Another 3 Phase Question: Application Specific

Discussion in 'General Electronics Chat' started by Screamtruth, Mar 17, 2011.

  1. Screamtruth

    Thread Starter Member

    Apr 17, 2006

    This has been covered in this, and many other, forums; however, I have a more application specific twist to the question.

    This concerns the current 'return path' when two phases, from a shipboard generator, are connected to the primary coils of a 'single phase' transformer.

    Specific Application:

    I have three phases coming from a generator on a vessel; I connect two of the phases across the primary input of a 480VAC/220VAC step down transformer. As there is no 'neutral', or third tap/lead on the primary side of the transformer, what/where is the return path for the current (primary side)?

    I have always assumed that the current returns via the two connected phases, but I cannot visualize this.
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    It would have a hard time returning on the unconnected phase. Strive as best you can however, to ensure that the third phase is drawing somewhere to keep the system balanced.
  3. billbehen

    Active Member

    May 10, 2006
    since they are out of phase, positive current is coming from one phase and (somewhat) returning through the other as it goes negative. but as mentioned above you'll have an imbalance. regular two wire AC is a full 180 degrees out of phase (push - me -pull you) but 3phase is only 120 deg
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
    I have also always wondered this, but never been curious enough to ask. Now I am installing a transformer and the question comes up; how ironic.

    So, since the phases are imbalanced, what is the effect on the rest of the equipment on the primary 3phase system? Also, what effect does the imbalance have on the equipment on the secondary side?

    I am installing (3) 3hp 480V inverter drives in a cabinet and I need to power 3 120V motor cooling fans, drawing .8A each. I have a 500VA control voltage transformer (480>120) that I am planning to use to power the fans. Do you expect that will work? I never though of the effect the 120degree phase shift might have on a single phase motor that is meant to be run 180 degrees out of phase. Or, the effect on the drives due to the imbalanced loading. hmmmm.... Please advise, It's not too late to rip it out; I haven't put power to it yet. What would you do to load the 3rd phase? just put a big resistor on it?

    ...sorry if this is considered highjacking; I'm pretty sure it's the exact same topic though
  5. subtech

    AAC Fanatic!

    Nov 21, 2006
    I think there is something inherently confusing when someone says "I'm connected across two phases of a three phase system".
    If the output of the generator is three phase, then there are three single phase sources available from the three conductors. The first is from phase A to phase B. The second from phase B to phase C, and then C to A. In short, when you have connected the primary winding of your stepdown transformer to any two of the three available conductors, you've connected to a single phase source.
    As far as the current flows, what goes in must come back out.
    If your transformer is connected A to B, and you measure 10 amperes of current in the phase A conductor, then that is what you will measure in the phase B conductor as well.
    Last edited: Mar 17, 2011
  6. Screamtruth

    Thread Starter Member

    Apr 17, 2006

    Great explanation; visualizing the A-B, B-C, A-C concept really made the difference for me.
  7. Screamtruth

    Thread Starter Member

    Apr 17, 2006
    Same topic, for sure.

    No worries!