Low voltage high current transformer welder ?

[eneuro]

Hello,
Just discovering the world of welding and testing spot welder made of huge transformer like this:

So, for spot welding its around 3V secondary is fine and it works using custom thyrystor's driver on this huge transformer primary.

But what about using it as classic AC transformer welder?
Do we need some kind of voltage step up to fire electric arc to start welding, so no way to use 3V secondary output?
Maybe preheating welded elements to lower down air break voltage could make possible classic welding using such low voltage?
I do not need it for production use, but weld something from time to time with small electrodes, while I can use spot welder to "glue" steel pieces together and use classic welder, but just thinking of it as additional feature, while I have custom spot welder driver so no problem to add another mode since I can easy reflash AVR MCU and add a few lines of C code more to existing spot welder software...

[T3sl4co1l]

Arc welding needs over 20V, and preferably more like 60-100V open circuit for maintaining the arc best.  You really need a few kW to do it well.  It can be done with 1kW or less, if you have fine rods.

Tim

[G7PSK]

The OCV (open circuit voltage) of an arc welder needs to be around 80 Volts but the welding voltage will be much less somewhere in the region of 24 volts for a larger machine but down to 4 volts in some small welders, these can only use cellosic rods as types such as basic will need higher striking and running voltages.
I suspect that the transformer shown in the picture would have far to low a duty cycle for MMA welding and would only be of use for spot welding but saying that 3 volts is quite high for a spot welder, the 180 KVA Skiaky that I used to have was only 1.5 open circuit  it went down to under 0.4 volts when welding on 8mm plate the weld time for thinner material was to short to measure.

[calexanian]

This all depends on a number of things. Some of our spot welders only have a few volts open voltage, but have special alloy tips for use with dead soft nickel, etc, etc. It depends on that material, thickness of material, length of desired pulse or AC voltage applied, and a number of other variables. If you can be a bit more specific on what you need to stick together i ma be able to help you more. Most classic hand operated spot welders and of the AC variety and are operated by a finger trigger that is just a momentary switch on the primary, or a foot switch and the weld time in controlled by the operator.  Small welders for battery tabs and such are typically DC pulse, or capacitive discharge type, typically through a step down transformer or coil and either charge the capacitor bank up to a adjustable voltage, or vary the pulse time. Your industrial 480 volt powered auto body type welder will have about an 80 volt open voltage while my generic hand op one is about 20 to 25 volts depending. It has no problem welding two sheets of 18ga mild steel together and I use a 10 amp variac and small ground tips to use it for battery tabs and such with very good results. I spring loaded the jaws, and tied a string to a piece of wood as a pedal to close them with a foot switch to make the weld. its a $150 poor mans micro spot welder.

[eneuro]

The OCV (open circuit voltage) of an arc welder needs to be around 80 Volts but the welding voltage will be much less somewhere in the region of 24 volts for a larger machine but down to 4 volts in some small welders, these can only use cellosic rods as types such as basic will need higher striking and running voltages.

Thank you for this tip, but do you mean http://en.wikipedia.org/wiki/Cellulose_electrode  ?

Probably this 3V secondary will be cut by half, so lower resistance at 1.5V higher currents and still I will be able if needed to reconfigure to 3V simply by making different connections.
Thinking about AC voltage multiplier and found in other forum one, but it was for classic welders with much higher open circuit voltage in the range of 24V and currents below 200A I guess only.

Most classic hand operated spot welders and of the AC variety and are operated by a finger trigger that is just a momentary switch on the primary, or a foot switch and the weld time in controlled by the operator.

Experimenting with automatic trigger where AC mains current sensor on transformer primary detects closed circuit on its secondary and after 1 second run preset spot welding.
Two potentiometers are used to control 230VAC 50Hz pulse width 20ms-1ms (energy) and total pulse time in myltiply of 20ms full mains sine wave periods.
I have classic fire button too, which can be used to start spot welding, but also to setup via aka 1 wire interface (it is pulled up via10k resistor to 5V MCU Vcc) internal parameters of spot welder-overwrite those manual potentiometer settings without touching them.

[G7PSK]

The OCV (open circuit voltage) of an arc welder needs to be around 80 Volts but the welding voltage will be much less somewhere in the region of 24 volts for a larger machine but down to 4 volts in some small welders, these can only use cellosic rods as types such as basic will need higher striking and running voltages.

Thank you for this tip, but do you mean http://en.wikipedia.org/wiki/Cellulose_electrode  ?

Yes. Cellosic rods are any rods that contain Cellulose. Whereas Basic are those containing limestone. With both types the proportions can vary so hence the generic names. If you want more information look at the AWS (American Welding Society) web site or the TWI (The Welding Institute) web site. http://www.aws.org/w/a/

http://www.twi-global.com/