Why tansformer rating in KVA and motor or generator rating in KW?
Synchronous generators are rated in kVA and not kW because power delivered by it that is 0.732VIxcos(theta) (Watts) depends on the load pf cos(theta) but its insulation and and conductors are designed according to 0.732VI (VA).
Transformers are also rated in kVA because they have two types of losses,copper loss(dependent on I) and core loss (dependent on V).Hence the insulations and windings have been designed to withstand as per V and I rating.
But motors are rated in kW because motor shaft can only supply real power in b.h.p(brake horse power) or kW as per load torque requirements and the motor characteristics and it cannot supply any reactive power.
Transformers and generators are rated in kVA. Motors in kWatt. Machines are rated in terms of their output type. For generators and transformers output is electrical that is apparent power. S=P+jQ. P and Q are complement to each other. So referring only P or Q is incomplete while telling about electrical power. So resultant VA. But in motor output is mechanical. Mechanical rotation will be done only by real power. As you know reactive power only sets up the magnetic field. It takes no part in mechanical rotation. So motor has the rating of only real power that is Watt.
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