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Current measurement issues

Discussion in 'General Electronics Discussion' started by Pexy, May 24, 2018.

  1. Pexy

    Pexy

    139
    3
    Feb 21, 2016
    Greetings
    I built a spot welder as my final work in highschool. I made measurements of all the parameters except short circuit current of the secondary. I used fluke 334(clamp meter) and could not get a reading after shorting out the secondary but was able to do so at the primary side. Is it possible that the current is higher than 600A? Or is it not working because the secondary voltage is too low but i don't think that's the problem. Any other suggestions of what is the problem here? Also could you give me new ideas on how could I meassure it?
    Thanks
     
    Last edited by a moderator: May 25, 2018
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    We can't comment on the expected current levels without details of the transformer.

    Measuring the primary side and using the usual transformer calculations (for a ballpark figure) should work though.
     
  3. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    A clamp on current meter measures current, voltage has nothing to do with it.
     
  4. kellys_eye

    kellys_eye

    4,281
    1,144
    Jun 25, 2010
    Primary current times primary voltage = watts. Assuming 'perfect' transformation, x watts in the primary = x watts in the secondary.

    Measure transformer primary volts and current (at full load) and do the maths (for ball park indication of whether you're exceeding your meters 600A range).
     
  5. Pexy

    Pexy

    139
    3
    Feb 21, 2016
    Yes, that's what i thought. Thanks.
     
  6. Pexy

    Pexy

    139
    3
    Feb 21, 2016
    if that's the case: primary volts= 240V times primary current(full load)=10.6A = about 2300W
    So 2300 divided by 1.5 (secondary voltage) = about 1500A
    So it probably exceeds 600A

    Here's the file with more information:
     

    Attached Files:

    Last edited by a moderator: May 25, 2018
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Doh! You think? Looking at the PDF file you included, I noticed that the no-load primary current is 5.5 A, about half the full-load primary current of 10.6 A. That's terrible! Looks like you didn't use enough steel in the transformer core, or not enough turns of wire on the primary, or both.

    Many hobbyists building low-voltage, high-current, welding power supplies use a salvaged power transformer from a microwave oven, carefully sawing off the high-voltage secondary but leaving the primary intact. Then a few turns of heavy-gauge wire threaded through the core suffices for the secondary. Plenty of videos on the Internet show how to do this. An oven with at least 900 watts microwave power rating is desirable, but more power means a bigger transformer and more current available for the secondary winding before core saturation occurs. You can also use multiple transformers to increase power capability or to accommodate 120 V AC primary (wire the two primary winding in series) if your available power is 220 V AC. Bear in mind that standard convenience outlets can only supply 15 A at 120 V AC, or about 1800 watts. Not sure what house wiring in Croatia supports at 240 V AC, but 15 to 20 A is probably realistic, so a maximum power of 4800 watts may be reasonable.

    For measuring secondary current, you could try splitting the secondary into two (or more) parallel lengths of heavy-gauge wire and shorting the ends together. Clamp the ammeter around just one of the paralleled wires and assume the current in the other wire(s) is the same.
     
  8. Pexy

    Pexy

    139
    3
    Feb 21, 2016
    I used a microwave oven transformer and modified it like this so I don't know why the numbers are so terrible. Would the numbers 4A for the no load primary current and 12A for the full load make more sense? First time i used a cheap Ebay panel power meter to measure this things. Later I borrowed the fluke and got this results. I also can't measure the current the way you suggested.
    Thanks
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Sure you can. Just get two (or more) lengths of heavy-gauge wire, maybe a foot or so long and clamp the ends together on each end. If the wire happens to be insulated, strip off about an inch of insulation on each end before inserting the wire into the pressure clamp. Now separate out one of the individual lengths far enough to allow your clamp-on meter to clamp over just that one. Use this arrangement to "short out" your secondary.

