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Tranformer Calculations and Rewinding

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  • Tranformer Calculations and Rewinding

    Hi !

    I am taking a course of transformer calculation and rewinding and the course requires that I provide calculations, provide all jigs to wind the transformer coils and wind the coils. The instructor suggest I get small welder and proceed with calculations/complete rewind.

    Anyone knows were I can get a small welding machine ?? I am in the Toronto area.


  • #2
    I would suggest starting with a microwave oven transformer.
    Most microwaves are discarded due to a failed magnetron, not the transformer.
    I have salvaged many transformers but have yet to try my own advice.
    Usually, they end up being sold as scrap to the local junkyard at around 15 cents US a pound.

    Remove the high voltage winding and replace it with a fewer turns of large gauge wire.
    You will end up with a high current transformer which can serve as a simple AC arc welder.

    There is a lot of information on the web

    But please read the following warning I came across on the web.

    Good luck with your course.

    Rewinding MOT's
    You can remove the secondary from a salvaged microwave oven transformer. Cut off the high Voltage and the very low Voltage winding using a hack saw or power tool. Hammer out the rest of the winding using a bolt or blunt punch. Remove the Iron magnetic shunts. Take care not to damage the primary (low Voltage) winding as you will be using it. A new output winding consisting of about 10 or so turns is wound onto the core. Now with a 10 amp variac you've got a variable power supply.
    It is advisable to use a variable voltage transformer (or use two MOT's with primary's connected in series) with microwave oven transformers because of the way they are designed. The person making the transformer KNEW that the transformer would ALWAYS be connected to a load (ie. the Magnetron). This allowed the designer to put very little windings on the primary winding. If you connect one of these transformers directly to the mains without a load on them they will blow a fuse because they will draw a very high magnetizing current. If you must use them without a variable voltage transformer use two of them with the primaries in series. This will solve the problem. You can connect the outputs in series too.
    Don't mess with the high voltage end of theses beasts as they can kill, and have killed. Remove the high Voltage winding.
    A web search will obtain lots of information on MOT's
    Miller Dynasty 350, Dynasty 210 DX, Hypertherm 1000, Thermal Arc GTSW400, Airco Heliwelder II, oxy-fuel setup, metal cutting bandsaw, air compressor, drill press, large first aid kit, etc.

    Call me the "Clouseau" of welding !


    • #3
      Hy Burnt !

      Thanks for you help in my project .

      Following your suggestion I make a simple evaluation wath I can get from the microwave transformer.

      I took a MOT remove the secondary, and coiled 10 turns in the core. Then connected the primary to the 117V line, I could measure the 10 turn coil voltage as 9.98V. Following calculations, I found one turn per volt, and the max induction in the core as 1.7teslas.

      The proposed arrangement has 2 MOT cores, using only the "E" sections of the cores. The "I" section will be left for other appication. The magnetic arrangement is dwaing in the attached drawing:

      Click image for larger version

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      A=35mm, B=63mm, C=17.5, D=103mm.

      The welder transformer will have 230V primary and 55V secondary, or 230 turns in the primary coil and 55 turns in the secondary. (very easy easy ??)

      As the transformer window dimensions are 17.5mm and 103mm, = 18.03square centimeter, and discounting 20% for insulation an some clearence, the available space for the coils could be 14.42sq cm. This means 7.21sq cm for the primary coil and other 7.21sq cm for secondary coil.

      The the size of primary wire was selected for such space. Following the AWG WIRE TABLE, the #16 wire is suitable for the primary. Secondary coil may be four wires #16 in parallel ( or a single wire #10AWG). The windings are to be with insulation between layers.

      Using the transformer in a sevice duty of 10% and with a air forced ventilation the copper current density could be 12A/mm2. Based on that, the secondary can carrier up to 62.4A.

      The next step is to calculate the max current in secondary with the sticking electrode and and the current in the arc. This calculations are based in the dimensions and geometry of coils arrangement.

      The leakage inductance was found as 0.048H and the leakage or dispertion impedance in the primary as 18.2 ohms... Primary maximum current as 12.3A and secondary maximun as 52.3A.

      Finally the arc current under 25V is 46.74amperes.

      It appears to be smaall current ...

      I will try to push the secondary over the primary, maybe I can get litle more arc current.

      Reducing the distance betwen the primary and secondary the arc current may be 50A and the sticking electrode current as 60A.

      The above calculation was done supposing the transformer with two coils, one primary and one secondary coil only. Primary and secondary coils are assembled side by side.

      If we split the secondary in two coils, one coil with with 11 turns and other with 44 turns (total 55 turns) and assemblind these coils at both sides of the primary the leakage inductance will be reduced and the arc current may be reach 64A and the sticking electrode current as 72A.

      What you think?

      What you think?
      Last edited by ggiill; 10-04-2014, 05:04 PM. Reason: recalculation of the coils arrangement


      • #4

        What happens if we use wire 15 rather than 16 ? Can we get more welding current ?



        • #5
          Hi !

          Lets calculate the transformer using wire #15AWG, ALL cooper wire.
          The stell core will be as the above drawing, 2 "E" sections from a old microwave transformer, assembled as the above figure. The stell core cross section is 22.05sq centimeter. A fan blowing the coils is required. Primary 230V, secondary 55V .

          a - two coils, one primary with 230 turns of 15AWG wire, with one 0.1mm Mylar insulation between the wire layers, one secondary with 55 turns of FOUR #15AWG wires in parallel with 0.1mm Mylar between layers. The maximum current out of the secundary occurs when you stick the electrode into the work piece, reaching 60A. This current is limited by the leakage inductance (dictade by coils geometry). When you are welding, the welding current is about 53 A under a 25V arc.

          b - Three coils, one primary and two secondaries. The primary with 230 turns of 15AWG wire, with one 0.1mm Mylar insulation between wire layers. One secondary coil with 45 turns and other with 10 turns. Both winding with FOUR #15AWG wires in parallel and with 0.1mm Mylar between layers. The maximum current out of the secundary occurs when you stick the electrode into the work piece, reaching 73A. This current is also limited by the leakage inductance (dictade by primary and both 2 sec coils geometry). When you are welding, the welding current is about 63 A under a 25V arc. It should be remembered that one #15AWG wire limit is 17.6A under a duty of 10%. Both secondary coils to be in series connected.
          A fan blowing the coils is required .
          It should be remember that there NO space for the variable magnetic shunt for adjusting the welding current.
          I have tryed the #14AWG for the coils, BUT no space inside the transformer window (1.75x10.3cm) to accomodate the coils.
          Last edited by ggiill; 10-15-2014, 11:45 AM. Reason: english spelling


          • #6
            Hi Ggiill


            Well you may find a burned buzzbox in a scrapyard or even in a weldind equipment repair shop.

            Some small buzzbox have the core welded, and the shoop do not open the welded core to replace the burned coils. it is cheap to buy a new buzzbox.

            But for you with the purpose for learning electromagnetics, get a burned machine, cut the welds, open the core, rewind, weld the core again, and make your experience.

            ANYONE knows a welding equipment shop near Toronto ? or has a burned buzzbox ???