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Up Topic Welding Industry / General Welding Discussion / Adding HF to a scratch-start Tig inverte
- - By Ian Taylor (*) Date 07-22-2003 10:32
Guys

I have recenly purchased a DC, scratch-start Tig inverter m/c on which I am learning how to Tig weld. I have no problem scratching up the arc however after using a unit with HF I realise how good that type of starting is.

My questions : is it possible to "add on" a HF pack maybe in series in the torch line. Might it be possible a build an "add on" pack (I have no problem building electrical/electronic circuitry as I have good contacts in that industry).

Regards
Ian
Parent - - By ssbn727 (*****) Date 07-23-2003 05:06
Welcome Ian!

I have a couple of questions for you.

(1) What make is the inverter? (2) Have you called the manufacturer of the inverter to ask the same question?

Now, here's my two cents worth. (warning: this might take some time!)
Inversion is the opposite of rectification. Rectification is the conversion of alternating current into direct current, and is achieved using rectifiers or diodes, example: four diodes or SCR's in a bridge connection for single phase, and six for three-phase input.

Inversion converts direct current into alternating current. However, an inverter is not simply a device like a diode, but a network of MANY components designed to achieve this conversion.

The great majority of arc welding today uses direct current, and since the mains(input) are invariably alternating current, some method is required for changing from AC to DC. Inversion is only part of what happens within the inverter power source. Many other electronic and electrical conversions and transformations are required to convert the mains(input) supply into one suitable for welding.

Now let's get into inverter design and operation. This design is used to explain the principles involved in the operation of welding inverter circuits and not to be confused as the only design used in industry today. This is why it is important to contact the mfgr. of the inverter in order to find out if you can do what you mentioned, and to find out if warranty issues can play a role in your decision to move further.

The inverter is an electronic network for converting DC to AC, and the frequency of the current must be increased from that of 60 Hz(Hertz)coming from the mains to up to around 60kHz.
It has been proven that increasing the frequency gives more controllable and adjustable qualities since it is easier to change electronic circuits than to vary large inductances made of iron and copper.

First the incoming mains are passed into a rectifier which converts AC to DC. In order to improve the power factor of the power source a large filter capacitor immediately follows the rectifier stage. This has the advantage of providing a fast response energy store (the mains supply is relatively slow). Then the inverter circuit converts the incoming DC obtained from mains (input) frequency (60Hz) to square wave AC again but, at a much higher frequency than that of the mains supply. This AC at high frequency is passed into a transformer bringing down the voltage and increasing the current to that used in a welding circuit. The electricity is again rectified to DC, and passed through the filter inductor where, by Lenz's law, it is smoothed before passing to the welding terminals for connection to the electrode or GTAW(TIG) torch.

Many units today have a DC or AC(HF) output. This type of power source may have "back to back" or "push -pull" connection of more than one inverter, but that depends on the arrangement. (you should be able to figure out the difference between "back to back and "push-pull" as it relates to parallel and series connection.)
This is perhaps the simplest and most common: The DC welding output is connected to a network of four high-current semi-conductor switches (thyristors or transistors) in a "bridge network". The "switches" are switched on and off in pairs, 1 and 3 together, or 2 and 4 together, by a timing circuit. The output is AC(HF), the current square wave, suitable for welding aluminum, and this network is a further form of inversion.

