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REWIRING A MOT (Microwave Oven Transformer)

Discussion in 'PROJECT OF THE DAY --- WHAT DID YOU DO IN YOUR SHOP TODAY?' started by petcnc, Mar 20, 2017.

  1. petcnc

    petcnc Greece Active User Active Member

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    A friend of mine seeks a Power Supply Unit (PSU) capable of 13.8V at 50 Amps. As the ready-made ones are a) rather expensive and b) unreliable (Chinese) he asked if I could make one for him.

    I love challenges and I could not resist this one! So I sought information on Low voltage High current PSUs.
    I found out that the most expensive part to make a high current PSU is the transformer as you need at least 800Watt power out of it! Thank God there are plenty of 800 watt transformers available for free in old microwave ovens. In a day I found a microwave unit with an as-good-as-new transformer.

    P3191591s.jpg

    These transformers have a 230V (Europe) thick wire primary and 2000V thin wire secondary to produce the high voltage needed for the microwave magnetron!

    As I only need the primary I cut and removed the secondary using my hammer and an old wood chisel.

    P3201625s.jpg

    P3191594s.jpg

    After removing the secondary wires (hammering them out of the transformer) I had a transformer body with a primary winding ready to accept my new High Current-Low voltage secondary.

    P3191592s.jpg

    I started by winding 5 turns of an insulated piece of wire for a secondary and after connecting the primary to mains I measured the voltage it gives. It gave 4.5V AC. That means my transformer’s secondary winding gives 0.9 Volts per turn.

    P3201636s.jpg

    Then it was easy to calculate the number of windings for the needed voltage.
    I needed a voltage around 16 Volts (to have a margin for rectification diode drops) so 17 to 18 turns were enough.

    Next thing was the calculation of wire thickness.

    I need a wire capable of 60 Amps to play safe, so I consulted the AWG tables to have an idea of proper thicknesses.

    AVG.jpg

    The closest wire, suitable for my needs, was a 7core 2 AWG (6,5mm) capable of 66A. With 1mm insulation the wire was 8.5mm diameter, too thick to be of use for the transformer I had!

    Wiring.jpg

    It would have space just for 5 turns giving, as per test wires, 5 x 0,9 = 4,5 Volts!!!!

    A different approach needed that:

    1 would make use of all available space without any air gaps
    2. give me the 17 to 18 turns I need for the voltage
    3. make use of material I already have

    I have inherited a few copper sheets 200 x 60 cm and 0.6mm thick that collect dust for more than 30 years in the basement. So I calculated if strips from the copper sheets could be used to rewind the secondary.

    P3191595s.jpg

    Some calculations later, I estimated that the 0,6mm thickness strip will be more than adequate for my needs, As:
    a) Fits in the required space

    Wiring1.jpg

    b) Is capable to withstand some 100Amps (26.5 x 0.6 = 15.9 mm^2, better than 1 core 6AWG as you can see below)

    AVG.jpg

    Apart from the copper strips I needed some heat resisting insulation to put between the strips. I found the best insulating material in my wife's kitchen. It is a roll of “silicon impregnated non stick oven paper” (I don’t know the proper english name of it)!

    P3191602s.jpg

    It withstands temperatures up to 220 C, it is very strong although it has a thickness of 0.04mm, so I will put 4 layers of it just to play safe.

    P3191604s.jpg

    Next step was to cut the foil in 27mm strips. That was done with some help from my trusty mini lathe, a steel core and a razor blade.

    P3191607s.jpg

    P3191609s.jpg

    P3191611s.jpg


    How much length of copper I needed?

    Calculating the core of the transformer and adding the thickness of each strip together with its insulation made a nice spreadsheet with all the details.

    WindingMaths.jpg

    To cut the copper strips I used an old drill powered sheet metal cutter.
    Not the best way to do it but it did the job with just a few areas to file.

    P3191596s.jpg

    Two strips 2m x 26.5 mm after an hour of measuring, marking and cutting were lying on the floor.

    P3191599s.jpg

    The right angle extension marks the starting point of the “wire” to make the connections later.

    P3191600s.jpg

    Rewinding was a matter of careful folding the copper strip over the 4 insulation strips around the center of the transformer.

    P3191613s.jpg

    Wooden wedges helped pressing the sides to keep the windings from unfolding

    P3191614s.jpg


    An hour or so later the first 2 meters strip was in place.

    P3191615s.jpg

    Time to folder and solder the next one.

    P3191616s.jpg

    Another hour or so the rewinding finished, new windings were tested for continuity, short circuits etc. and as the results were fine, Coil winding “cool cure” varnish poured throughout the windings and the transformer was ready.

    Transformer back side after varnishing.

    P3201628s.jpg

    Transformer front side. You can see start and finish ends of the secondary high current coil.
    At the bottom the 2 wires that go to mains, to test the transformer

    P3201629s.jpg

    Testing the transformer a few hours later. All is OK.
    15.63 Volts AC

    P3201633s.jpg

    Plenty of Current also (estimated around 50 Amps)!
    Unfortunately my 100A amp meter is on its way from far east and I do not have the equipment to measure the amps produced.
    A 40 amps circuit breaker connected to the secondary, goes off instantly while I short the low voltage circuit through it.

    P3201639s.jpg

    Waiting for the rest of the parts to arrive from China to construct the PSU I will leave the transformer in peace for the next weeks!

    Thanks for reading.

    Petros
     
    Last edited: Mar 20, 2017
  2. strantor

    strantor United States Active User Active Member

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    I love it! Very meticulous and the copper strip was a great idea to maximize space utilization. Wish I had some strip like that laying around.
    I have a bit of a MOT fetish myself. I will pick up a microwave if spotted at a garage sale for <$15 or so.
    You might be interested in my last MOT project which I posted about before: Show Us your Welders!
     
