How to control 350A DC current? (Welder mod) - Page 1
Forum section: electronics / projects

author: TheD312
author_rank: Contributor
timestamp: November 11, 2017, 10:25:41 am
content: Hello guys,I have a old german MIG welder (Gloos GLC 356) and I would like to make a little mod to it. I want to built a Pulse welding mode into it. With Pulse I do not mean that it welds for 2 sec then stops etc. but that the current changes between two values with a certain frequency.I want to make this thing arduino controlled so you can set a upper and downer current and a frequency at which those currents alternate on.Ex: I1= 100AI2= 140Af = 1kHzSo the welding current alternates from I1 and I2 at 1kHz.But now comes the problem: How can I controll up to 350A of current (@ about 20v) within a frequency range of 30Hz to 10kHz?My idea is to use a Power IGBT like the Toshiba MG600Q1US65H that can control 1200V up to 600A. According to the datasheet the collector current is kindof proportional to the gate volage. With kindof I mean that the characteristic curve is not linear, but that could be solved with software of the Arduino.In the datasheet I can see that I would need about 5v to 8v at the gate to controll currents up to 400A. So I could use the arduino to control the gate voltage and frequency.Do you guys think this IGBT method would work? Or how would you do it? The IGBT method is quite cheap as you can get one for 20 bucks online.Greatings from Germany

author: Siwastaja
author_rank: Super Contributor
timestamp: November 11, 2017, 11:17:53 am
content: Those IGBTs are unsuitable to run in linear mode - they are designed for switching only. SOA is not even specified in the datasheet. Most likely, the linear mode would be a poor choice due to huge dissipation, so you'd basically need to build a switch mode supply capable of welding currents.Sorry, but this won't be a project on your level based on the impression I get from your post. You should try to pick an easier power electronic design task first. Expect a few years of learning time.You can do this (heck, you can do anything you want if you are willing to learn!), but if you want to tackle this design, you have a lot of homework to do, no one is going to explain the required 500-1000 pages of information piece by piece on a forum.Not wanting to discourage you, just stating the fact this will be a daunting task. Doing a few weeks of reading on modern power electronic design could help you ask the right questions.

author: Benta
author_rank: Super Contributor
timestamp: November 11, 2017, 12:29:40 pm
content: Better idea might be to switch between two taps on the transformer.

author: brainwash
author_rank: Frequent Contributor
timestamp: November 11, 2017, 01:51:17 pm
content: Theoretically, you can do that if you use a fast PWM and add another PWM layer to get the desired frequency. Basically, you write to the PWM register at the frequency you want to work. For standard Arduino I think you can get as high as 62.5kHz. Your control loop must be able to run at multiples of the desired output frequency, it would set the duty cycle for the fast PWM.Code:#define dutyCycle1 (100A/1200A)....#define dutyCycle2 (400A/1200A)...void timerTask(){if (buttonPressed){setFastDutyCycle(counter % 2 ? dutyCycle1 : dutyCycle2);}else{setFastDutyCycle(0);}counter++;}setFastTimerFrequency(62.5kHz...);setTimerTaskFrequency(1Khz..., timerTask);I don't know if you can daisy chain timers on AtMega328 but you can do that with an ARM (STM32, Tiva C, ...). This allows more control and you can use the MCU for something else.This is all theoretical, there are many considerations and a high chance of blowing expensive IGBTs or setting things on fire.

author: TheD312
author_rank: Contributor
timestamp: November 11, 2017, 02:14:03 pm
content: Thanks for your answers so far.@ SiwastajaYes I am not the smartest person when it comes to power electonics but I also know something@BentaYes I could do that but for that would be more complicated and involve much more parts I think@ brainwashI do not really want to use PWM to get a smooth DC voltage without too much ripple. Of course I could mount the tranny before the smoothing caps but I still don`t quite like the idea of PWM.I would like to switch the secondary dc current from the transformer, not the pri windings.One other Idea I have is to maybe use a LOT of simple high current NPN transistors in parallel which would control the collector current based on the Base voltage/ current. Theoratically that would actually be possible wha?!I saw a different welder schematic from the same company using one of those transistor "arrays" with about 130 of them mounted on a watercooled plate

