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Rating of any equipment is defined by it's output power ( in this case motor has output of mechanical power ) so we need to use active power units ( Hp or KW )
Motors are having inductive loads which creats a lagging powerfactor.Hence it is denoted in KW/HP
Motor is a rotating Machine/Equipment,there is an air-gap between Rotor & Stator(There are some leakage Flux = P.F of Motor OR Phase displacement Between Voltage & Curren),there comes the lagging Effect of KVA. As we know KVA Cos(PHI).So it is rated in KW,not in KVA.
2nd :- Motor is designed for0.8 Lagging Power Factor.
Rated motor power is mechanical power. So, naturally can't be expressed by electrical units. It has this same unit like electrical power but has different physical explanation. Rated motor power [kW] * mechanical conversion efficiency = Electrical active power [kW].
No induction motor has fixed PF. Power factor in induction motors is depending on rotational speed.
motor is defined by KW and KVA, motor is actual consume KW power and caused reactive power in network by KVAR
so motor defined by KVA=KW+KVAR not KW only
motor has fixed and defined power facter i-e also rating is mentioned in kw on motor
In usual characteristics plates you be able to obtain several data or terms, typicals are electrical power in KW or HP, cos fi or power factor, Voltage, Intensity, Frequency, etc.. sometimes for different suplies as different frequencies, or voltages values. You can obtain of the characteristics plates differents values depending the connection of the motor Y or D.
With the characteristics of active power I mean KW or HP and cos fi you easily could obtain the value of KVA in the next formula P=S*cos fi with the dimensional equation [KW]=[KVA]*1
You would deduce that you should obtain different values of KVA depending cos fi, with the same value of power or in KW or HP.
;-)..
it is rated in KWh because motors have define power factor while transformer and alternator manufacture have no idea what type resistive load will connect
Motor has fixed Power factor, 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.
