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Why transformer is rated in KVA?

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