3phase power calculations

  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. uart

    uart 2,776
    Science Advisor

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