    I would use largeish copper pressure clamps for this exercise, stuffing one wire of the secondary into one side of the clamp and stuffing the paralleled wires into the other side. Repeat for the other secondary wire. It is only a temporary arrangement to allow you to accurately measure the short-circuit secondary current. You can probably use the copper pressure clamps to connect the secondary to your "welding rig" when you get to that point.

    Since you used a MOT (Microwave Oven Transformer) for your project, I can think of only ONE reason why it does not appear to work: there is an excessive insulating gap between the "E" and the "I" laminations. That would also explain why your primary current is so high without a load on the secondary.

    Most commercial power transformers I have seen (I don't know about MOTs) used an interleaved scheme to stack up "E" laminations alternately with "I" laminations. This method of assembly provides a much better magnetic path through the core than just butting a stack of "E" laminations up against a stack of "I" laminations... unless the mating surfaces are polished very smooth to eliminate an air-gap and held under heavy compression while they are welded together. You probably (hopefully) learned in school the reason for using laminated steel strips, insulated from each other, to isolate and minimize the flow of eddy currents induced by alternating current in the primary and secondary windings. It is important that this be done in such a way as to minimize any air gaps in the magnetic path.

    The interleaved "E" and "I" technique of transformer core assembly is not the best approach to this. A single long, thin, strip of grain-oriented silicon transformer steel, painted on both sides with insulating thin enamel, arranged in a coil around the the windings is better. Variable transformers or Variacs are made this way and the construction is very efficient both magnetically and electrically. It is costly to manufacture this way though.

    One of the videos I saw for disassembling a MOT indicated these were originally made by welding the "I" stack of laminations to the "E" stack of laminations. The video suggested gluing the two pieces back together, after breaking the weld by cutting or grinding, and then removing the high-voltage secondary. I didn't watch the whole thing to see exactly how the "I" stack was re-mated with the "E" stack, but if I were doing it I would make sure the mating surfaces were smooth and clean and use a very thin layer of either contact cement or superglue.

    I happen to have an old microwave oven in the garage that I want to salvage the MOT from. I will let you know in a separate thread how that turned out. Meanwhile, why don't you check the magnetic path between the "E" and "I" stacks of your MOT to see if there is a huge "glue gap" that you can fix?
     
  10. Pexy

    Pexy

    139
    3
    Feb 21, 2016
     
  11. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Haven't started a MOT project yet, but I did check the garage awhile ago to make sure the microwave oven was still there. It is a really nice 1200 watt General Electric branded oven that is poorly made. There are several interlock switches that have to be either closed or opened before the magnetron will energize, and these switches are operated by unreliable mechanical interfaces to the two door latches that simply will not stay aligned.

    I could defeat all these switches and operate the microwave without safety interlocks because, fortunately, the manufacturer included a full schematic wiring diagram in a little envelope taped inside the cover. So, modding it is no big deal for me, but it would not be very safe for my wife to use because she could start the oven without fully closing or latching the door.

    So, I dragged the beastie as part of full load on a Pensky one-way rental truck from Dayton, OH to Venice, FL in December 2016 and then immediately purchased a smaller 700 watt microwave oven from Wally World. The smaller oven suits us just fine for heating up a cup of coffee, a cup of soup or tea, or a frozen pot pie. Still, I keep postponing the inevitable tear-down and salvage of parts from the defunct microwave. This particular oven replaced, about ten years ago, a similar Panasonic microwave oven that finally died of old age. I left that one in the basement of our house in Dayton, but may go back and salvage "stuff" from it before we sell the house. Microwave ovens, like so many electronic consumer items today, are considered disposable rather than repairable although parts are readily available. The problem is the cost of parts and labor (time) to repair exceeds the cost of replacement with a new one. That's why the mom and pop tv and radio repair shops have all but vanished here in America.

    But I will take that puppy apart and salvage what I can. It is taking up valuable space on a table in my garage that I can put to better use... just as soon as I find my "round tuit".
     
    Pexy likes this.
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