Need help identifying 3-phase connection

Discussion in 'General Electronics Chat' started by GenStudent, Mar 16, 2014.

  1. GenStudent

    Thread Starter New Member

    Mar 16, 2014
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    I am trying to construct a circuit diagram for a diesel generator power system. I have read quite a bit online about 3-phase, but I haven't found an explanation for what I am seeing:

    The generator has only three leads. Between lead "A" and lead "B" I read 208 volts. Between "A" and "C" I read 120 volts and between "B" and "C" I read 120 volts.

    I don't think these voltages are consistent with normal delta, high-leg delta or open delta, delta with a center tap on one phase, nor with a wye system. Incidentally, the generator powers 120V lights, some baseboard heaters of unknown voltage, and a big motor that is labelled "3 phase."

    Can anybody tell me what connection arrangement I have?

    Thanks in advance.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    There is on more lead someplace. Follow it back from the three leads to the motor.
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    And the generator model is what?
     
  4. GenStudent

    Thread Starter New Member

    Mar 16, 2014
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    Good idea to start with the model number! I'll scrape off some of the dirt and grime and try to find it.
     
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
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    It would appear you have a three phase four wire 120/208 'Wye' connected genset but you are missing one leg.

    Connection 'C' appears to be your common center point for the wye and you are missing a 'D' connection for the third phase.

    I have a old military genset that can be reconfigured for a number of different voltage and phase combinations and if I am remembering right in one of the 3ph connections for the 120/208 output one of the lugs that is a phase in another configuration becomes the center point connection for the wye setting.
     
  6. Zerotolerance

    Member

    Sep 18, 2011
    56
    10
    It seems that your C leg has blown a fuse or something. If you had a WYE configuration you would have 4 wires total. I am thinking you are missing the fourth wire though somewhere.
     
    Last edited: Mar 17, 2014
  7. inwo

    Well-Known Member

    Nov 7, 2013
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    Right you are.
    I learned something today.:D

    I don't understand it though.:confused:
    Would need more than the 6 leads to get 120/240 single phase.:cool:

    "Phase Conversion in Generators:
    (1) Reconfiguring Coil Connection
    A three-phase generator can be converted to a single-phase one by altering the connection between its stator windings inside or outside the generator head. For instance, in the case of a 3-phase generator, you would have 6 leads. Larger generators commonly have 12 leads from six coils and all the wires come out of the generator making it easier to configure the generator in a variety of ways as follows –

    • Connecting the coils in series will convert the generator to a single-phase one.
    • By connecting opposite coils in series, you can double the output voltage.
    • A parallel connection will double the flow of current. "

    http://www.dieselserviceandsupply.com/Generator_Phase_Conversions.aspx
     
  8. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Six leads is all you need to get 120/240 single phase from a 120/208 or 120/120/120 three phase genset.

    For the 120/240 single phase setup all you do is configure it into the 120/120/120 delta setup then shift your bottom phase left or right. (_/\ or /\_)
    From that the bottom phase connections that were moved left or right make up one side of the 120/240 and the other two, still in their open ended delta configuration, make up the opposing 120 so when connected end to end you get 120 - 0 - 120 with the proper 240 across the two ends. ;)
     
    inwo likes this.
  9. Zerotolerance

    Member

    Sep 18, 2011
    56
    10
    Glad you got this figured out!
     
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