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Re: Massive Confusion.

Tesla List wrote:
> 
> Original Poster: "Coiler" <mycroft-at-access1-dot-net>
> 
> Someone that is using a welder as a ballast, I would like to talk to you
> about your current measurements. Something is very confusing about my
> setup.
> I am using a lincon 225A welder as ballast for my pig. I shorted the output
> of the pig, and took current readings with a borrowed current probe.
>
> At this point, I got worried, as I only have a 5KVA pig, and this represents
> almost 12KVA input power, if you belive the readings. What bothers me, is
> this
> is 50% of the setting.. and the welder wants a 50amp breaker. Mind you, my
> 30A fuses were fat-dumb-happy at this setting.
> So I cranked it up to 225A setting. At this point, I showed 90A on my meter
> and finally my 30A fuse popped. (note that for this, I bypassed the pig)
> This makes *no* sense to me. [though the readings are consistant with
> what I measured via a resistor in line] its almost like the readings are
> 100% of the true readings.
>
Current setting is the nominal OUTPUT current of the welder. A typical
welder puts out 30-50 Volts open circuit, so, the primary current will
be 1/4 the output current. You are measuring more, but there are other
factors at work as well.

As to kVA's vs amps, etc. You have just demonstrated the problem of
power factor. You essentially have hooked a big inductor across the
power line. Lots of current flow, but not much power, because it is
essentially out of phase with the voltage, so the watts are low, even
though the voltamps are high.

As to your 5 kVA pig and the massive current. Recognize first that
distribution transformers are massively overdesigned to sit in the hot
sun, etc... Next, that 5 kVA is the rating of the transformer doing what
it was designed to do, transform.. The idea is that the transformer is
probably something like 95% efficient (its probably on the nameplate
somewhere), so when 5 kVA is flowing through it, 5% of that, or 250
watts is being dissipated internally. Given the large mass of the
device, it would take a long time to heat up noticeably.  The losses are
probably resistive in nature, that is, the loss is proportional to the
current squared. 5 kVA, at 240 Volts, is about 20 Amps. If you are
running 50 amps through it, you are now dissipating 2.5^2 = 6.25 times
the nominal power: about 1.5 kW. Again, you might notice it getting warm
after a while, but, it has a pretty good radiating area, and a lot of
mass, so you might not.

As to the breaker not tripping. Your 30 amp breaker can run at a 20%
overload (e.g. 36 amps) all day long and not trip, per specification.
Run the current up to 145% and it trips a bit faster, run it to 200% and
it will trip mighty fast (or it should). Circuit breakers are designed
to interrupt short circuits with large overcurrents, not minor
overloads, otherwise they would "nuisance trip" when motor loads start,
etc. From this behvior do houses burn down with bad extension cords,
christmas light strings, etc. 

When you are welding, the actual load varies quite a bit, because the
inductance is only a sort of feeble constant current source, with the
current really being controlled by the skill of the welder, as she
controls the arc length. In welding, the current control is kind of like
a "coarse heat range setting", more than an actual current regulator.

I note that the newer, fancier welders do actually have a constant
current source (or actually a controlled V/I characteristic, depending
on the kind of welding, etc.) created by a switching power supply. All
an attempt to make it easier to weld well, and a major advance over two
car batteries, jumper cables, and a couple of coat hangers.
 This leads to 2 questions:
> 1) will someone that uses a Lincon welder and can measure the current please
>    check their values at the above settings.
You can get a chart of the V/I characteristics of the Lincoln welder
from Lincoln (or their dealer). It is essentially a resistance and an
inductance in series with an ideal transformer.
A generic chart is in most welding textbooks.

> 2) Just how far can I push a 5KVA pig without it going *POOF*.

A long, long ways. Let temperature be your guide. If you want the clever
way, measure the winding resistance, which varies in a nice predictable
way with temperature. This gives you sort of an average winding temp, as
opposed to the hot spot temp, which is your real limit. 
> 
> I admit I am a little disappointed.. I was hoping to be able to start at
> about
> 2KVA, but if these numbers are correct, the minimum I have is 4.8KVA :/
> 
> Michael Baumann
> Coiler, Homebrewer, Nerd. mycroft-at-access1-dot-net
> http://www.access1-dot-net/mycroft

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