Why is a Motor Rated in kW instead of kVA?

Electrical Technology

Why are Electric Motors Rated in kW or Horsepower (hp) and Not in kVA?

We already know that transformer ratings may be expressed in kVA instead of kW. The same applies to generators and alternators, which are also rated in kVA while motors are rated in kW or hp (horsepower). This is because the manufacturer and designer don’t know the actual consumer power factor (P.F) while manufacturing transformers and generators.

The power factor (PF) of transformers and generators/ alternators depends on the nature of the connected load, such as resistive, capacitive, and inductive loads. These factors affect the overall power factor of transformer and generator, as they consume reactive power according to the load. The scenario is different in the case of the rating of electric motors.

On the other hand, an electric motor has a fixed power factor, i.e., the motor has a defined power factor (P.F), and the rating is mentioned in kW on the motor nameplate data table. That’s why we rate the motor in kW or HP (kilowatts/horsepower) instead of kVA. In clearer terms, the motor only consumes active power in watts as input and provides mechanical power in hp (horse power) or kW at the motor shaft as output.

In addition, an electric motor is a machine that converts electrical power into mechanical power. In this case, the load is not electrical but mechanical (motor’s output), and we take into 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 with any power factor due to its design. These are the simple reasons why a motor is rated in horsepower (hp) or watts (W) instead of volt-amperes (VA).