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Rating Of Ac Machine


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Hello NCT,

Power KW = KVA x Power Factor

The power factor is depends on the load connected to the alternator/transformer..

Alternator are power sources and the transformers are just altering the level of voltage. We can load any kind of load.

You may be aware that there are three kind of loads namely resistive, inductive and capacitve loads..

Power factor for resistive load will be unity, for inductive load will be lagging and for capacitve load will be leading..

You can load the alternator and transformers with any of the above kind or in combination of that.. so your system power factor is not fixed and may vary time to time..

That is why we are mentioning the rating in KVA..


Practically arthing resistance of a pit connected shall be less than three ohms for effective earthing..

but by forming earth grid we can get combined resistance value of less than one ohm..




why the transformer and alternator rating are in KVA can anybobody give reply.


What is acceptable earthing earthing resistance of pit connected for induction motor.


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Hello NCT


why the transformer and alternator rating are in KVA


The major limitation on an alternator and a transformer is thermal. As the losses increase, the temperature of the insulation rises and this shortens the life of the insulation.


The losses are primarily iron loss and copper loss. The iron loss for a given wound component is fixed for fixed voltage and frequency. If the voltage or the frequency is altered, the ion loss will be alter as well.

The copper loss is a function of the square of the current in the device, so both the transformer and alternator have a maximum current output. Any further increase in current will result in a higher power loss and associated rise in temperature. So, the maximum output of a transformer and an alternator is current. In order to include the voltage component in the output is rated as the product of the RMS voltage and the RMS current. This, divided by 1000 is the KVA.

- A transformer rated at 100A at 60 volts is a much different size from a transformer rated at 100A at 400V, so it is necessary to rate in KVA rather than Amps.

If the load poser factor is reduced, the maximum KW output is also reduced.


Best regards,

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