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3phase power calculations

  1. Apr 11, 2011 #1
    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. jcsd
  3. Apr 11, 2011 #2


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    Yes, assuming that your phase to phase voltage is 400 volts RMS then that is correct for the KVA-hrs. If you want to know the kW-hrs you'll also need to know the phase angle.
  4. Apr 13, 2011 #3
    I believe this is wrong if 400V is your phase to phase voltage.

    Power in a three-phase system is 3*phase to neutral voltage*current on one phase. So this would be 3*400V*130A.
  5. Apr 14, 2011 #4
    3-phase system:

    Power = 3 x Power of 1-phase = 3 x phase to neutral voltage x current in one phase x power factor

    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.
  6. Apr 14, 2011 #5
    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.
  7. Apr 15, 2011 #6
  8. Apr 23, 2011 #7
    which power do you want to measure apparent power, active power, or reactive power?
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