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Mystery MOT watt rating

  1. Feb 16, 2014 #1
    My apologies if this has already been covered. I did a search, but couldn't come up with anything.

    My question is this. If you come across a random transformer with no markings, how can you tell it's watt rating? I have acquired several MOT's, and wish to use them in a DC arc welder. I want to parallel MOT's with similar wattage, but I have no idea what microwaves they came out of. Any help would be great!

  2. jcsd
  3. Feb 16, 2014 #2


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    A random transformer with no markings and no info about where you got it can be as dangerous as the arc welder held directly at your face. Surely MOT's and NST's have high watt ratings but you will have to be accurate about the input voltage and any large deviation from the rated wattage and voltage values can be disastrous especially while handling arc welding voltages.
  4. Feb 16, 2014 #3
    Yes, I am well aware of the dangers MOT's present. Am I to understand that there is no way to test it to determine it's watt rating prior to constructing the welder? Surely this can't be the case.
  5. Feb 16, 2014 #4

    Is there no one out there that is willing or able to give me any advice on this? At the very least, can someone point me towards another forum I could ask that could help me out?
  6. Feb 17, 2014 #5

    jim hardy

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    I've seen plenty of How-To forums on MOT welders. Google on MOT WELDER yields a lot of them.

    I'll take you at your word you're aware of the dangers of MOT's. They're killing tinkerers.

    NEVER energize a MOT that has not had its high voltage winding physically sawed off and removed.

    Basically the rating you're after will be proportional to the weight of the bare core after you've removed all the copper windings and the magnetic shunts.

    The cross sectional area of the core determines its volts per turn. The size of the 'windows' determines how many turns of what size wire it can encircle. The two together determine how many volt-amps it can handle.

    So pick transformer cores with similar cross-sectional area and window area. They'll have similar weight.

    You'll be winding your own secondaries so you can match their voltage for parallel-ing. Leaving the magnetic shunts in should help them share current, and help give you the voltage droop a welder needs.

    repeat, for the uninitiated:
    NEVER energize a MOT that has not had its high voltage winding physically sawed off and removed.
  7. Feb 17, 2014 #6


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    And for calculating wattage of an unknown transformer without ratings, it's not easy. I don't know but this might help you. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/tracir2.html
    You must have an idea of what MOT you have, where you got it. Arc welding transformers must be sufficiently rated for high amperage. I hope you know what you are doing.
  8. Feb 17, 2014 #7
    Thank you guys, this is the type of information I was looking for. I appreciate the help.
  9. Feb 18, 2014 #8


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    You are very much welcome. And don't forget Jim's warning.
  10. Feb 18, 2014 #9

    jim hardy

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    This may help you do some estimates from measurements you can make.

    Generally, transformer iron is run at flux density of up to around 12-14 kilogauss
    The cross section of the iron core therefore determines how much flux is encircled by the turns, therefore the volts per turn. Volts per turn is a handy number to know for a core.
    (there are exotic alloys capable of higher flux density but so far as i know they're too expensive for kitchen appliances. You might find them in a really good portable welder, though.)

    The area of the openings in the core , that is the windows where you put the windings , determines how many turns of what size wire you can fit on the transformer.
    Therefore that number determines how many amp-turns it is practical to attempt with that core. Core window size in square inches is a parameter used by transformer designers .

    Product (volts/turn) X (amp-turns) yields Volt-Amps,
    so cores with similar cross section and window size will have similar VA capability.

    When taking the windings off the transformer, you can count the primary turns and get a number for the core's volts per turn capacity. Be aware that many MOT's are pushed pretty hard on that parameter because they're not intended for continuous use,,,, if your core hums and runs hot add 5% or 10% more primary turns to reduce flux and magnetizing current..

    Of course were talking about old style MOTs that run at line frequency. These new inverter types are a different animal altogether.

    old jim
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