transformer and motor or generator rating

Why tansformer rating in KVA and motor or generator rating in KW?

  • Bhupesh Sharma
  • Bhupesh Sharma
  • |
  • 03 Mar
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  • 5 Answers

text limit upto 4000

  • 1

    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.

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  • 0

    Transformer =  KVA
    Motor = K

  • 0

    The simplest reason is that power= VI X power factor. But the power factor depends upon the load and not the transformer. So for transformer rating P=VI is taken in kva. Kilo used for ease.

  • 0

     In transformer Copper losses ( I²R) depends on Current which passing through transformer winding while Iron Losses or Core Losses or  Insulation Losses depends on Voltage.
    So transformer rating in kVA, Not in kW.

  • 0

    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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