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  3. brino

    brino Active User H-M Supporter-Premium

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    Nice "roll your own" transformer! :encourage:
    That copper sheet is a great solution to fitting the required secondary winding.
    Great work.
    -brino
     
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  4. petcnc

    petcnc Greece Active User Active Member

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    Very nice welder you have got there! 3 MOTs together! I like it
     
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  5. brino

    brino Active User H-M Supporter-Premium

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    It occurred to me that this would be great for a number of other projects too, like a high-current automotive battery charger.

    I'm going to have to grab a couple more microwave ovens.......

    -brino
     
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  6. petcnc

    petcnc Greece Active User Active Member

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    I'm not sure you need that high amperage to charge a car battery...
    On the other hand it will fully charge a 100AH battery in 2 hours! But is it good for the battery?

    You also need High Current secondary. The idea to use copper strips for the secondary came to me as I was looking on how to make a 2 roller system to compress a round wire flat so it could fit the transformer!!!!!:rolleyes:
     
    Last edited: Mar 21, 2017
  7. ELHEAD

    ELHEAD United States Active User H-M Supporter-Premium

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    Impressive. Great idea. Nothing like thinking outside the box.
     
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  8. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    A nice job! I have fought with winding high current windings in the past and this looks like a much easier approach. I have some aluminum sheet about 10" wide which was a winding in a distribution transformer using the same design as yours, 1mm x 200mm sheet. It looks like you have about 1.5 milliohms/m of resistance or about 6 milliohms total resistance so you should be seeing a voltage drop of about .3 volts @ 50 amps which is very respectable.
     
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  9. petcnc

    petcnc Greece Active User Active Member

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    RJ I noticed that the secondary winding of the transformer were not copper but some silver coloured wire, most likely aluminum.

    I suspect it is doable with aluminum sheet but I have no info on resistances and magneticaly induced currents on aluminum.
    On the other hand I have no aluminum sheets available to test it!
    It would be interesting to have some data on an approach like this using aluminum strips.

    As for resistance measurements, I measured a total resistance of 0.2 ohms for all 4.5 meters of copper strip.
    I'm sure I do not have the proper equipment to give accurate results on low resistance measurements so I mention it as a rough measurement only.
     
    Last edited: Mar 21, 2017
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  10. petcnc

    petcnc Greece Active User Active Member

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    Thats the secondary I removed from the MOT

    P3191592ss.jpg

    Copper like paint outside but aluminum inside?
     
    Last edited: Mar 21, 2017
  11. strantor

    strantor United States Active User Active Member

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    Aluminum windings are common. After seeing what you did with the copper strip I had my own idea to do it common kitchen aluminum foil. Several strip in parallel per turn, to achieve the thickness of your copper strip. It would have slightly higher resistance than your copper, assuming it's actually just aluminum and not something else.
     
  12. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    Keep in mind that the resistivity of aluminum is about 50% higher than for copper. You would want to go down two wire gauges if you substituted aluminum for copper. Aluminum foils is quite thin, around 1 mil. To get equivalent current capability to Pete's .6mm copper, you would need somewhere around 40 layers of foil.

    If it were me, I would use aluminum flashing from the DIY. It is about 8 - 10 mils thick and comes in rolls up to 50' long and in widths of 6" and 12" and is fairly reasonably priced. Actually less expensive than aluminum foil, pound for pound. I would pop it in the oven on high for a half an hour to anneal it. It takes around 650 -700ºF to anneal.

    Copper flashing is also available from the same sources.
     
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  13. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    Digital multimeters are notoriously bad at measuring low resistances. Without special instruments, the best way to get accurate low resistance measurements is to apply a known current and measure voltage. I will apply something like 12 volts to a series string of a precision resistor and the unknown resistance and measure the voltage drop across the known resistor to calculate the current and measure the voltage drop across the unknown resistance. The current is Vprec./Rprec. and the unknown resistance is Vx x Rprec./Vprec. I would use a known resistance of something like 1 ohm so the current is equal to the voltage drop. I have a number of 50 watt 1% resistors which will take that kind of current overload for a short period. Even the low cost multimeters do a fairly good job of measuring millivolts.
     
  14. strantor

    strantor United States Active User Active Member

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    Yes this is a really good way and I have done it also. DMM measures Ohms with .1Ω resolution, but measures millivolts to .001mV (.000001V) resolution, where with some 3rd grade math you can turn it into a .000001Ω value. If you do it often enough to where keeping up with a separate corded power supply and meter is cumbersome, you can get a cheapo milliohm meter. I have this one and its not the best but it's worth $82. Comes with kelvin clips and all. It's a chinese clone of a $600 meter
     
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  15. MetalMuncher

    MetalMuncher United States Active Member Active Member

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    Interesting project! :)
     
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  16. hman

    hman Active User H-M Supporter-Premium

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    Petros, that's an absolutely wonderful and innovative solution you've come up with. Congratulations!
    Rick Sparber (a member of this forum) has done quite a bit of R&D on measuring low resistances and small changes in near-zero resistance, in connection with his design of touch-down probes for machine tools. He's also come up with a relatively simple stand-alone milliohm meter. It's based on a "free" Harbor Freight multimeter and a quad op amp. Here's a link to his article:
    http://rick.sparber.org/electronics/ramp.pdf
     
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  17. petcnc

    petcnc Greece Active User Active Member

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    Thank you for your kind words!
    Rick's article is very impressive! It goes to the "to do list".
    Petros
     

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