generator sizing

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jojo

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Philippines
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Electrical Engineer
gud day!!! i just want to know the formula in getting the kilowatt size of an alternator when the given is the total kVA load.. what i know form other engineers is that the
kilowatt size of the genset = (120%)(total kVA of the load).. is this formula correct for single phase as well as for three phase systems? thanks..
 
Re: generator sizing

Your formula is backwards, at best. Sometimes, KVA = (120%) of KW, but it is physically impossible for it to be the other way around.

I can't draw pictures and post them. So I'll describe something, and you draw it on paper.

Draw a short horizontal line segment. At the right end point, draw a short vertical line segment. Connect the left end point of the horizontal line segment with the upper end point of the vertical line segment. You have drawn a right triangle, and the right angle should be on the right side of the figure.

Label the horizontal line segment "KW," the vertical line segment "KVAR," and the hypotenuse "KVA." Label the angle at the left edge of the figure "theta."

"Theta" is the phase angle by which the voltage leads the current (in an inductive circuit, which we see more often than a capacitive circuit). The cosine of theta is the "power factor" of the circuit.

To get KW, if you know KVA, you multiply KVA times the power factor. But this necessarily means that just knowing KVA is not enough. You need two facts (i.e., lengths of sides or measures of angles) about any right triangle, in order to calculate the remaining facts. KVA is just one side (the hypotenuse), one fact. You need one more.

Please note that a power factor of 83.3% will give you the expression that I put at the top of this post: If pf = .833, then KVA = (120%) of KW.

Please note also that KVA, being the hypotenuse of the triangle, can never be shorter than either of the two sides. That is why the expression you gave is impossible.
 
Re: generator sizing

Jojo
It depends on the load. If you have a lighting load then you would be OK. However if you have large A/C units then you need to take a different approach. The Gen must be able to start the A/C unit as well a run it. Starting on most any motor is about 4 to 6 times its FLA.
Suggest you contact a MFG and get more information.

[ August 18, 2005, 04:49 PM: Message edited by: bob ]
 
Re: generator sizing

hello ... thanks for the info. ive just read your messages.. thats why i ask in this forum because im doubtful of the formula that the kilowatt rating of the generator= (120%)kVA load.. because in our electrical engineering, PF= kW/kVA. i was just curious if the 120% in the formula they gave was a constant.. thnks. you've really enlighten me.
 
Re: generator sizing

Not a constant, more like a fudge factor to avoid calculating the load and sizing the generator for it. Each manufacturer will have information about the capabilities of each of their models.

Jim
 
Re: generator sizing

Originally posted by jojo:
hello ... thanks for the info. ive just read your messages.. thats why i ask in this forum because im doubtful of the formula that the kilowatt rating of the generator= (120%)kVA load.. because in our electrical engineering, PF= kW/kVA. i was just curious if the 120% in the formula they gave was a constant.. thnks. you've really enlighten me.
Hi Jojo,
the formula pf=kw/kva is right..solving the kva value would give you kva=kw/pf..in this case,you will always get kva greater than kw value because your divisor is always less than 1 except when the pf is unity of course.

Best regards,

Danilo
 
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