Why Motor rated in kW instead of kVA?

DWQA QuestionsCategory: Induction Motor & GeneratorWhy Motor rated in kW instead of kVA?
Guma asked 7 years ago
3 Answers
beem answered 7 years ago

We already know that Transformer rating may be expressed in kVA instead of kW as well as Generator and Alternator rated in kVA. 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, capacitive, and inductive load etc which affect the overall power factor of transformer and generators as the consume reactive power according to the load as well.
At the other hand, Motor has fixed Power factor, i.e. motor has defined power factor (p.f)  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 more clear words, Motor only consume active power and provide mechanical power in HP or kW at motor shaft.
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.

Anil answered 7 years ago

We know that Transformer rating may be expressed in kVA as well as Generator andAlternator rated in kVA 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 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.

INAA answered 7 years ago

Because kilowatts are a rational and easily usable unit of power. So instead of saying a motor is 1.34HP (power delivered to the shaft, not power consumed), you say it’s 1KW. Horsepower is a pretty archaic unit of work, and doesn’t fit well with any current system of units. You will note that Europe seems to largely spec their automotive engines in KW as well. I love that. It makes sense. I wish we in the US would do that.