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Homework Help: Kva rating calculation

  1. Jul 12, 2010 #1
    Hi there,

    Can some one assist me as to how to answer this question...

    An alternator is connected to a 12kw load and operating at a power factor of 0.8 lag. what is the kva rating of the alternator required to drive the load?

    i dont know what formula to use so Any help would be great. cheers
  2. jcsd
  3. Jul 12, 2010 #2
    What limits the power output of your alternator?
  4. Jul 12, 2010 #3
    it doesnt say? this is why i dont understand? is it asking what the apparent power value is? if so...
    Apparent pwr= true power/ power factor

    is true power 12kw?
    if so....

    12000/0.8= 15000va

    does that seem right?
  5. Jul 12, 2010 #4
    You've been givena 'trick' quesiotn. The limiting factor in a simple alternator is the current. Current generates heat. Heat causes things to fail. How does the current change with the power factor for a linear load? When the load is linear the current and voltage are still sinusoidal.
  6. Jul 12, 2010 #5
    thanks for the responses, but the last one lost me. im very ammature when it comes to this stuff.

    Its actually a multiple choice question with the answers being...
    b)apparent power

    my guess is apparent power. and i think (a) is there to trick you, as the above calculation comes to 15kva

    what do you think?

    thanks again
  7. Jul 12, 2010 #6
    Sorry to throw you in the wrong direction. 15kw is correct.
  8. Jul 12, 2010 #7


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    Gold Member

    If you can remember trigonometry, that is the reasoning behind maths that applies here, as per the 'power triangle' (attached).

    Attached Files:

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