author: oldway
author_rank: Super Contributor
timestamp: November 11, 2017, 02:30:38 pm
content: Quote from: Benta on November 11, 2017, 12:29:40 pmTo switch between 2 taps of a 3 fases 50Hz transformer at a 1Khz frequency is not possible.Here are the schematics;https://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0ahUKEwiNoMbB2bbXAhVQKuwKHTKEDAgQFgg6MAY&url=http%3A%2F%2Fp.weld21.net%2Fhy%2Fc-seriee.pdf&usg=AOvVaw2soCYjNdYxXZH7g2EbltENThere is not output capacitor, so the first thing to do would be add a big electrolyyic capacitor to modify this MIG welder?

author: FrankBuss
author_rank: Unknown
timestamp: November 11, 2017, 02:43:04 pm
content: I guess switching 100 A at 1 kHz with long welding wires is a good way to get a visit from one of these buses with the big antennas from Bundesnetzagentur, because your neighbors only hear noise in their radio receivers when you use it.

author: TheD312
author_rank: Contributor
timestamp: November 11, 2017, 03:42:13 pm
content: @oldwayIts actually not the machine in your schematic, mine is different without a display.Here is a link of the actual schematic:https://www.mikrocontroller.net/attachment/280042/GLC_256_GLC_356.pdf@ FrankBussI´m not sure if anyone (even the people living on the land) hearrrs radio at those times

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 11, 2017, 04:04:41 pm
content: May I ask why you want to do this?Just interested is all.

author: TheD312
author_rank: Contributor
timestamp: November 11, 2017, 04:11:19 pm
content: @MjolinorI thought it would be a nice project to learn a bit of electronics and the overall weld result when welding with pulsed MIG is better.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 11, 2017, 04:18:54 pm
content: Quote from: TheD312 on November 11, 2017, 04:11:19 pmOK, cheers.Why is pulsed between two values better than pulsed on/off?

author: hlavac
author_rank: Frequent Contributor
timestamp: November 11, 2017, 04:28:09 pm
content: You dont want the arc to die i guess

author: brainwash
author_rank: Frequent Contributor
timestamp: November 11, 2017, 04:28:32 pm
content: This entire "project" gives me the chills. Forget I gave any advice, you need to worry about a lot of other stuff (leads inductance, protection, filtering, failsafe, monitoring, cooling). You really don't want to learn "electronics" on a 145kg 380V beast.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 11, 2017, 04:33:06 pm
content: Best not guessing really.The arc will have a decay time. If you are running at 1 kHz, 50% duty cycle then it has 0.5 ms to die. I haven't done any calculations around it but that seems very fast to me and you can control that decay time by other means anyway.On / off is much easier to do and with research and some calculating you can make it sustain the arc for long enough.

author: TheD312
author_rank: Contributor
timestamp: November 11, 2017, 06:40:30 pm
content: Yes, you don`t want the current to completly switch off so the arc stays on.If you use pulse the arc can "bur" deeper into the meaterial, the beed gets smoother and it also prevents splatter.Cooling would probably not be the biggest problem since the machine has a internal water cooling unit so i could hook up some copper pipes to the water circuit which then are mounted to a big aluminum (or aluminium, not sure here) heatsink containing the power devices.But summerizing this whole concept I too start to belive that the idea of integrating a pulse mod probably will never happen.Mounting 50+ T03 Transistors on a heatsink seems a little bit complicated, I would need a big heatsink and connecting all those pins to some sort of bus bar...The only good idea is the IGBT method but like someone here wrote previously that IGBTs are only good for pulses not linear mode.Another thing I thought about:For the continous welding current mode (no pulse) I thought that maybe I could use the same IGBT but at a very high freq like 50+ kHz, if that goes through a good amount of capacitors it probably wouldn`t even be noticible during the welding process.

author: viperidae
author_rank: Frequent Contributor
timestamp: November 11, 2017, 09:01:31 pm
content: Can you switch the different primary windings? Looks like each phase already has 6 windings to switch between.

