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Some questions on transformers, coils, and short circuits.

  1. May 23, 2016 #1
    Hi, I'm trying to wire a transformer which steps my mains voltage (240VAC) down. I just finished wiring the the primary to the plug, and thought I should test my connections by plugging it in. The coil was not inside the core or near the secondary, it was just lying on a piece of paper.
    I plugged it into an extension cord and then switched it on. Immediately, I heard the familiar mains hum coming from the coil, but I noticed its connection with the live wire sparking and switched it off within a second.
    I removed the plug from the extension and tried to switch it on again, which did not work. Upon further inspection, I found that the fuse had blown and I replaced it. I also touched the coil and found it was rather hot.
    My questions are the following:
    1. Firstly, why did the fuse on the extension cord blow, and not that of the plug which the coil was connected to? They were both 13A Bussmann fuses. If it makes any difference, the extension cord also had two chargers connected to it (Both of which are fine, by the way).
    2. Second, how do I know this won't happen again if I place the coil in the transformer with the secondary (which will still be disconnected)? It seems to me that the coil is causing a short circuit now, but what's gonna stop it from doing that if I place it into the transformer?
    Any help would be greatly appreciated.
     
  2. jcsd
  3. May 23, 2016 #2

    jim hardy

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    You seem to be ignoring fundamentals of inductance.
    That's not unusual, it is not well taught .

    Inductance is flux linkages per amp, nΦ/I

    Coil lying of the floor?
    With no iron core, how can it possibly make enough flux to oppose line voltage?
    It takes a lot of amp-turns to push flux through air.
     
  4. May 23, 2016 #3

    phinds

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    Yeah, what Jim said.

    Basically what you've done is run a wire (a short circuit) from one terminal to the other. A bunch of turns is almost irrelevant with no core.
     
  5. May 23, 2016 #4
    Thanks, I haven't learned this at all, actually. That should explain why it won't do the same thing if I put it in a core. Do you know why the fuse on the extension plug blew, rather than the one on the coil plug?
     
  6. May 23, 2016 #5

    jim hardy

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    Wire an incandescent lamp in series with your coil to limit the current.
    Then insert your core and watch the lamp dim.
    Note it really dims when you get rid of any air gap in the core.

    Then search on "Magnetic Circuit"
    and learn what are
    permeability
    MMF (amp-turn) (Oersted)
    Flux (Weber) ( Maxwell)
    Flux Density(Tesla) (Gauss)

    old jim
     
  7. May 23, 2016 #6

    jim hardy

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    Fuses in series - first one to melt wins the race.
    Let's play 20 questions -
    Are they the same part number ?
    Are they glass fuses?
    Can you see the elements?
    Is the blown one only melted in middle and sagged from heat?
    Or is it completely vaporized and left black inside the glass?
    Is the not-blown one sagged from heat as if it only barely lost the race?
    What's its resistance? How many milli-ohms does it read?
    How many milli-ohms does a new one read ?
    If they're not glass so you cant see inside, can you cut them open and look ?

    Here's some Bussman ferrule fuses
    as you see they come fast acting, time delay, dual element, very fast acting,....
    http://www.cooperindustries.com/content/public/en/bussmann/electronics/products/bussmann-circuit-protection/fuses_and_accessories/ferrule_fuses/quarter-inch-fuses.html [Broken]
    each with its own melting characteristic.

    Good time to study up on 'Fuseology"

    old jim
     
    Last edited by a moderator: May 7, 2017
  8. May 23, 2016 #7

    berkeman

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    Thread closed for Moderation...
     
  9. May 23, 2016 #8

    berkeman

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    Sorry, you do not have the experience to be doing this project with only help from the Internet. This thread will stay closed as a dangerous discussion, Please find an experienced local person near you who can tutor you in person on dangerous projects like this.
     
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