In Summary: In an inverter-based power source, the incoming mains or input at 60Hz (50Hz in europe) is first rectified to DC before being fed to the inverter circuit. This circuit then reconverts the DC supply to an AC supply, but at a much higher frequency than that of the mains(input) with ranges of 6-60kHz depending on the "brand". Since the "input transformer is now operating at a high frequency, it's size and weight are greatly reduced compared with that required at 60Hz. The high frequency transformers use cores of ferrite, not iron laminations, and after transformation to the correct voltage, and current levels, the high frequency AC is again rectified to DC, and supplied to a welding output "smoothing" inductor filter, the size, and weight of which is also reduced because of the high frequency.
Machines offering DC and AC output generally incorporate an inverter/commutator circuit immediately before the output terminals. The inverters use either frequency modulation or pulse width modulation(PWM) as the means of control. Current and voltage feedback signals from the welding output are used within the cloed-loop control systems to maintain the required welding parameters. By varying the electronic controls or software as in the case of microprocessor-based controls, it's possible to provide machines with both static, and dynamic characteristics for a wide range of welding processes (SMAW, GTAW, GMAW, FCAW, pulsed or not etc.).
Inverters are characterized by their small size, and low weight compared with conventional machines, and offer high efficiency and high power factor. Their flexability, fast response, and excellent welding properties will extend their use over the coming years.
This information was quoted in part from: "The Science and practice of Welding" Volume 1 "Welding science and technology" 10th edition, by A.C. Davies, published by Cambridge University Press.

Now that I read your question again, I think by reading how the AC(HF) is produced within the power source just before the output(welding) terminals, you may have a problem with regards to preventing high frequency feedback into the internal control circuits or microprocessor if you do'nt consider a method of preventing this possibility. I maybe incorrect in this assumption because I do'nt know which brand of inverter power source you are using, and do not have the schematics available for your inverter in order to determine this possibility. Losing your warranty would be a shame also. So, I think it's best to contact the manufacturer's technical support division in order to find out further as to whether or not you can do what you had in mind. I hope that the excerpts from the quoted book and myself have been able to explain how inverter power sources work in order to guide you in the right direction.

Respectfully,

SSBN727 Run Silent... Run Deep!!!
Parent - - By Ian Taylor (*) Date 07-23-2003 08:00
Thanks SSBN727, what an excellent reply !! - I now understand a bit better how inverters work !!

I was hoping that the answers might have been something like "no problem, go and buy black box A from manufacturer B and just plug it in" but like everything in life its not that easy.

The unit is secondhand and is quite old so there are no warranties with it. I'm trying to find the manufacturer but I fear they may be no longer in business - but I shall keep on trying. (the manufacturer may be called "Electronic Welding Systems" - the unit type is EW130X - it is a 3-130amp, DC only, 110/230V inverter unit. I have contacted the distributor and they are looking into this question.)

From your reply it looks like I have to hard wire some circuitry into the unit - I shall let one of my electronics mates have a look inside. I had been pondering the feedback issue regarding the HF getting back into the unit and I guess that might be a showstopper.

Anyway, I shall persevere with this as it would be an excellent add-on if I can do it safely (ie. without danger to myself or the machine).

Regards
Ian
Parent - - By ssbn727 (*****) Date 07-23-2003 08:20
Hello again Ian!

A word of caution then, if you're going to add-on from the inside.
Make sure that you tell your friend about the capacitor circuit that may still have energy stored!!! Whoever it is that's going to work inside must know this because, if you or whoever, does not "BLEED"(discharge) the capacitor circuit before any work is performed, then the person performing the work inside the machine will get the shock of their lives!!! NO JOKE!!! Be extremely careful when working inside any inverter-based welding power source!!! I'm sure your friends are very capable of "re-engineering" the machine in question. Do'nt get me wrong but, just in case that person is'nt aware of this potentially fatal danger inside, please be adamant about this!!!! DO NOT PERFORM ANY WORK UNLESS YOU FIRST DISCHARGE THE CAPACITOR CIRCUIT!!!

I CANNOT OVEREMPHASIZE THE DANGEROUS POTENTIAL FOR SOMEONE UNFAMILIAR WITH THE DESIGN OF THESE TYPES OF POWER SOURCES!!!

Respectfully,

SSBN727 Run Silent... Run Deep!!!
Parent - - By Ian Taylor (*) Date 07-23-2003 08:57
Yes, I fully appreciate your comments. I will attach this thread and talk it through with my electronics mate although I'm sure he is aware of the consequences - but I agree - you can never be too cautious.