author: brainwash
author_rank: Frequent Contributor
timestamp: November 11, 2017, 09:09:59 pm
content: Slow switching (<<20ms)mightwork but the entire unit is an old design which runs at 50Hz. Newer designs are inverter-based (AFAIK) and have all those custom modes built-in. Having used both transformer-based designs and inverter ones I would never go back to 50Hz. I have only experience with stick welding though, MIG might be different, but I doubt it.

author: jbb
author_rank: Super Contributor
timestamp: November 11, 2017, 09:42:53 pm
content: Oh my, that welder is a beast. Real old-school. Heavy, chunky, reliable, provides good cover in a firefight, what's not to like?It has 50Hz input and a 6-pulse rectifier, giving a 300Hz voltage ripple (6x 50Hz) on the output anyway. It might be possible to convert the rectifier to phase-controlled thyristors, but that wouldn't offer very fast control anyway - maybe on 50Hz (remember it's necessary to avoid DC currents on the transformer windings).Unfortunately, for advanced modes I think a switching-style welder might be required.

author: Circlotron
author_rank: Super Contributor
timestamp: November 11, 2017, 10:07:57 pm
content: Suggest you try a little 10amp benchtop version first. Not enough to weld with but the fires and explosions and damage to your wallet will be manageable. Will also demonstrate just how hard it is to make power electronic stuff work properly in real life. Also, IGBTs are not as suitable for low voltage stuff as mosfets. IGBTs have a minimum voltage drop of 1 to 1.5 volts, mosfet voltage drop only depends on current and on-resistance.Controlling welding current by varying the conductivity of an IGBT or other device simply diverts some of the welding power into the semiconductor device instead. You will VERY quickly have two arcs instead of only one.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 11, 2017, 10:38:22 pm
content: I sort of think that a good learning start here may be a knackered 3 ph inverter. These rectify the supply and then chop it to three phases. Usually they are quite easily identifiable parts and they are just micro controlled for the three phases. Removing the micro control and sticking an Arduino in there is easy enough and they are availabe faulty fairly cheap up to some quite high powers.

author: brainwash
author_rank: Frequent Contributor
timestamp: November 11, 2017, 11:26:38 pm
content: I think a good first project would be mapping out the arc: how the arc strikes, voltage, current, with a 1ms resolution. It might seem trivial but it's not. Remember, you can't do controlled welding without good monitoring. If the project succeeds, you will learn a lot of stuff on how to use the machines, 90% is operator performance. And it might even take off - as a semi-commercial product - with those using old-style machines.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 12, 2017, 09:02:49 am
content: Given that the available power for welding is controlled by the welder already then maybe the easiest thing to do is to make a bypass switch that dumps into a low value high power resistor.Running that at 1 kHz should be easy enough, efficiency isn't on your agenda at all and it will do exactly what you want by wasting the extra current.

author: TheD312
author_rank: Contributor
timestamp: November 12, 2017, 09:22:55 am
content: I don`t really wanna experiment with inverters, just wanted to mod the machine I have, actually its secondary side of the transformer, without using any resistors etc, just some power switching devices.The thing obviously is that I can`t get some tranny that can switch 400A permanently "in one brick" so I will probably dump the idea or wair for a few years when I will know more about power electronicsThe other thing with controlling the transformers primary winding would indeed be a very good idea.I also have a "Messer Griesheim Wiggi150G 150A Tig welder (Also a 20+ year old 40kg massive metal box) which does exactly that. It has 2 primary windings (each for one phase) and those are shitched by Triacs I think.That would be a nice, pobably working thing but I´m not the smartest person when it the voltage exeeds non- hurting- level and power factor control...There probably also would have to be some sort of voltgae feedback to the arduino...Whoever wrote this: With 230v I might even get 3 arcs not just one

author: fcb
author_rank: Super Contributor
timestamp: November 12, 2017, 09:28:50 am
content: Mjolinor’s suggestion is a good one to try.Build an active load to soak some of the current at your frequency. It stands as much chance of working as trying to switch modify the welder control.Much easier to construct and test also, although cooling will be ‘fun’.Personally I’d stump up and buy a professional piece of kit (I had a 3ph 2013 ESAB TiG for a project - that was a complex bit of kit when you took the sides off). You’ll spend a huge amount of time and money on developing a modification, and things go south quickly at those currents.

