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Power Equations
3

Power Equations

Power Equations

(OP)
I have a problem.
We have an unbalanced 3 phase load from 2 distribution boards fed from a 60kVa ups.
Does anyone know of the equation for working out kVa of an unbalanced load from the 3 different line currents. The voltage is 415v (U.K.)

Regards
Gavin

RE: Power Equations

if your 3-phase load is not ballanced and not grounded, you will need to know the impedances of each phase, and all 3 currents. if your load has a ground (3-phase, 4 -wire), sum up the currents and multiply by 415V. What is your load?

RE: Power Equations

Apparent power (KVA): S(3ph)=P+jQ=VaIa*+VbIb*+VcIc*, of course, each of these phase voltages and currents are phasors and (*) is the conjugate of...

RE: Power Equations

Or if you don't know about conjugates:

P(3ph)=Va*Ia*cos(a)+Vb*Ib*cos(b)+Vc*Ic*cos(c)    

Q(3ph)=Va*Ia*sin(a)+Vb*Ib*sin(b)+Vc*Ic*sin(c)

Where a is the angle between Va and Ia, b is the angle between Vb and Ib, and c is the angle between Vc and Ic.

RE: Power Equations

For kVA, simply multiply each phase current by the appropriate phase voltage(L-N, or L-L Voltage/1.73) and sum. For circuits with similar PF values, this will be reasonably accurate.

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