From the wording you use do you imply that maybe I don't have to hard wire the HF circuitry - do you thinks its feasible to add-on outside the unit ? (maybe in the torch line ?)

Parent - - By ssbn727 (*****) Date 07-23-2003 12:05
Hello Ian,
I would suggest that you hard wire the HF cicuitry so that there is'nt the potential for HF feedback to the inside control circuitry because, if the control circuitry is affected by the HF feedback, then the "chopping" of the AC square wave may not be consistent or other components can be affected causing erratic output where it counts most.
High Frequency feedback shielding can be applied to the welding circuit whether it's done from the inside or as an add-on unit. This all depends on how much room you have inside the machine to work with because, in order to make a HF circuit as an add-on or not you have to consider the parts necessary with respect to their size, complexity, etc.
Remember in the first reply that it mentioned how the DC welding output is connected to a "bridge" network that consists of 4 high current semi-conductor switches (thyristors or transistors) that are controlled by a timing circuit that switches the semi-conductors on and off causing the output to become AC(HF), current square wave, etc. this is simply a further form of inversion. Well, yes this is true but, because of the HF, you have to make sure that there is'nt any HF feeding back to the machine possibly from the work cable. not to be confused with the machine ground, and now that I mention this, make sure that your mains, and machine are properly grounded so that there is'nt the possibilty of the HF output feedback to any other HF sensitive equipment such as: computers, telephones, etc.
Your electronics mate can probably come up with some good ideas as far as how to go about properly shielding for HF feedback, whether you decide to install the necessary components from the inside or there is'nt enough room, and you have to "add-on" from the exterior as a separate unit which, in essence is another inverter. The timing circuit is crucial because, if it's not calibrated to the proper settings based on whether the machine is using FM or PWM (frequency modulation or pulse-width modulation), then you may have erratic HF output.
HF intensity also has to be considered so another circuit may need to be added to control the intensity. The bottom line is, your electronics mate has to consider these factors in order to successfully achieve what you want because, I'm here in Pittsburgh, PA, in the USA and you're over there somewhere else in cyberspace. This means that I cannot without looking at the schematics or the interior of the machine, give you the best advice on how to decide which way would be the most feasable for your application. In other words, I do'nt want to lead you off the side of a cliff without a parachute, so to speak! No offense intended please! Let us know how it works out because, I'm curious how you will go about it!

Respectfully,

SSBN727 Run Silent... Run Deep!!!

P.S. I noticed that the machine you have has an output of 130 amps.
My only question is whether it is cost effective to do this, because of the low current output or better to purchase a new machine that already has built in DC or AC(HF), and a greater amount of current output, plus also giving you the capability to perform GMAW(MIG-MAG), and flux or metal cored arc welding, and pulsing, etc.,with the use of microprocessor-based controls enabling you to set, and store customized welding parameters at your convenience!
Another factor to consider is whether or not the existing machine has enough of a duty cycle because the newer machines are capable to having anywhere from 60 to 100% duty cycle ratings at almost maximum current output! If your machine has a much lower duty cycle than at least 60% then you might find yourself waiting for the machine to cool down more than having enough welding time with the existing machine.
This is a very important factor to consider!
Parent - - By Ian Taylor (*) Date 07-23-2003 13:37
Good morning SSBN727,

I really appreciate your comments and advise and am becoming more and more intrigued with this project. I accept that it might be easier to buy a better unit but I think I will continue this investigation untill it becomes impractical (electrically) or cost prohibitive. My electronics mates are also becoming interested and are suggesting they will try and do this for interest rather than for commercial gain (ie they will do it for free - well, almost !)

I have found out that the unit was made by Lincoln Electric Italia and I will be asking them for info when I get their details (fax and/or e-mail). (I think it may have lift-start but I'm not sure !)

The unit has 130amp at 35% duty, 100amp at 60% duty and 90amp at 100% so it should keep going at the power I want to run it at (40 - 80thou Cr-Mo tube).

