Why Motor rated in kW instead of kVA

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Why Motor rated in kW instead of kVA

  • Motor is rated in kW since it specifies the capacity of the motor to drive its load. It is the active power (kW) that is of interest when a motor drives a load.
  • The motor converts the active power that it draws from the mains into mechanical power that the load consumes/demands.
  • In addition to active power, the motor draws reactive power also, but this is not converted into useful work. Reactive power is used only by the motor to magnetize the air-gap. Thus, a motor is rated in terms of kW.
  • 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.
  • We know that Transformer rating may be expressed in kVA as well as Generator and Alternator rated in kVA Designer doesn’t know the actual consumer power factor while manufacturing transformers and generators.

Why Motor rated in kW instead of kVA

  • 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 Motors, etc. 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.

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