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Dr. Dark Current

Mon Oct 20 2008, 05:35PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Is buying the transformer an option? Used (ebay,...) welder transformer would be perfect for this application if you can get super-thick bar/wire for the secondary.

Speaking of batteries, the best would probably be to use several "D" NiMH cells in parallel, they can provide huge currents at the voltage you need, so no inverter necessary.

Steve Conner

Tue Oct 21 2008, 09:21AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'm surprised nobody recommended the old favourite, a car battery and a carelessly dropped wrench

newwave

Tue Oct 21 2008, 12:55PM

Registered Member #1758 Joined: Tue Oct 14 2008, 09:46PM
Location: noth eastern usa
Posts: 7

thanks for all the replys, my favorite is

a car battery and a carelessly dropped wrench

but seriosly I am exsperimenting with robotic underwater water welding, I hope that helps I can't really give out more input on the load sorry. the powersupply has to come from my boat with a second set of batteries and an a second 300 amp alternator installer. I really don't know wich current is best at the end of the circuit AC or DC I figuered I would try them both and see what gave the best result

Dr. Slack

Tue Oct 21 2008, 02:51PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

So at that voltage you're talking about spot welding.

It would be much nicer to run relatively thin cables down to a water-cooled transformer, than seriously thick cables out of your boat down to the workpiece.

Here's a serving suggestion.

Find a large MOT core and run it at 100Hz and 1.5v per turn, that would be fairly clear of saturation. The higher frequency means more volts/turn, but also slightly higher core hysteresis loss. Cut all the windings off. Pass a staple bent from copper rod through to use a single turn secondary. Thread another 16 turns of thinner bar for a 24v primary, which you can run as a full bridge from 24v batteries. Control the output power by controlling the bridge drive.

I recently put a few 7.5mm2 turns through a MOT core. I stripped the insulation off some twin'n'earth (Romex), twisted 3 cores together to get the cross section and the flexibility, then insulated them with heatshrink - reasonably thin and robust. So it can be done, but the packing density's not great. You can wind better if you grind open and re-weld the core.

newwave

Wed Oct 22 2008, 06:53AM

Registered Member #1758 Joined: Tue Oct 14 2008, 09:46PM
Location: noth eastern usa
Posts: 7

thanks dr slack,
taking the ocean as a mot cooler will it be sufficiant enough to cool the mot? or do should I think about nitrogen for cooling purposes. as dr kilovolt said

the MOT will work just a few tens of seconds at this current...

I need to run load for atleast 1 to 15 min. at a time.

this is a picture I mocked up

[Edit: Fixed double post]

Dr. Slack

Wed Oct 22 2008, 08:47AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

taking the ocean as a mot cooler will it be sufficient enough to cool the mot?

Well, perhaps not the ocean in direct contact, I doubt salt water would do anything good for corrosion, although with very low voltage there shouldn't be a safety or shorting problem, and with AC, electrolysis shouldn't happen. Maybe a closed plastic container of oil, either vegatable or mineral, whose thermal capacity and convection will keep the windings fairly cool, and keep the water out. Usually on this forum, people are putting transformers in oil for kV insulation rather than kWatt cooling.

BTW, before you get jumped on by a mod, there's no need to double post to add a diagram. Although there is no attach button on an edit, you can add the diagram to the "attachments" board and edit a link to it.

Steve Conner

Wed Oct 22 2008, 10:06AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Spot welding or arc? The requirements are quite different. For arc welding, you can actually weld with a decent sized car or truck alternator, modified slightly. Lots of info on welding forums about this: off road enthusiasts like to put them on their jeeps so they can weld the chassis back together if it cracks in the middle of nowhere.

For spot welding, you may have to turn to some of the crazy ideas in this thread

newwave

Wed Oct 22 2008, 10:17PM

Registered Member #1758 Joined: Tue Oct 14 2008, 09:46PM
Location: noth eastern usa
Posts: 7

thanks dr.sack, for all of your help.
steve McConner: It will be based on spot welding, not arc. I also been to alot of welding sites and forums, I learned alot from them them. I also learned alot here.
again I would like to thank all of you again.
I believe I have all the nessasary info, to start building. I will let you all know how I made out after testing. I should be able to have results or another question by sunday. thanks again sincerly Newwave

I finished the curcuitry up yesterday, and tested it out. It works but I have to cunstruct a set of preasure jaws. thanks again everone for all your help.

HV Enthusiast

Sun Oct 26 2008, 01:01PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Just as reminder, double posting is not allowed.
MODEDIT: Fixed several double posts in this thread.

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Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob