Questions about my homemade Arc Welder

Discussion in 'General Electronics Chat' started by slidr, Mar 30, 2015.

  1. slidr

    Thread Starter New Member

    Mar 30, 2015
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    Background:

    I scavenged a 1100W transformer from a discarded microwave with the intention of making a stick welder (for fun and perhaps minor repairs, nothing I would trust anyone's safety to).

    I cut the welds on the transformer, removed the secondary (and primary in the process because it was in the way), replaced the primary just as it was, and created a new secondary by winding what I assume was 10 or 8 AWG wire (from a cheap set of jumper cables), and wound it 8 times around the transformer. I then resealed the transformer with silicone adhesive.

    Questions:

    1. I plugged in the transformer, and used my multimeter to take a voltage reading on the leads coming from the secondary. I got a reading of 9.5V. Can I then assume that the transformer is outputting approximately 116A (1100W transformer divided by 9.5V)? Or would I only be able to measure the current by buying/using a clamp meter?
    2. I am using a welding ground clamp and 400A electrode holder, which are wired to the leads coming from the secondary coil, and am unable to produce an arc with 3/32 6011 electrodes, on the thin metal that was once the cover of the microwave I scavenged the transformer from (I am able to get a spark by pecking and scraping the metal, but no arc). Does anyone know what the issue could be? I realize this isn't a welding forum, but just curious if anyone would know. I may be able to find the answer myself if someone could possibly answer question #1 for me.
    3. A majority of the tutorials I have viewed on turning a microwave oven transformer into an arc welder have used anywhere from 2-6 transformers, while a certain few have used only 1 and achieved success. What benefits, if any, would you get from using more than 1 transformer? A better/longer duty cycle since the work/heat would be spread over more than one transformer? Or were multiple used perhaps because of the tight space needed to wind the secondary coil with a thicker gauge wire, and not being able to get as many turns as desired?

    I don't have any pictures right now since I'm at work, but can take some when I get home and post them if you think they may be of any help.

    Thank you for reading!
     
  2. cornishlad

    Member

    Jul 31, 2013
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    I don't think 9.5v is enough to sustain an arc. When I had one I believe the open circuit output was 50v.
    Also the transformers are a bit special - they have a highly inductive source impedance so the high level of current after the arc has struck is out of phase with the volts. Can't explain the relevance of that in a short reply but put simply there'san element of wattless power. Your normal transformer will appear as a short circuit. Perhaps that would work as a spot welder ?
     
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
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    To make a basic stick welder you need more than just a transformer and two windings sets.

    First off your open circuit voltage is way too low to be of any use. Typically a stick welder has a open circuit voltages of around 60 - 80 volts AC that pulls down to around 20 - 30 volts when an arc is struck. For very small rods you can get by with a bit under the 20 volt arc voltage but the arc is very hard to maintain.

    Second you need a way to keep your acr current fairly constant while trading off a high open circuit voltage for a lower higher current arc voltage which for stick welding either requires a fairly complex electronic circuit or some sort of adjustable inductance in series with the secondary winding since you do not have a transformer that is capable of being easily modified for constant current use.
     
  4. slidr

    Thread Starter New Member

    Mar 30, 2015
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    Thank you for your response.

    I had no idea about the special transformers, I will do some research on that!

    "adjustable inductance in series with the secondary winding"...does this mean a way to adjust the current to accommodate different sized electrodes? Or is this referring to a special transformer, as mentioned by cornishlad, above?

    If I were to rewind the secondary around 40 times with a thinner wire to achieve 40-50V (OC), would this mean that my amperage would drop to around 20-30A? Or would multiple transformers have to be used to attain a higher voltage with a large current (and I'd assume a larger breaker too!)?
     
  5. #12

    Expert

    Nov 30, 2010
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    Examine the welding accessory, "tickler".
    An inductance in series with the arc will tend to raise the voltage enormously when the arc breaks.
     
  6. MCU88

    Member

    Mar 12, 2015
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    Madness. Dangerous too. You can buy an brand new welder now for under $100 on eBay.
     
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  7. #12

    Expert

    Nov 30, 2010
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    I like your idea but, if everybody bought everything in finished form, there would be no use for this website and we'd all be as bored as we used to be.:(
     
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  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a bunch of turns of thinner wire for a secondary will not work, you need heavy wire to carry the arc current. you wound 8 turns that gave you 9.5 volts, if you want more voltage, wind more wire, at around 1 turn per volt. (actually a little over 1 volt.) I have a cheap welder that does like yours and runs about 24 volts open circuit, wont sustain an arc also. I got another from harbour freight rated at 180 amps, it runns about 60 volts open circuit and works fine.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The SMAW welder is a constant current variable voltage, as mentioned, the voltage collapses when the arc is struck, typically this is done by a loosely coupled secondary, you may have noticed a crank on the front panel on many, this moves a laminated shunt between pri & sec. windings.
    Max.
     
