There are two type of losses in a transformer;
1. Copper Losses
2. Iron Losses or Core Losses or Insulation Losses
Copper losses (I²R) depends on Current which passing through transformer winding while Iron Losses or Core Losses or Insulation Losses depends on Voltage.
So the Cu Losses depend on the rating current of the load so the load type will determine the power factor, that’s why the rating of Transformer in kVA, not in kW.
Designer doesn’t know the actual consumer power factor while manufacturing transformers and generators i.e. the P.F (Power factor) of Transformer and Generator/Alternator depends on the nature of connected load such as resistive load, capacitive load, and inductive load as
But Motor has fixed Power factor, i.e. motor has defined power factor and the rating has been mentioned in KW on Motor nameplate data table. That’s why we are rated Motor in kW or HP (kilowatts/ Horsepower) instead of kVA.
In addition, Motor is a device which converts Electrical power into Mechanical power. In this case, the load is not electrical, but mechanical (Motor’s Output) and we take into the account only active power which has to be converted into mechanical load. Moreover, the motor power factor does not depend on the load and it works on any P.F because of its design.
It’s important to know whether the transformer is rated in KVA or KW but while connecting the transformer in a system it is essential to know about its per unit impedance due to which the calculations of the line becomes simpler.