for obtaining 277 volts single phase. Th is voltage is normally used for operating fluorescent lamps or similar devices requiring 277 volts. For further details showing KVA and voltage combinations for various auto transformer connections refer to the Acme catalog.

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

Load connected with the transformer always varries i.e. it may be sometime resistive load or inductive load or capacitive load. As a result p.f. sometime unity, sometime lagging or leading and it is not equal. We know that KW=KVA*COSθ As p.f. varries so KW varries. So on the nameplate of   the transformer KW rating is not appropriate.Again cu loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss deoends on volt-ampre(VA) and not on phase angle berween voltage and current i.e. it is independent of load power factor. That is why rating of transformer is in KVA and not in KW.