Multi-tap inductor fried by shorting taps

Discussion in 'General Electronics Chat' started by Chaos7692, Aug 6, 2016.

  1. Chaos7692

    Thread Starter New Member

    Aug 6, 2016
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    Had a situation at work today that I'm trying to fully understand. We test generators by applying loads, resistive and reactive. The reactors have multiple taps to be used at different voltages to achieve the same ratings, so the lower the voltage applied, the fewer turns used in the inductor. An operator burned up nearly 20 reactors, and I had to figure out why. The problem I found, and almost certainly the cause, was that the contactor for 346v was stuck closed, while running 277v. So basically the 277v was applied in parallel to tap 1 and 2, with neutral at the other end at the 5th tap. What I want to know is why this burned up the reactors, and how do i calculate the current through them? At first thought, it seemed since it was the furthest from the neutral tap that was mistakenly used, it should have been the same amount of current. My guess was it has to do with phase shift.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    I am unable to visualize what you are talking about. Can you provide us with a diagram and the winding details of the inductors like wire size, number of turns and so forth. Didn't they know something was wrong when they destroyed the first one, or do you have robotic automatons doing the testing?
     
  3. tcmtech

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    Nov 4, 2013
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    A multi-tap inductor sis basically the same thing as an autotransformer so when you have two taps shorted together they are basically the same as a shorted winding set in a normal transformer.

    So after they guy fried the first 3 - 4 didn't it occur to him that just maybe there is something wrong with the test rig and cooking the next 16 - 17 was totally unnecessary. :rolleyes:
     
  4. #12

    Expert

    Nov 30, 2010
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    I guess that's the price of hiring uneducated people for the lowest possible wage.
    We used to call them, "(production) line monkeys". They don't know electronics, they know how to do the "procedure" they are reading.
     
  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Exactly
    Lets use this as an example inductor:
    [​IMG]

    How much real current and power caused the damage is tricky to calculate because the DC resistance of the wiring has to be considered and could result in very high currents in the shorted section being reflected back to the source.
     
    Last edited: Aug 6, 2016
  6. tcmtech

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    Nov 4, 2013
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    The people I've worked with like that were typically in management positions or quickly moved into them.

    I think I have told the story here a time or two about one of my old bosses who ordered a bunch of new bandsaw blades only to find out that once he uncoiled them from their shipping package they were all 'welded backwards' so he gave the company who we ordered them from a raging phone call about it and sent whole lot back? :oops:

    I don't know how the rest of the world works but where I have worked apparently 'those who can' stay working on the line and 'those who can't' rise to management. :mad:
     
  7. #12

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    Aha. So that was my mistake!
     
  8. crutschow

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    Mar 14, 2008
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    A tcmtech noted the inductor looks like an autotransformer with the output shorted so its current was limited mainly by the inductor winding resistance and other circuit resistance with only the leakage inductance adding a small additional impedance to the current.
    I would suggest adding a circuit breaker to limit the circuit to the inductor below it's maximum current rating.
     
  9. #12

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    Are you kidding? Somebody in a suit would have to put down his martini and sign a paper to pay for that circuit breaker.
     
    nsaspook likes this.
  10. Chaos7692

    Thread Starter New Member

    Aug 6, 2016
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    Wow, lots of responses. To clear up the mistake, the reactors were all applied at the same time. It was a 3 phase generator, using 1 reactor per phase, and 6 steps of fairly low kvar. Each reactor was fused, and they did blow, just not before the reactors and wiring burned up. The diagram posted above is a good example, with taps 6 and 7 having 277 applied in parallel, and neutral at tap 1.
     
  11. #12

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    3 phase circuit breakers exist.
     
  12. Chaos7692

    Thread Starter New Member

    Aug 6, 2016
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    The entire circuit was protected, each phase, and each reactor had a fuse, rated at various currents depending on the kvar of the reactor and a main 3 phase breaker at the generator, rated at 250 amps. This is precisely why im trying to figure out how exactly the wires and inductors could have burned up before blowing the fuse. Its possible that the fuses didnt even blow until the wires shorted together, but not likely because not every segment had wires burned all the way through, but they all had blown fuses.
     
  13. ian field

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    Oct 27, 2012
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    The common term is: "kicked upstairs".
     
  14. nsaspook

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    When those taps shorted the inductor stopped being mainly a energy storage device with it's inductance/magnetic field and changed into a energy dissipating (resistor) device moving power into the shorted segment DC resistance from the generator with currents maybe less than an direct phase to neutral line fault condition (that would have blown the fuses quickly) before the fuses blew and after significant damage had occurred .
     
  15. ian field

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    On an inductor in an AC system; shorted turns have pretty much the same effect as core saturation on the peaks of the waveform.

    In a DC system; it reduces the total winding resistance in proportion to the number of turns between the shorted taps.
     
  16. nsaspook

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    I wasn't talking a DC systems, only the equivalent DC resistance of the shorted core winding. As you say with the shorted taps the amount of EMF from mutual induction is limited by the loss of some amount of reactive impedance in the rest of the inductor when the taps of the inductor are short circuited.

    http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/tracir.html
    tracir4.gif
     
    Last edited: Aug 7, 2016
  17. Chaos7692

    Thread Starter New Member

    Aug 6, 2016
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    I think I have what I need, thank you for your replies. But by all means continue the conversation if you want, I'm only beginning to understand inductive loads. Now I have to try to explain it to management, who of course know nothing. This failure will cost at least $10,000 in parts, and probably 6 weeks of down time. That's why I'm trying to gather all the info I can before they ask. Thanks again!
     
  18. Chaos7692

    Thread Starter New Member

    Aug 6, 2016
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    Okay, so it seems I have an engineer at work arguing that the problem described, should not have caused the reactors to burn up, because the taps that were shorted were 277v and 346v while running at 277v. Had it been the other way around, running 346v it would make sense. This would certainly be the case in a resistive load, but it's not a resistive load.
     
  19. nsaspook

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    What type of engineer is this person?
     
  20. Chaos7692

    Thread Starter New Member

    Aug 6, 2016
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    He's got his P.E. Pretty sure electrical
     
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