I think that I would probably mount the components in a separate box attached to the outside of the inverter and hard-wire through the casings so it won't be a "plug-in" add-on unit.

I'll probably be asking more and more questions as time goes by but to date I must thank you again for your input to this thread.
Parent - - By ssbn727 (*****) Date 07-24-2003 07:12
Hello again Ian!

Good idea about configuring the device to the machine!!!
A thought also entered my mind about adding a sort of on-off device for the "exterior component" in order to "switch" from AC(HF) to DC.
Just a thought...

In response to your mention of your friends being interested in helping you with this project, I think that it is very admirable that your friends feel this way because, too often in this crazy world of ours, we find that greed tarnishes our relationships with the people we would normally trust!!!

I am humbled by your gratitude and look foward to reading more from you sir!!!

Respectfully,

SSBN727 Run Silent... Run Deep!!!
Parent - - By Ian Taylor (*) Date 07-24-2003 08:54
ssbn727

I have re-read the thread several times now and your last comment ("adding a sort of on-off device for the "exterior component" in order to "switch" from AC(HF) to DC.") got me thinking that maybe we are slightly at cross purposes.

I only want to add HF into the DC only output to aid arc-starting (ie non contact starting) as I will only be welding steel with the unit.

Your last post implied (to me at least) that you were talking about adding full AC capability to the machine ? Am I correct ? (or when you say "AC(HF)" do you mean just the HF added to DC ?)

Regarding switching, if its electrically OK (and OK from a safety point of view) I would add a button to the torch to switch the HF in to start the arc and on release of the button the HF would be switched off thus allowing conventional DC welding to be carried out.

Regards
Ian
Parent - By ssbn727 (*****) Date 07-25-2003 16:37
Hello Ian!

I was just offering some options to you without any prior knowledge of what exactly you were going to configure... The design of the attachment you are planning is dependant on what your consensus with your electronic's mates, eventually becomes!

If your current output remains DC, then your only concern will be that of shielding the interior of the power source from HF feedback because, the components that make up your control circuitry inside the power source may not have been originally designed with HF feedback shielding in mind for such components!!!

If you are going to use the HF as an attachment from the exterior in order to just initiate a welding arc without making contact with the material, you then have to design and configure a device that has some sort of sensing/timing circuit that can sense when the output current from the welding power source is turned on or a voltage sensing connection that will detect when the welding circuit voltage closes (OCV to CCV) from the welding inverter, in order to set the amount of time the HF will stay on before ultimately, the HF gives way to to the transferred arc, and shuts off...

In other words, build it by either adding a HF transformer, a HF capapacitor, spark gaps, timing cicuitry, intensity circuitry, etc. or you can put together another less complicated inverter connected with a sensing/timing cicuit, intensity circuit, etc. - that is drawing electricity from a separate source of electrical input, in order to work as a separate power source exclusively for generating the HF which will then aid in transferring the DC arc across the air gap to the work without making contact at all!!! I would'nt suggest that you draw your input watts to run the separate HF generating power source from the welding inverter because, the duty cycle for the welding inverter power source will decrease

Where you should concentrate on HF shielding is in the area of grounding the components. Examples are:. welding power source, the HF generating device. Also make sure that the mains are also properly grounded, and the input cables are properly shielded, input cables are as straight and short as possible (no coiled primary cables!!!), and finally. if you and your mates can find a way to protect the welding power source's interior control circuitry from HF feedback, then everything that you were attempting to achieve with your concept will work out just fine!!!
Please let me know how it works out...

Respectfully,

SSBN727 Run Silent... Run Deep!!!
Parent - By ophdeheezy Date 11-19-2005 22:35
Sorry for the late reply. I was wondering if this project was successful. I would like to convert my scratch start into a HF as well.
Up Topic Welding Industry / General Welding Discussion / Adding HF to a scratch-start Tig inverte

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