Fluke 374 testing a 3 phase bridge

This topic contains 8 replies, has 3 voices, and was last updated by  Damion Pomeroy 7 months, 2 weeks ago.

  • Author
    Posts
  • #8349

    Chauncey Orourke
    Participant

    Hi I would like to know how to use my Fluke 374 to test a 3 phase bridge that I think has failed?


  • #8350
    John C
    John C
    Galco Representative

    Hi Chauncey, thanks for asking Galco today.

    Ok does the rectifier module have any signs of physical damage (cracks in the case)?
    Are there 3 terminals along the center and 2 terminals on the end of it?


  • #8351

    Chauncey Orourke
    Participant

    Nope, no signs of cracks in the case.
    Yes it appears to have 3 down center and 2 terminals at 1 end.


  • #8352
    John C
    John C
    Galco Representative

    OK then plug the test leads into the bottom of the 374 Fluke meter, now put the red lead on the minus terminal, and the black lead on 1 of the 3 in center. You should read a resistance. If you swap the leads the meter should read infinite or open. Next go to the plus terminal with the black lead and put the red on the center terminals. It should read resistance again and swap should read open.


  • #8353

    Chauncey Orourke
    Participant

    The center terminal of the 3 in the center of the module always reads 0 to the minus terminal but none of the other do.


  • #8354
    John C
    John C
    Galco Representative

    If this is true, then the module has a shorted rectifier in it and it has to be replaced.


  • #8361

    Damion Pomeroy
    Participant

    Hi was reading this post and cant figure out why you put the black lead of the meter on the plus terminal, isn’t that backwards?


  • #8362
    John C
    John C
    Galco Representative

    That’s because the plus and minus terminals are not the input power terminals of the rectifier. The AC connects to the 3 terminals in the center of the module, the plus and minus terminals are the output power terminals. If you search a diagram for a “3 phase rectifier” click images and you will find diagrams showing all the common anodes connected to the minus terminal of module. Conversely, you will also notice all the cathodes connected common go to the plus terminal. This is because the AC terminals are the power source or input. So the anode of 1 of the rectifiers with its cathode connected to the plus terminal, will forward bias when the AC is positive dropping about .4 to .6 volts and the rest of the voltage is on the plus terminal now. If you have a 480 volt power source, then the diode drops its .4 and the other 479.6 volts in on the plus terminal, minus terminal is same way but opposite polarity.
    This help?


  • #8363

    Damion Pomeroy
    Participant

    I found some diagrams and I see what you mean, thanks John


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