# Kva rating calculation

1. Jul 12, 2010

### tobiascarter

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
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2. Jul 12, 2010

### Phrak

What limits the power output of your alternator?

3. Jul 12, 2010

### tobiascarter

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
=15kva

does that seem right?

4. Jul 12, 2010

### Phrak

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.

5. Jul 12, 2010

### tobiascarter

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...
a)15kw
b)apparent power
c)2w
d)3000w
e)11000va

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

6. Jul 12, 2010

### Phrak

Sorry to throw you in the wrong direction. 15kw is correct.

7. Jul 12, 2010

### Zryn

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

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