Dummy Loads For Checking Welder specs

[Cursitor Doom]

Can anyone suggest a suitable dummy load for this purpose? Needs to be
able to handle up to 200A for at least half a minute. I suppose the
ideal thing would be an arc light bulb out of a cinema projector, but
I'd prefer something more readily obtainable aready lying around the
house/garage. I want to test current draw under load and see if
there's any intermittent breakdown in output (which I suspect there
may be with these newer inverter welding sets).

[Roger Hayter]

Are you allowed to run it into a short circuit? That way the only heat
dissipation is in the equipment.

--
Roger Hayter

[Cursitor Doom]

Obviously short circuits occur during MIG welding every time the wire
hits the grounded metal. At that point, the wire is sputtered, the
circuit becomes momentarily open until the wire feed makes contact
again and this is continually going on at x cycles per second. As far
as I know, anyway. The situation with ARC/MMA is fundamentally
different as plasma is the dynamic resistance. I don't know the answer
but discussing it does at least help to clarify my thinking and might
give you or someone else here a better idea of the problem...

[Fredxx]

Most welders have an open circuit of 20V so at 200A we're dissipating
4kW and require a resistance of 0.1 ohms.

That's some load.

Now if you could get a few of these:

https://www.ebay.co.uk/itm/Boiling-Vessel-Cooking-BV-Army-Nato-complete-with-Cable-FV706656/184592417164

I think you would need 4 or 5 for your 200A

[Paul]

You'd start with an article discussing the electrical characteristics.
Which you already know some of them.

https://www.twi-global.com/technical-knowledge/job-knowledge/power-source-characteristics-121

For example:

https://weldtalk.hobartwelders.com/forum/weld-talk-topic-archive/welding-processes/12428-how-to-test-the-output-of-a-welder

"You can load bank both CV ( MIG etc ) or CC ( stick / tig ) machines"

Here is an example of a load bank (this looks low-tech and probably
for non-inverter welders). At least the scale of the box and
the vent hole for the fan cooling on the side, hints at
what sort of size is involved. Some of these load banks are
intended for trailer-style welders. (Back home, the gentleman
next door worked for the highways department and repaired
their trucks, and he had a good sized trailer-mount welder.)

https://www.millerwelds.com/accessories/load-banks/load-banks-m08200

*******

You could start with some wirewound resistors.

http://www.farnell.com/datasheets/1927175.pdf?_ga=2.227992665.558601560.1612919039-1466950013.1612919039

Manufacturer: ARCOL
Manufacturer Part No: HS50 R1 F

You can construct a heat tunnel, using four heatsinks with
inward facing funs. Fasten a 120mm fan to the end of the tunnel,
Drill and tap the heatsinks. Screw down the aluminium power
resistors to the heatsink. Some thermal paste between heatsink
and power resistor helps (Arctic Silver).

The hardest part of the project, is connecting the
resistors together. The resistors are, after all, only 0.1 ohms
and you will be connecting ten in parallel to hit 0.01 ohms. If
the welder is 100A 100W, that load would work. If the welder
was 200A 400W, that load would work (the resistors will get
damned hot), One reason you are selecting a large number of
resistors, is so they can be reconfigured to create other
resistance values, according to welder characteristics.

To be a decent welder, the power numbers are probably different
than that. Maybe the damn thing puts out a kilowatt. Make sure
of your power numbers before incinerating something.

The resistors are available in more powerful values, but
the price is proportional to the increase in power rating
(and consequent reduction in delta_T heat rise above ambient).
The 10W resistors are expensive and piddly (you might use
them without heatsinks, still with a fan, but at the
price, no way!). The 100W resistors are expensive. And so on.
The seller sometimes tries to "move" components not selling well,
or components where they have decent quantities. I got a real
good deal on ceramic caps once, and regret not buying a lot of
them while I had the chance.

I used to get power resistors for 1/8th the price of what I
see today. The local electronics store used to sell them,
and it made buying power resistors "painless". But it
still doesn't solve the infrastructure issue. You have
to make sure you're doing a good job of dumping the
heat, into the air. The wire gauge to the ends of the
resistors must be "extra fat". The electrical load I
constructed has a fan too. But my electrical load is
not in the same class as yours, so my wiring is a joke.

The one I show above, is the "fancy" version. Because it is
encased in Al, and has screw holes, it begs to be bolted
to a heatsink. There are cheaper ones that rely on their
own casing for cooling, but then you need more of them.
If you're not careful in your selections, the solder could
even begin to melt. Some are also fuseable, and the
material in the resistor will melt (it's part of the design).
So read the datasheets carefully.

The above resistor is 200ppm or so, so not nearly as bad
as the tempco of copper wire.

If the "resistor bank" is 0.01 ohms, the wire interconnect
to the ends of the resistors must be very good quality, otherwise
"the cabling is the resistor" and not the resistors. Copper
has a high tempco, and if the copper resistance is
greater than the resistor resistance, the tempco of the
copper interconnect dominates the resistance behavior.
(Resistance of load will change during operation, just
like light bulbs change radically and nichrome has a tempco.)

