Power transformer capacity is rated in kilovolt-amperes (kVA). The output rating for a transformer is determined by the maximum current that the transformer can withstand without exceeding its stated temperature limits.

Power in an AC circuit depends on the power factor of the load and the current, so if any AC electrical equipment is rated in kilowatts, a power factor must be included to make its power rating meaningful. To avoid this, transformers and most AC machines are rated in kVA, a unit that is independent of power factor.

In addition to its kVA rating, the nameplates of transformers typically include the manufacturer’s type and serial number, the voltage ratings of both high- and low voltage windings, the rated frequency, and the impedance drop expressed as a percentage of rated voltage. Some nameplates also include an electrical connection diagram.

Power transformers are generally defined as those used to transform higher power levels than distribution transformers (usually over 500 kVA or more than 67 kV). The kVA terminal voltages and currents of power transformers, defined in ANSI C57.12.80, are all based on the rated winding voltages at no-load conditions.

However, the actual primary voltage in service must be higher than the rated value by the amount
of regulation if the transformer is to deliver the rated voltage to the load on the secondary.

The efficiency of all power transformers is high, but efficiency is highest for large transformers operating at 50 to 100 percent of full load. However, some losses are present in all transformers. They are classified as copper or I2R losses and core losses.

Copper losses, also called load losses, are proportional to the load being supplied by the transformer. These losses can be calculated for a given load if the resistances of both windings are known. As in generators and motors, the core loss is due to eddy-current induction loss and hysteresis (molecular friction) loss, caused by the changing polarity of the applied AC.

If the cores are laminated from low-loss silicon steel, both eddy-current and hysteresis losses will be reduced. Nevertheless, well-designed transformers in all frequency and power ranges typically have efficiencies of 90 percent or more.

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