Electronics type advice again please

  1. johnrev

    johnrev Member

    Messages:
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    Location:
    Norfolk, UK
    Hope I'm not being a pain but am hoping eddie49 or mike109444 or one of the other electronics wizz's on here could answer a few questions about transformers for me.

    I know MIG is a CV process and that a mig transformer uses a tapped primary to adjust power settings (voltage) and I know a stick welder uses a CC transformer which generally has a shunt controled output power arrangement (amps).

    1) Are there other differences between the two types of transformer? The one from my SIP 140 looks massively larger than the one in my Topmig 150 even accounting for the shunt system.
    2) Could a transformer from a stick set be used as a CV mig power unit if the shunt is removed and the primary is fed via a PPM type supply rather than via the usual tappings?
    3) I assume the secondary of the stick unit would be too high a voltage for mig so could this be reduced by removing some of the secondary winding?
    4) How would this affect the output power ie if the stick machine was, say, 140amps at 60v would it be correspondingly higher output at 30v?

    All this is just something I've been musing about and may not actually try it but you never know how bored you might get eh!

    Thanks
    John
     
  2. hotponyshoes Member

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    I'm far from an expert but seeing as you can get machines that mig/stick/tig all in one I would think its possible
     
  3. voipio Member

    Messages:
    1,001
    Cambridge, UK
    Seems like quite a bit of work. The shunt would be required to be set to the minimum position and a number of taps added to the windings. If, by PPM supply, you mean mains waveform phase controlled i.e. light dimmer style, transformers do not respond well to that type of supply, so not to be recommended.
     
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  4. johnrev

    johnrev Member

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    115
    Location:
    Norfolk, UK
    Thanks guys. The new multi-function machines seem to be the electronic types that use IGBT's and such to 'make' the welding current rather than transformers so they work in a different way and can be controled to give all sorts of outputs.
    Removing the shunt is no problem but not sure if this leaves you with a standard type transformer or not. The phase control circuit for a welding transformer has been shown on here a few times and is actually a pretty simple thing. Must be better than trying to rewind a transformer to put in taps for switched power.
    I'm sure there are many ways that all this is not a good/workable idea but just had me thinking how feasible it might be. I just love fiddling around with this stuff.
    Thanks again
    John
     
  5. johnrev

    johnrev Member

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    115
    Location:
    Norfolk, UK
    I just thought I would add here that I have unwound some turns from the transformer secondary and voltage now is no down from around 60v to around 35vac with the shunt system removed.
    I bought one of the cheapo phase control modules from Banggood which is the same one shown on another thread here and rated at 10,000w. I know it's not an ideal application to use it for a highly inductive welding transformer but wanted to see how effective it is at controling the voltage. Results are good with output from 0 to 30v and no signs of heating after a few minutes with no load.

    The buzzbox transformer does buzz well and sounds unhappy at very low volts but comes 'on song' as the voltage rises. Gets noisy again at high volts but then it always has sounded 'angry' in use. I think within it's useful voltage range it is happy enough.

    I've left the test 100w lightbulb in parallel with the tranformer primary which gives a nice indication of the input voltage. I don't know if this has any effect but did read somewhere that it's a good thing to have a bulb as a non-inductive load connected- not sure why or if this actually helps at all?

    I would like to stress-test it all now by doing a little welding and see if anything explodes! Could I just connect up a ground and electrode holder and try running a bead? Striking an arc will obviously not be easy at the reduced voltage available but I can try with some really thin rods I guess. Is trying a stick even a feasible idea or should I just wait till I have made up the rest of a makeshift Mig setup before trying it out?

    I'd be grateful for input from some of you guys.

    Cheers
    John
     
  6. hotponyshoes Member

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    3,395
    Location:
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    I have no idea about the electronics.
    And mig/stick are completely different so what works with one might not work with the other.

    But I was testing it I would rather risk destroying a welding rod than risk a feeder/mig torch/spool of mig wire so testing the stick first makes sense to me
     
  7. hobby_machinist Member

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    56
    Location:
    Croatia
    Long story short you can only modify it from stick to mig or vice versa by chaning the number of turn of primary and/or secondary coils, so completley rewinding or removing few windings from transformer. Take in considiration that when unwinding the transfomer you can damage the wire insulation and thats game over, also your duty cycle has now changed and you can fry the transformer by burning just one rod
     
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  8. johnrev

    johnrev Member

    Messages:
    115
    Location:
    Norfolk, UK
    Oh dear! hobby_machinist are you saying that what I've done by removing turns from the secondary only is no good for adjusting the transformer output? I thought that the turns ratio was all that mattered and that the primary being as standard would mean it was still going to work OK. I did wonder if less turns on the secondary without increasing the wire guage might have an effect on duty cycle or even it's capacity to output a useable current for welding but figured it was worth a try!
    I was careful when removing turns and it was easy to do since the wires were pretty loosely glued and seperated easily. I can see each turn and there's no damage to any insulation.

