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3phase power calculations 
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#1
Apr1111, 04:58 AM

P: 24

I have never really studied the heavy side of electrical engineering. We are currently running a machine that runs off 3 phase supply. I want to measure the kW or KVAh on this machine using a CT which is all already set up. The Current at maximum on 1 phase reads at about 130A, and the phases are balanced. So I wanted to know how to calculate the power of the system regarding the fact I'm only measuring 1 phase? Although it seems to me that the current flowing in 1 phase is the tatal current used by the machine in any case, so I would calculate 400V*130A*sqrt3 = Power and just times this by hours and divide by 1000 to kVAh.



#2
Apr1111, 06:12 AM

Sci Advisor
P: 2,751

Yes, assuming that your phase to phase voltage is 400 volts RMS then that is correct for the KVAhrs. If you want to know the kWhrs you'll also need to know the phase angle.



#3
Apr1311, 06:30 PM

P: 18

I believe this is wrong if 400V is your phase to phase voltage.
Power in a threephase system is 3*phase to neutral voltage*current on one phase. So this would be 3*400V*130A. 


#4
Apr1411, 07:47 AM

P: 28

3phase power calculations
3phase system:
Power = 3 x Power of 1phase = 3 x phase to neutral voltage x current in one phase x power factor or Power = sqrt3 x phase to phase voltage x current in one phase x power factor as phase to phase voltage is sqrt3 x phase to neutral voltage, both equation will result same. 


#5
Apr1411, 10:29 AM

P: 18

Wasn't sure about the second equation but yeah this looks correct.
And I can't edit my last post but in "3*400V*130A", instead of 400V I meant "400V/sqrt3" if 400V is indeed the phase to phase voltage as mentionned earlier. 


#6
Apr1511, 08:13 AM

P: 1

there have some already product, that can use CT, measure curren,voltage,PF,Wh etc. there I suggest you can find the meter here http://www.cqbluejay.com/view_50.html



#7
Apr2311, 08:02 PM

P: 1

which power do you want to measure apparent power, active power, or reactive power?



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