How did all my fuses blow?

Discussion in 'General Electronics Chat' started by strantor, Aug 26, 2011.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    At work I have a machine with 9 small Variable Frequency Drives powered on a common DC bus from a very large Variable Frequency drive through sliprings. There are 50A main DC bus fuses and 16A fuses on each of the small drives. all the small drives are in parallel. see drawing attached. twice in the past week, all of these fuses have blown simultaneuosly. I can't figure out WHY. The HOW is already figured out: I found a short between + and - near the sliprings. In my quest to figure out WHY, I contacted the manufacturer and they said to check for a short near the sliprings; I told them I already found that and they basically said "well, there you go" and didn't give me a WHY. In my thinking, if a short occurred near the sliprings, only the main bus fuses would blow. if the short occurred on one of the small drives then the smaller 16A fuses for that drive (that drive only) would blow; worst case scenario, both the 16A fuses (for that drive) and the 50A main bus fuses would fuses blow. I cannot think of a scenario other than all 9 small drives simultaneously shorting out and blowing their fuses withinin the fraction of a second that it takes the main bus fuses to blow. any insight?
    FYI, while running, these small drives (all 9) draw about 2A combined, whether accelerating, decelerating, or running constant speed.
     
  2. ian123

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    Aug 24, 2011
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    was the short on the bottom of the slipring
     
  3. strantor

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    No, actually it wasn't on the slipring at all, it was the incoming power cable, right before it terminates to the brushes. the rotating part of the slipring had rubbed the insulation off.
     
  4. praondevou

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    Jul 9, 2011
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    The small inverter DC inputs were also shorted and before the short occured they were energized. What kind of rectifier do they use?

    I could imagine that somehow the energy stored in capacitors inside the small inverters can go back to the input (which was shorted).

    However, if it has a simple fullbridge rectifier this shouldn't be possible....

    You could open a spare one to see if that's a possibility.
     
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  5. strantor

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    Ah! I think you've got it right. The recifier is in the large inverter. The small inverters are taking 650VDC straight into their DC bus, no rectifier involved. So the caps in the small inverters are what blew the 16A fuses.

    Thanks praondevou!
     
  6. praondevou

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    oops , I was thinking the small inverters are "normal" AC/AC frequency inverters, powered with DC...

    Yes if they are DC/AC converters then the caps are the problem.
     
  7. R!f@@

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    If the Rectifier located in the big drive?
     
  8. strantor

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    I was thinking of installing Diodes to prevent this, but I strongly suspect that this will cause problems with regeneration and may blow the drive instead of the fuses. see attached.
     
  9. strantor

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    ..........yes it is...........
     
  10. praondevou

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    yes, if the drive doesn't have a resistor or similar component to consume energy while the motor is working as a generator it will elevate the internal drive DC-bus, in worst case it'll blow. If the big drive has a regulated rectifier (with feedback), it would probably just decrease regulation to maintain it's DC output at the desired level when a small inverter feeds energy back to the bus.
     
    Last edited: Aug 26, 2011
  11. R!f@@

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    Hmm..U have to use high speed schottky diodes if you u wanna isolate 'em liittle ones.
     
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