author: TheD312
author_rank: Contributor
timestamp: November 12, 2017, 10:01:49 am
content: I don`t want to use resistors, I think those would dissapate more power than my IGBT idea.But how about this:If I would use something like this in the picture:http://ccipower.com/sites/default/files/LoadConfig1.png(Imagine pri transformer windings instead of a motor)And some of these semikron (or parts form different manufacturers) "diode and thyristor modules" togeher with a circuit for phase angle control that would actually theoratically work, doesn`t it?!But then I will probably have problems with the output voltage since it is goig to be lower on 50% power than 100% power...Then the switches switching different winding taps could be cranked all the way up and only those SCRs would contoll the output current.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 12, 2017, 10:17:51 am
content: Whatever method you use the power has to be dissipated unless you are going to use multiple transformer windings and the problem with that is it will not work at 1 kHz, the magnetic flux time constant is too slow. The welder is not designed to vary the power it puts out so rapidly so you "dump" either by increasing the resistance in the power path or as I just suggested, with a parallel short circuit. I said resistor because it is easier to understand, in reality you would probably use a tank LC circuit with resistance.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 12, 2017, 10:37:47 am
content: Any linear i.e. dissipative mean is absolutely stupid, but could be acceptable, or at least tried.The proper solution basically involves designing the complete welder from scratch, in switch mode.But, the following could be tried: (I can't think anything simpler than this)Don't try to create a pass regulator; do it by shunting instead.Drive the said IGBT using a proper optoisolated gate driver IC and the Arduino. Switch fully, i.e., Vgs=15V or 0V, with a good gate driver.Only use it to drive an extra resistive shunt load. Use a super beefy resistor bank. You probably can't use off-the-shelf power resistors since the cost would be prohibitive, but roll up something. Try not to be too inductive. You can, for example, weld steel sheets together in series and parallel until you have good surface area and the resistance you want.For example, if assuming:Welding voltage (total Vdrop in welding cables, connections, and most importantly, in the arc): 20V (verify this assumption)-> this is the output voltage at the welder terminals where you are going to connect the parallel shunt load to.Current: 350A.Amount of currrent change you want to aim: Let's say 100A.R = 20V/100A = 0.2 OhmsDissipation in R: (100A)^2 * 0.2 ohms = 2000W (at 50% duty cycle, 1000W on average)Welder open circuit voltage when no arc exists: 50V (verify this assumption!!)Maximum current in this case (assuming infinite welder as a voltage source): 50V/0.2ohms = 250ADissipation in R in this case: (250A)^2 * 0.2 ohms = 12500W (But in reality, no, since the welder open circuit voltage drops; but you'd need to model that first)The result would be that, without feedback, the current is highly variable with conditions, but it would besomething.You would likely need to add some logic so that you'd turn on the pulsing only after the arc is there, otherwise you would be dissipating huge amount of energy all the time, and making the arc starting more difficult due to the limited open circuit voltage left. At the simpliest, this could be the Arduino connected to the welding trigger switch, with some turn-on delay (typical time it takes you to get the arc going, maybe 500-1000 ms?) in the code.

author: TheD312
author_rank: Contributor
timestamp: November 12, 2017, 12:40:07 pm
content: Mhh...@ SiwastajaWith your comment in mind I actually drew up a simplified schematic how I imagine that. So you say that this would actuelly somehow work?Actually it kindof makes sence.If the pulse is not used the IGBT simply stays switched off. When the pulse is on the IGBT switches based on the timig so that a curent drop accurs at a set frequency. I guess it can be adjusted with PWM at a certain frequency. So the PWM will have to have a certain frequency and the Pulse will also have a frequency how often "the PWM occurs". I have no idea how I could solve this with software (actually someone wrote it before) but I thing it is a good poit to tal about.I could hook up the arduino to a already exiting shunt so the pulsing starts when the curcuit senses that lets say >10A are flowng.I think fabricobbling together a 2kw resistor shouldn`t be a too big problem...

