Transformer rating in kVA not in kW explained why

Transformer rating in kVA, 3 phase tranformerThe power rating of a device is the maximum magnitude of power that can be allowed to operate by the device.

Active power, Reactive power, and Apparent power are rated in W, VAR, and VA respectively.

Active power = V I cos ɸ  W  (power factor, pf = cos ɸ  )

Reactive power = V I sin ɸ  VAR

Apparent power = V I  VA

For a transformer, its power rating is in KVA (Apparent power) not in KW (active power). Apparent power is the resultant of Active and Reactive power. Apparent power has no reference to the pf or the phase angle of the electrical system.

Transformer power loss and rating

The main reason behind the transformer rating is its power loss. A transformer has two types of power loss, copper loss, and Iron loss.

Copper Loss

Copper loss occurs due to the ohmic resistance in the primary and secondary winding. The copper loss varies with the amount of current through the windings.

Iron Losses or Core Loss.

Iron loss occurs in the transformer core due to the alternating flux. It consists of eddy current loss and hysteresis loss.

The copper loss of a transformer depends on its current and the iron loss depends upon the voltage. Hence the total loss depends upon the voltage and the ampere, irrespective of the power factor or the phase angle. The winding temperature and oil temperature of the transformer rise with the amount of loss or heating of the transformer. Hence the temperature rise of a transformer can be also referred to as it is proportional to the apparent power.

While discussing in practical terms, a manufacturer has no idea about the type of load or load power factor that will be used with the transformer. The load can be either resistive, capacitive or inductive. A transformer is a device which transfers electrical energy into another circuit, not a device like motor, bulb, etc., which has fixed power factor values. That’s why a motor is rated in kW.

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