Why Motor rated in KW instead of KVA?

The induction motor and the transformer both works on the same principle that is Faraday’s law of electromagnetic induction.The motor is rated in KW instead of KVA and the transformer is rated in KVA instead of KW.

The transformer and alternator are designed on the basis of apparent power(kVA). The power factor at the incoming supply side of the transformer depends on the types of loads connected to the secondary of the transformer i.e. the load may be resistive, inductive or capacitive. The designer does not know at what power factor the user will operate the transformer.

At the other hand, the motor has fixed power factor at its full load current. All the motor data- voltage,current and power factor are given on the nameplate of the motor. The motor is designed to drive the mechanical load of particular rating.

The motor draws active power from the mains and converts it into mechanical power. The reactive power drawn by the motor to meet its magnetizing power requirement does not converts into the useful work.

The motor is designed to deliver the rated KW or the shaft power at the rated voltage, current and power factor. When the motor is 100% loaded it delivers the rated KW at the rated voltage,current and power factor. The power factor is specified for the rated full load current of the motor. The active power drawn by a three phase induction motor is

P = KVA x Power Factor
P =√3 V I Cosφ

Where,

V = Line voltage

I = Line current

Cosφ = Power factor

The following motor data is engraved in the nameplate of the motor.