3-Phase of an Electrical Machine

Discussion in 'General Electronics Chat' started by Management, Oct 18, 2009.

  1. Management

    Thread Starter Active Member

    Sep 18, 2007

    I would like clarification on nomenclature when it comes to report out specs of a generator that goes to a 3-phase system.

    If the specs of a generator is reported to have an voltage of 10 kV and a PF of 0.9, is that voltage the line-line voltage or the per phase voltage? I'm thinking phase voltage but not sure.

    Also, it is reported to have an output power of 6 MW. Since it says megawatts I am going to assume that this is real power and in RMS, correct? Is this power into the load?

    So if I wanted to find the current per phase it would be the power into the load divided by the product of the PF and voltage per phase, correct? Or should I divide the power by 3 first?
    Last edited: Oct 18, 2009
  2. Management

    Thread Starter Active Member

    Sep 18, 2007
    No one?

    I read that if they don't state what it is that I should assume it to be line-line voltage, Vab.

    So if this is the case then the phase voltage is:
    Vph = Vab / (sqrt(3)*ang(30º)

    Is this correct. I need to know the phase voltage and the phase current. If someone can confirm this or correct me it would be great appreciated.

    Also, if the power is 6 MegaWatts, then the phase current would be (6 MW / 3) / Vph or is it 6 MW / Vph.

    Is this correct?
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Without seeing the actual machine specs, it's a guess that the voltage is phase-to-phase.

    I can't remember the 'official' calcs for phase current, but if you measure all phases you are measuring the same current twice. Think of a single phase load drawing 100A, if you measure both connections they read 100A each, it's still drawing 100A not drawing 200A.
    (100 x 2) / 2 (feed + return) = 100A

    With three phase it's something like
    (100 x 3) / 2 (feed plus return) = 150A equivalent on single phase.

    6MW @ 10KV = 600A single phase or 400A three phase??
    (Please allow for power factor & me being half asleep...)
  4. Management

    Thread Starter Active Member

    Sep 18, 2007
    So if they give you a power spec should I assume that it is 3 phase and not the power per phase? Does it make a difference? Was reading this site's 3-phase power section: http://eece.ksu.edu/~starret/581/3phase.html#8 It says to assume it's 3 phase. and it's equations says that it is the sum of each "phase power".

    This website said: Power (single phase): P = Vp×Ip×cos φ

    So should I divide that 6 MW by 3 before trying to find that current?

    Also, I found that I can just consider the voltage value and get Vp = Vab/sqrt(3). And then get Ip from the above equation where P is 6 MW / 3.

    Do you agree with this approach?
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    I would think ...

    The 10kV is probably line-to-line [phase-to-phase]. Probably quoted 6MW is real power otherwise it would 6MVA - but it seems strange one would quote MW rather than MVA...?


    Power per phase=2MW

    At unity pf, Iphase=2e^6/5774=346Amps
    At 0.9pf, Iphase=346/0.9=385Amps
  6. Management

    Thread Starter Active Member

    Sep 18, 2007
    Great! You confirmed what I said above. Thanks a lot.