    As it's now done and not reversable I guess it can't do any more damage to try it out with a thin stick as hotponyshoes says. Got my doubts I'll even get an arc struck but I'll give it a try and see what happens. I'll keep a close eye on the temperature of the windings and see what gets hottest quickest.

    Thanks for the comments both and I will report back once I've played some more. Any more views from anyone would be welcome.

    John
     
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  9. eddie49 Member

    John, you have started a very interesting thread! You are combining the "can I swap an Arc to a MIG" ( or vice-versa ) question with the "can I regulate a MIG with a light dimmer" concept.
    For the latter, Lidl manage it with the Parkside PFDS 120 A2, an £80 gasless transformer MIG that provides a fully-variable 25 to 120 Amp output using a Triac controller on the input, instead of the usual crude two or four "MIN/MAX" and "1/2" switches. They were recently on sale again, and I bought one so that I could reverse-engineer the circuit diagram.

    This article by Lincoln Electric has a good explanation of why Arc ( MMA ) has to be CC and MIG needs CV:
    https://www.lincolnelectric.com/en-...stant-current-vs-constant-coltage-output.aspx

    Now how you get it from a transformer is not that simple. As you have said, with an inverter it's easy. The feedback for the pulse-width modulation controller that drives the gates of the MOSFETs or IGBTs can come from a voltage comparator across the DC output, or a current-sense pickup coil on one of the output leads. That allows both MIG and MMA functionality in one box.

    1) I think that you could spend a lifetime learning the finer points of transformer design, so that you could specify one that runs CV and another that does CC. It has to do with pri/sec turns ratio, the relative resistance of the two windings, inductance vs. reactance, and the magnetic reluctance of the transformer core. You may notice that a MIG transformer core often has an air-gap, covered by a line of welding, to control the core saturation.

    An Arc welder at 60v and 140 Amps needs an 8.4kW transformer, but your 150 Amp Topmig running maybe 30v DC needs only 4.5kW, so the Arc transformer is lots bigger and heavier.

    2) It would "work", but to get true CV you would need to drive the PWM primary controller with voltage feedback from the secondary output.

    3) Yes, as you have found, you'd need to unwind half the secondary winding.

    4) You could probably pull some more amps out of your "half" secondary before the transformer as a whole would heat up too much. It is still an 8.4kW transformer core, with an 8.4kW-capable primary winding. However, if you tried to take the theoretical new secondary current - 8,400 Watts divided by 30 volts = 280 Amps, the secondary wire would burn out.

    At low settings, the Triac in the dimmer is only letting through a tiny bit at the end of each AC half-cycle. The waveform will look more like a sawtooth than a sine wave, so its not surprising that the transformer grumbles!

    I think the lightbulb is a good idea. It provides an immediate resistive "bleed" for the inductive delay and kickback from the transformer, which may help prolong the life of the "dimmer". In addition, to actually provide regulation, the Triac has to stop conducting at the end of each half-cycle, waiting to be triggered on again at some variable time from the zero crossing point. Just like an SCR ( thyristor ), once triggered a Triac will stay conducting as long as current flows. Without the lightbulb, the inductance of the transformer primary may prolong the current flow into the start of the next half-cycle, so the Triac may not get the chance to turn off.

    Rather than a spiky arc electrode, I would start testing with a smooth resistive load on the secondary. Some 12v 60w car headlamp bulbs in series and parallel will get you started with a few amps, but what you need for a serious test is a Load Bank, such as this one made by Tec-Arc:
    https://www.wellyweld.com/products/calibrator-1000i-ac/dc-73466.aspx
    For a slightly cheaper version, Google "hillbilly load bank":
    https://www.facebook.com/5mweldingc...put-297-amps-from-a-stocks-m/563264813823854/
    Two electrodes in a large bucket of very salty water! Check the current with a clamp-on DC ammeter. Watch out for the chlorine...
     