author: Siwastaja
author_rank: Super Contributor
timestamp: November 12, 2017, 01:21:25 pm
content: Yeah, could basically work. But the devil is in the details (and in the safety).However, this is certainly orders of magnitude easier than trying to use semiconductors as pass elements in their linear range at that power level; or designing a proper switch mode converter.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 12, 2017, 01:33:56 pm
content: Without doubt it is one for the "McGyver" folder is this.

author: TheD312
author_rank: Contributor
timestamp: November 12, 2017, 01:40:47 pm
content: Maybe I will taccle that design if I have some time and my Arduino programming skills will be greater.I probably could make a 0,2 ohm resistor out of some simple wire, I just don`t know how to integrate iit into the watercooling system. Otherwise it might get very hot in that machine.I guess 1kW loss will at least heat a bucket of my worksop space in the winter

author: drussell
author_rank: Super Contributor
timestamp: November 12, 2017, 03:02:57 pm
content: Why would you use the resistor in shunt? Why not just make a small value, high power resistor and put it in series with the welder output, then use your IGBTs to short out the resistor when it should be on full, and have them off when your control control says it should be in resistor mode.You could use multiple resistors with IGBTs across each to get several levels of power reduction easily then also, if you choose. This way the resistor(s) dissipate the vast majority of the power instead of transistors in linear mode dissipating it all.

author: TheD312
author_rank: Contributor
timestamp: November 12, 2017, 03:15:48 pm
content: Because if I will have a lets say 0,2 ohm resistor in series with the output it will cause a voltage drop of 20v and will get hot likeAnd I guess (depending on the weld result) 60-80% of the time I will use constant mode.I could make a watercooled resistor by winding copper pipe (or different material) so it gets the needed resistance and then let water flow through it. I´m just not sure how hot it wil get. With 1kw being dissapated with it probably even the special coolant will vaporize

author: TheD312
author_rank: Contributor
timestamp: November 12, 2017, 04:36:20 pm
content: Does someone know how I can calculate how hot a resistor will get hot when a certain current flows through it?Lets say I want to use 15m of aluminium wire (what would give me about 0,2 ohm) and a peak current of 100A is flowing through it?I think I will actually try to use a big resistor to generate some current drop on the output.I just would like to know what will be the best way to make such a resistor that will stay somehow cool at 1kw of power being dissapated.I know that this is not going to be a efficient way but maybe I will give it a try once.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 12, 2017, 04:41:03 pm
content: 1 kw is quite easy to imagine, it is one bar of an electric fire. That will run all day and dissipate 1kw.To make it suitable fro your need you need to make it suitable for 20 volts? That is about 1/10 of the elements normal working voltage.Find one, unwrap it, fold it 10 times or so and there you have it, 10 (or so) parallel resistors that will dissipate 1/10th of 1 kw each.Please note, I have drunk wine so do your own working out, I may be talking crap, I can do that so well sometimes.

author: TheD312
author_rank: Contributor
timestamp: November 12, 2017, 07:14:27 pm
content: Yes I did see it somewhere on youtube before. But what he did is a "Power source" that outputs ~1kA of current in a very short time, so it is not really a design for me.So lets imagine that I would try out the Method with the IGBT switching a big resistor parallel to the output.Is there a specific type of IGBT or can I just use any random IGBT that is rated 300A or even better 600A (they´re quite when you get them used on Ebay) like the one I talked before?Like this MG600Q1US65H:http://www.rtdigbt.com/Article/UPload/201303/toshiba/MG600Q1US65H.pdfThen I don`t need two "overlapping" PWM frequencies to controll things right?I could use the set switching frequency and the current going through the resistor could be set by the gate voltage?!

author: oldway
author_rank: Super Contributor
timestamp: November 12, 2017, 09:53:32 pm
content: If I had to solve such a problem, I would add a buck converter that would provide an adjustable lower voltage from the secondary dc voltage.A 200A converter would be enough.With Power Mosfets, I would switch output between the two voltages at the needed frequency and duty cycle.