    Last edited: Mar 30, 2015
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Another couple of options is a Spot welder or a High Frequency induction heater.;)
    Max.
     
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  11. cornishlad

    Member

    Jul 31, 2013
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    here's a YouTube video of a basic stick welder:
     
  12. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    you can make an excellent spot welder with it. IIRC, it takes a minimum of 35 volts to maintain an electric arc.
    use several taps on the secondary to get a range of voltage suitable for various thickness and alloys
     
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  13. ian field

    Distinguished Member

    Oct 27, 2012
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    My commercial unit has a capacitor tapped across part of the primary - I assume this has something to do with resonating and getting a high enough unloaded voltage to strike the arc.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    ....Or make a MIG welder transformer out of it, ~35v secondary. Heavy duty Bridge, No shunt.
    Max.
     
  15. slidr

    Thread Starter New Member

    Mar 30, 2015
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    Thanks for the suggestion. Surprisingly, I was not able to find such a device related to welding, but it is the same thing as an Armstrong Oscillator?

    Thanks. I don't need a welder, or else I would probably buy one. I just want the satisfaction of knowing that I could make one, or at least try to.

    Thanks, I'm afraid that with the gauge of wire I was using, 8 turns was all I was able to fit inside the transformer (I have a feeling it was due to the unusually thick insulation on the wire).



    http://www.allaboutcircuits.com/vol_2/chpt_9/6.html

    About halfway down the page:

    Another application is in current control for AC arc welders, which are nothing more than step-down transformers supplying low-voltage, high-current power for the welding process. A high voltage is desired to assist in “striking” the arc (getting it started), but like the discharge lamp, an arc doesn't require as much voltage to sustain itself once the air has been heated to the point of ionization. Thus, a decrease of secondary voltage under high load current would be a good thing. Some arc welder designs provide arc current adjustment by means of a movable iron core in the transformer, cranked in or out of the winding assembly by the operator. Moving the iron slug away from the windings reduces the strength of magnetic coupling between the windings, which diminishes no-load secondary voltage and makes for poorer voltage regulation.

    I believe this is what a few posters were referencing earlier?
     
  16. Rocket.Man

    New Member

    Jan 1, 2012
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    Keep in mind the higher the secondary voltage the easier it is to get the rod started welding. That same voltage makes it easy to keep the arc going. My Lincoln welder is 75v that will give YOU only 14.6 amps on your transformer not enough to burn a 3/32" rod.

    30v will be hard to get the rod started and hard to keep it going but this will give you 36.6 amps. This might be ok for 1/16" thick metal.

    If you have a 1500 watt variac it will step up the transformer primary coil voltage about 10% depending on the manufacture.

    Buy 7014 rods, 3/16" diameter, they are easy to weld with, they almost weld themself.

    If you can get your hands on 2 MOTS that are both the same 1400 watts you can build a nice welder. Put the 2 primary coils in series and run the MOTs on 240 vac. Put a 25v secondary coils on each MOT in series this 2 coils will give you 50v 36a. NOTICE, you must get the polarity correct on the 2 transformers primary coils in series other wise output voltage will be ZERO on the 2 secondarys in series.

    You can make an excellent spot welder with copper tubing and solder on 90 degree elbows. Make a U shape for the secondary if you have enough space make 2 Us connect them in parallel. The secondary will be about .9v at 1100 amps with a 1100w MOT.
     
    Last edited: Mar 30, 2015
  17. cornishlad

    Member

    Jul 31, 2013
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    I think that will be a power factor corrector. Commonly used on any mains device that has a high inductive component. Electricity supply companies don'tlike it and factories get charged extra if their current is out of phase with the volts.
     
  18. #12

    Expert

    Nov 30, 2010
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    The, "tickler" I saw was merely a coil in series with the welding electrode.
     
  19. ian field

    Distinguished Member

    Oct 27, 2012
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    There was an article years ago for something like that for electric train sets, to break down oxide & tarnish on the tracks. An oscillator with high enough frequency to develop an appreciable voltage on a pretty thick wire secondary. The few turns of thick wire secondary loses negligeable voltage when mains frequency pulsating DC flows through it, and that current stalls the oscillator so its not active in normal operation - when crud insulates the track, the oscillator starts up and burns through it.

    The device has a special name, which I can't for the life of me remember, I keep hoping one will turn up while I'm looking for something else - but it hasn't yet.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A coil was/is often used in series on SMAW DC welders as a 'smoothing' choke to reduce the high DC ripple.
    Max.
     
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