How to calculate 3 phase Alternator line currents?

Jul 31, 2004 2 Replies
M

This problem has been bugging me as i used to be able to do this!



Ok we have 20KW @ 0.8Pf 200V/400Hz 3 phase power units for supplying power to aircraft. These are the manufacturers quoted figures.



We have a Resistive element load bank (no inductive elements)



The manufactures quote 72Amp maximum line currents per phase for this system.



Is this the true current (KW) or apparrent current (KVA) they have quoted?



As a learning curve for me, could someone show me the method of simply calculating the Kw, KVA and KVAR line currents that make up the 3, 4, 5 traingle (0.8PF) for this example of a 20 KW generator @ 0.8PF.


Any help apprehciated Regards Martin


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A

In your 3, 4, 5, 0.8pf triangle the sides are KVAR, KW, KVA respectfully.

The loadbank is good for about 25 KW (200 * 72 * sqrt(3)).

ARM

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T

kW is power not current

as you have specified a resistive load the PF is therefore 1.0 kW will equal kVA

for 3 phase watts = volts x amps x PF x 1.732 (type square root of 3 into google and see what happens)

for 72A x 200V x 1 x 1.732 = 24940 VA

factor in a "typical" industrial load (or a building full of computers) with a PF of .8

24940 x .8 = 19952 W

do a search for " 3 phase electric power formula" to get plenty of online references

now for extra credit figure out where the missing 48 watts went :)

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