Calculate KVA Rating for 12kW Load at 0.8 Lag

[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 don't know what formula to use so Any help would be great. cheers
------------------------------

 

[Phrak]

What limits the power output of your alternator?

 

[tobiascarter]

it doesn't say? this is why i don't 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?

 

[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.

 

[tobiascarter]

thanks for the responses, but the last one lost me. I am 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

 

[Phrak]

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

 

[Zryn]

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