Why Motor Ratings Given in KW and That of Alternator and Transformer Given in KVA?

 > A volt is not the same thing as watt.

 > A volt ampere is the unit of measurement of apparent power which is the product of supply voltage and load current.

 > The Watt on the other hand is the unit of measurement for true power which is the product of supply voltage, load current and the power factor of the load.

>  The reason that a motor is rated in Watts and not in VA is twofold. Firstly, it is in-phase component of the load (and supply the losses), while the reactive component provides the magnetic field. The in-phase component of current, multiplied by the supply voltage is true power in watts. Secondly the mechanical load of the motor is measured is Watts so it makes an absolute sense to express the output power of a motor in watts too, so that the two can be matched.

 > A transformer on the other hand is rated accordingly to the product of its rated voltage and rated current, in the other words in terms of its apparent power in volt amperes. This is because the transformer designer has no way to know what type of load (resistive, inductive, capacitive) is going to be connected to his transformer , so has to allow for the ‘worse case’ scenario, so that the windings do not become overheated. The only way to do this is to specify the max. rated current which, when applied by the rated voltage, results in volt amperes, not watts. kW is the output mechanical power of a motor and is expressed in kW.

 kVA is the net (apparent) power input to the transformer. This input power is the output + losses.

 kW = kVA x system power factor

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