    Last edited: Aug 31, 2020
  10. johnrev

    johnrev Member

    Messages:
    115
    Location:
    Norfolk, UK
    Eddie so glad you've chipped in. Even a quick read of the above has explained an awful lot which I didn't know. I had no idea there was so much involved in the CV/CC transformer design. Thought it was just about using the primary tappings (turns ratio) or phase angle control that made a CV transformer as opposed to the loose coupling and shunt of the CC type.. I admit there does seem to be a much greater physical gap between primary and secondary windings on my buzz box transformer compared to the migs but that's obviously a physical necessity anyway to give space for the shunt system. There's obviously a great deal more importance in the coupling and other aspects of the transformer design than I had realised.

    I don't have a clamp meter and can't justify spending too much for another 'toy' but will look out and see if maybe Banggood has a cheapo one. Really not keen on the saltwater bath thing. Have read about people directly using mains supply through one to weld with-Scary!

    Just to clarify if you would:-

    You say 'it would work but to get true CV.....' How 'true' does the CV need to be to make a workable Mig transformer? In all honesty I still am unsure why Mig needs CV. What would be the result of using a CC transformer in practical terms (I know it isn't something that can be done but just wonder why)

    Thanks again for taking the time to reply Eddie. Really appreciate you passing on your knowledge.

    John
     
  11. johnrev

    johnrev Member

    Messages:
    115
    Location:
    Norfolk, UK
    Just read the last again and not explained my question properly.

    Assuming there is not a black and white definition of the output of a particular transformer ie CC or CV. Am I correct that one transformer may give an output which is neither completely CV or completely CC? If this is so then what would be the actual result of trying to weld using a wirefeed system using an imperfect CV supply. My guess after reading the article you linked to above, is a 'farty' type of arc which is not pleasant to use.

    If this theoretical halfway CV transformer output is rectified, smoothed by large capacitance and regulated by a suitable inductance, would the situation improve to the point of being not bad or does the basic imperfect output still not allow a useable welder?

    John
     
  12. hobby_machinist Member

    Messages:
    56
    Location:
    Croatia
    As far as I know there is no written definition between CC and CV transformers, but what you can notice is the CC sources have higher OCV from 40-88v (im talking about standard transformer welders with AC or DC) and mig welders CV have lower like 30-40v max. Here is example: CC source (transformer DC stick welder 30-200a) - OCV around 80, votlage when welding is 25v and at 130 amps, CV (transformer mig/mag 25-275a) OCV from 13-37 BUT relation between voltage and amps while working is 25v at 210 amps. Maybe some sparky can explain this relation.

    The capacitor on welding machine is to smooth the arc when AC input voltage drops to 0 so it doesnt kill the arc, large inductor is usualy found in mig welder to also smooth the current (single phase or three phase). Three phase stick welders dont have large capacitors or induction coils because voltage never drops to 0 and stick can take some minor differences in current and voltage, but for mig you have to have inductors to make output current smoother because the wire is thin and if you have even minor difference in voltage and current its going to affect how you weld.

    CV is used on mig/mag systems and is allways fully rectified and then goes to inductor to smooth it out so there is no big spikes in amps or volts. If thats what you are asking

    Although they are called CV and CC power sources, they parameters will slightly change when welding
     
  13. johnrev

    johnrev Member

    Messages:
    115
    Location:
    Norfolk, UK
    Thanks for that hobby_machinist. Think I'm pretty much understanding what you say and I removed some turns from my secondary to reduce the output voltage for the very reason you state. Originally 60v and now near 30v so voltage is somewhere near.

    What I struggle with is the fact that a Mig transformer is designated as CV when the voltage output is not even close to constant in use hence the capacitance fitted to hold it up. I can only assume that a stick welder CC transformer is even less able to maintain it's voltage under load and the sag in volts is more dramatic because of the reasons eddie gave above. Maybe looser coupling ( I can understand this and can actually see it on my CC unit with it's wide gap between primary and secondary windings), less effective laminated core and perhaps more 'clunky' construction all round??

    Obviously since a transformer by definition produces a sinewave output this is only producing it's actual intended voltage for a tiny part of each AC cycle and for the most part is way different. It's RMS value and O/P voltage after rectification and smoothing must presumably be better maintained than a similar CC unit can manage- but how much better?