Another thing to remember, is at currents above 50A, you
start to feel magnetic deflection effects. This means that
some of your materials need to be stiff enough to resist
twisting or torque while powered. We got a demo of that
at a local technical school during open house. They had some
long cables with 100A running through them, and when the
power was switched on and off, the cables (heavy cables)
would deflect. Still not enough to be a danger, but still
a visible effect. In engineering school, there were calcs
highlighting the effect, but a visual demo makes a more
lasting impression. These are not wires that were "coiled",
so no multiplicative effects were present.

Paul

[Jeff Layman]

Perhaps a daft idea, but could you get a similarly specced welder which
has been dumped because its control circuitry is faulty, but the
transformer is ok, and use it in reverse? Then maybe measure the voltage
across that transformer's primary when your welder's output is connected
to its secondary. I assume you'd get around 230V, so what would happen
if you put a 3kW heater across it to load it? Or maybe a 3kW + 1kW if
Fredxx's mention of 200A @ 20V output is right? If the 230V (or
whatever) didn't change much with load, wouldn't that suggest the 200A
output spec is more-or-less as stated?

--

Jeff

[Robin]

On 09/02/2021 23:31, Fredxx wrote:
> On 09/02/2021 19:22, Cursitor Doom wrote:
>> Can anyone suggest a suitable dummy load for this purpose? Needs to be
>> able to handle up to 200A for at least half a minute. I suppose the
>> ideal thing would be an arc light bulb out of a cinema projector, but
>> I'd prefer something more readily obtainable aready lying around the
>> house/garage. I want to test current draw under load and see if
>> there's any intermittent breakdown in output (which I suspect there
>> may be with these newer inverter welding sets).
>
> Most welders have an open circuit of 20V so at 200A we're dissipating
> 4kW and require a resistance of 0.1 ohms.
>

that's about 13m of 2.5mm so 6.5m of T&E with the phases shorted at one
end? in a bucket of cold water if smoke is a worry ;)

--
Robin
reply-to address is (intended to be) valid

[Andy Burns]

Cursitor Doom wrote:

> Can anyone suggest a suitable dummy load for this purpose? Needs to be
> able to handle up to 200A for at least half a minute.

Where's Andy Moir when you need him?

[Owain Lastname]

On Tuesday, 9 February 2021 at 19:22:19 UTC, Cursitor Doom wrote:
> Can anyone suggest a suitable dummy load for this purpose? Needs to be
> able to handle up to 200A for at least half a minute.

5-6 cheap electric showers, in effect water-cooled resistances?

Owain

[jon]

Might need mains voltage for those.

[Fredxx]

ETP copper is usually taken as 1.7x10^-8

17 / 2.5mm = 7mohms/m so as you say ~13/14m. 4kW is an awful lot to
conduct through PVC to water in a bucket. I think I might treat the
cable to be a consumable in the experiment.

[Cursitor Doom]

On Tue, 09 Feb 2021 20:53:57 -0500, Paul <nos...@needed.invalid>
wrote:

Interesting! I can't say I've noticed any cable twitching when welding
using MMA at up to 320A, but then I wasn't looking at the cable at the
time!
I'm wondering if a trough of salt water would work? I seem to vaguely
recall some serious radio hams in the US used them for RF dummy loads
for their big linear amps.That might work if the electrodes were
spaced close enough to emulate the resistance of a plasma 'ball' as it
were. :-/

[Robin]

I should have made clear I looked upon the whole cable - copper as well
as PVC - as sacrificial

and also that right length is tricky as resistance will increase with
temperature so if crucial to maintain 200A for some seconds best shorter
- perhaps 10m

[Brian Gaff (Sofa)]

Is it AC then on the electrodes? I thought it might be chopped dc.
Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
bri...@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"Jeff Layman" <jmla...@invalid.invalid> wrote in message
news:s006ga$etl$1...@dont-email.me...

[Adrian Caspersz]

On 09/02/2021 19:22, Cursitor Doom wrote:

> Can anyone suggest a suitable dummy load for this purpose? Needs to be
> able to handle up to 200A for at least half a minute.

Starter motor on a relatively seized up non starting car/lorry?

--
Adrian C

[Jeff Layman]

I think DC is more commonly used, but it depends on what the welder is
being used for. Many welders seem to have switchable AC/DC output.

--

Jeff

[Paul]

Brian Gaff (Sofa) wrote:
> Is it AC then on the electrodes? I thought it might be chopped dc.
> Brian
>

We need one of those takeapart videos.

For a look-see.

Paul

[Andy Bennet]

On 09/02/2021 19:22, Cursitor Doom wrote:

400mm of 2mm diameter stainless steel rod will give you 0.1 ohm.
Stick it in a bucket of cold water and it should be good for a few minutes.

[newshound]

I'm glad someone did the sum, I was thinking much the same myself (but
with steel: hadn't thought of using stainless).