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Line Current in a 2 phase transformer

  1. Sep 25, 2013 #1
    I am a graduate engineer and have been set a small task to work out the current (primary side) in a 2 phase 30kVA 415:650 transformer. The 2 phases have been utilized from a 3 phase source and are hence 120 degrees out of phase.

    I understand had it been a 3 phase 30kVA transformer I could calculate my current by 30kVA / 415 / route*3 = 42A.

    If it was a single phase then simply 30kVA / 415 = 73A.

    However if I am only using 2 phases from a 3 phase source yet still need to produce 30kVA of power, how do I calculate the line current?

    Thanks in advance.
  2. jcsd
  3. Sep 25, 2013 #2
    Hello Anoor and Welcom :

    I think we will need a little more info -- you are describing a 30KVA trans but the Voltage is 415:650? And by 2 phase - is the secondary like an Open delta - or some other arrangement? Best bet is to post a diagram of the nameplate info.

    Lastly -It is not clear if you are actually working on this physically? In the USA you would not be a " qualified person" - working on this is extremely dangerous, does your University have a qualified electrician that you can work with?.
  4. Sep 25, 2013 #3
    I am not working on it physically its just in theory.

    Maybe its my poor explanation ill try again!

    If I have a standard 230/400V switchboard and want to connect a transformer it would normally be a 3 phase (400V) or single phase-neutral (230V). However if I wanted to connect a transformer to phases L1 & L2, instead of the neutral (giving me 400V). What would my line current be on the primary and how would I calculate it? I apologise if my ignorance doesnt allow me to adequately suscribe the situation! I have attached a pdf showing an extract of the schematic, I hope this helps!

    Thanks again.

    Attached Files:

  5. Sep 25, 2013 #4
    OK the diagram help a little. If you think of the "phase" as between the lines, as in L2-L3 - this is really a single phase question - IN the USA we refer to this as Line to Line. In this case there is no phase angle - so the Line current is KVA/V - also the transformer input or out put is only a 2 wire device - right?

    The lines L2 and L3 are only out of phase 120Deg relative to the neutral