    I wonder what type an 'ordinary' transformer would be. The type inside the wall charger brick or the audio amplifier power supply or the 240v to 110 volt yellow site transformer or the microwave oven? Would these be generally CV types I wonder or CC or maybe for these uses it doesn't matter. Or perhaps a 'standard' general purpose transformer would always be CV and the stick welding unit is actually a special case designed to be CC being a completely different design purely for that purpose?? If so are there other uses for such a saggy output transformer in other situations?

    I think my few remaining active brain cells seem to need to be able to 'see' things in a more practical way to be able to understand fully so I guess I just have to accept that the two types of transformer have to be designed by guys much smarter than me.

    John
     
  14. hobby_machinist Member

    Messages:
    56
    Location:
    Croatia
    Ordinary transformer like from 110 to 220 or 220 to 110 or old transformer type wall adapters should be CV because if large voltage spikes are allowed it would kill what is connected to it.

    If you increase electrode to work distance in stick the voltage goes up but...... Wait I think i got it. All transformers are limited by how much amps primary and secondary take and produce under specific loads and winding ratios. Let say we have stick welder with only one set of primary and secondary coils, the output is 25v 150a, we can adjust voltage by electrode to work distance, we cant increase it but by shorting or simply diging the rod in work, we can bring that voltage so low that it will short the circuit bringing voltage to zero and kill the arc, lets take mig welder with one set of windings with 20v and 200a output, have normal parameters with right wire and we try to short it but we cant because the wire is still burning, if we increase wire speed it will be crapy weld but as long as welder can output enough amps it will burn the wire and voltage will be more less the same, for real short circuit we have to use bigger wire that welder cant burn anymore like from 0.8 jump to 1.2 or 1.6. Amps increase by how much business they have to care of, so how much wire they have to burn in given amount of time.

    In conclusion electrical arc lenth determines the voltage in welding, amps are determined by the how much our consumer of electrical power is needing. The main reason why you cant use mig for stick is this: stick welder has 25v at 130a (this is perfect for 3.2mm 7018), mig has also 25v but at 210a, enough for 1.0 wire but for 3.2mm electrode too much and voltage is to low to maintain the arc with electrode.

    Other than output of welder or other types of power source the voltage between two points is limited by the distance, meaning you cant put 10000000 volts between points that are 2cm from each other because physics, and that brings us to the few moments before short circuit because the voltage is to high for that distance so it cuts the arc out (or there is not enough amps to burn the electrode so it sticks), or we can make the distance too big so arc cant jump anymore. I learned this in one of ElectroBoom vids about tasers but can find it now.

    There are formulas for efficiency (cos phi), duty cycle, OCV and the rest of that good stuff but you have to try whats best. I have seen some stick welders that have more less the same output but their efficiency and OCV are way different. If you want the prefect arc you have to put math and physics a side for a bit and experiment with different winding ratios, wire sizes, coil distance from primary to secondary or in three phase distance from multiple windings.

    Some of the best transformer stick and mig/mag welders were made by Uljanik Pula under licence from Esab but they were largely improved by Uljanik, they overbulit them so much that migs didnt need coling fans. Im hoping to get my hands on of them

    Maybe we could test these theories by conecting different types of welders to something that pulls a lots of amps like 24v truck starter motor and measure the current draw and voltage, or even better get someone that knows what he is talking about and explain us.

    I hope I didnt wrote anything wrong or confused someone. Excuse me and my grammar, Im writing this sleepy as dead cat and dont know much about electrical stuff other than it can shake you pretty bad and you can use it to melt and burn stuff
     
  15. johnrev

    johnrev Member

    Messages:
    115
    Location:
    Norfolk, UK
    Well you kept me awake ok. I'm now even more convinced I'll never really get a grip on the theory of these things. Like I said I need to visualise what is happening and I just can't manage it I'm afraid. Maybe 50 years ago if it had been taught at my colledge I could have grasped it better but not now.

    I can't really justify spending too much just to experiment. I really like to get a machine together with a pretty high expectation of it working without a lot of investment of time or money. That's the satisfaction to me. If I can gather knowledge and some learning along the way all the better but it's the building and using that I enjoy.

    Thanks very much for the explanations and for taking the time to try to explain to me. I think what I will do is put this to one side for now while I work on making the rest of the mig welder I'm putting together. I can then connect up this transformer and actually see what happens when I pull the trigger? I assume it will be a no-go and can then concentrate on finding a suitable mig transformer to fit instead. I'm always looking out for dead mig welders and something will come along in the end.

    Again thank you and all the best. Oh and I'd like one of those Uljanik Pula bullet proof migs too-my kind of machine :-)

    Take care

    John
     
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