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Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Hi All !
For long time I was trying to design a arc weder machine that can weld stell using commom arc welding electrodes. The basic specifications could be;
Power supply available : 230V, 60Hz No load output voltage at the welding electrode: 50~60V Arc on output voltage: 20~30V ac or dc Welding current (arc on current): 30~80A Short circuit current whem the electode is touching the work: 40~90A
My first idea is: to make a full wave rectifier that deliver 325V dc to a inverter , the inverter ( or any suitching device) will provide 20kHz or more to a transformer, the transforner will receive 325V and deliver 50~60V to welding electrode. the current will be regulated and ajusted by a simple inductor in series with the transformer secondary.
Any suggestion, schematic for the switching circuit, or even the "apparatus circuit" ?
I have built welding transformers in the range of 60~240A using 60Hz transformers and inductors as ballast, also transformers with magnetic shunt to control the arc current
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The most simple an unoptimal solution is to use a bridge (half, full) of transistors at maximum duty cycle, which drives a transformer through a series inductor. It is recommended to put the inductor on the primary side of the transformer. On the secondary you just have a full wave rectifier with fast diodes. The pros are - no feedback loop, simple control of transistors. The cons are - high reactive power in the transistors, higher losses, control of output current basically possible only by varying air gap in the inductor (frequency control is also possible, but efficiency of such solution is questionable)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I'm also interested in building an inverter welder, to run from a mobile generator (you can't run a 'normal' welder from a mobile generator, for reasons that were once explained to me by the designer of my generator over the phone, which involved phase angles or something when running inductive loads).
Is it possible to limit current using series capacitors, either on the primary or secondary, or would this cause other adverse effects?
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Hi Arcingnoob:
Thanks for the videos, they are realy goood and guive a precise procedure to dis assemble a MOT. Also the project with the 2 mot running a arc werder is a good one. However, as posted in the first place, the concept is a metal arc welder using a inverter. Thanks again Newton
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Dr Dark Current: Yes, your arquitecture match with my conceptual. Assuming that the output welding current under short circuit at the electrode may be limited at 100A, the aparent output power could be 100A x 60V = 6000VA. ( 60V is the no load voltge at the electrode) Assuming the converter efficience as 90%, the converter input could be 6000/.90 = 6700VA The 230V input after a full wave bridge rectifier and input capacitor filter will be 325Vdc. The load at rectifier, 6700VA/325V = 20.6A The R load at rectifier, 325V/20.6A = 15.8 ohms
If a capacitor of 5000UF is input, a ripple of 10% may be on the dc bus.. .
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Newton Brawn wrote ...
Hi Ash !
The generator arc welder is still working in the farms, and matches very well with the arc V/A characteristics.
But I am interested in a very simple inverter supplying AC and DC to the welding rod.
Thanks
Hi Newton. I think you mis-understood me. I didn't mean to recommend using a welding gen. set. I meant I tried running a buzzbox from a generator and it doesn't work.
I'm also interested in building an inverter welder, and the question I asked is 'would a series capacitor on either the primary or secondary of the inverter transformer work to limit current, or would other problems arise?'....My idea would be to vary the capacitance, either using a variable capacitor, or switching several parallel ones in and out of the circuit, in order to control current.
Another simple idea might be to vary the switching frequency, which I think could work, maybe also with a series capacitor to prevent saturation of the core at low frequencies.
I'm still a relative newbie when it comes to designing high frequency transformers, so maybe someone with more experience than me could advise on either of these ideas, and whether they could work or not?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I would add a rectifier and filter inductor after the transformer, for the following reasons:
If you weld with high frequency current, the inductance of the long welding cables will prevent you from getting the welding current you want. The current will vary a lot depending on how the cables are laid.
Likewise for the inductance of a large steel workpiece.
Even the thickest cables will get extremely hot from skin effect.
Stick welding electrodes are designed for use with DC (AFAIK)
The filter inductor ensures a continuous current, helping keep the arc lit, and allowing you to regulate the current with PWM.
Last of all, this is what's inside an inverter welder, I've examined the guts of a few. They are basically a SMPS minus the output filter caps.
Some of them feature an AC output for welding aluminium. They actually use a rectifier followed by another inverter to produce a low-frequency square wave output. This explains why they are considerably bigger and more expensive than DC-only machines.
You can get fast rectifier modules with multi-hundred amp ratings from Digikey.
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