Re: A Ballast Mystery

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jason Johnson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <hvjjohnson13-at-hotmail-dot-com>


> This is the common setup with the welder primary and
> pole xfmr LV winding in series.
>
> 1) with the welding leads OPEN, the pole xfmer HV
> horns OPEN, and the shunts fully inserted, the 240v
> current was about 0.15 Amp. No surprise there.
>
> 2) with the welding leads OPEN, and the pole xfmr
> horns SHORTED together, the 240v current was about 5.5
> Amps. (???)
>
> OK kids. With the welder leads open, how did the 240v
> current go up?

Because, with the welder leads open the primary of the welder
transformer has the same inductance in both cases, but in case two you
are lowering the impedance of the load (pole transformer), and so the
total impedance drops and voila: more current flow.

>
> 3) with the welding leads OPEN, and the pole xfmer
> horns SHORTED out with an ammeter/DMM, the 11.5KV
> current was 0.12 Amp. Power-wise, 240v at 5.5A is
> pretty darn close to 11.5KV at 120ma. Still though,
> how is the current going up? Why don't the open welder
> leads restrict the 240v current to 0.15A?
>
> 4) with the welding leads SHORTED, the shunts fully
> inserted, and the pole xfmr horns SHORTED with a DMM,
> I got 460ma from the 11.5KV horns. I'm sure the 240v
> current is too high to safely measure with my 10A max
> DMM.

Ok, when you short the leads, the secondary of the welding transformer
is essentially reflected through the turns ratio onto the primary of the
welder. This happens in all transformers to a degree, bu really only
notice-ably in high power factor tranformers, such as welders and pole
transformers. Thus we can view the impedance for the primary coil as a
combination of the two, and when you vastly decrease the impedance of
the secondary the same thing happens to the primary, thus the large
increase in current you see (the 7 amps to 28 amps you mentioned on your
page).


<< Jason R. Johnson >>
G-3 #1129
The Geek Group
http://www.thegeekgroup-dot-org/

"The two most common elements in the universe are hydrogen and
stupidity."
 -Albert Einstein


>
> I'm OK with items 1 & 4. Items 2 & 3 have me stumped.
> How can adding the inductance of the pole xfmr in
> series with the unloaded welder cause the current to
> go up? If anything, it should go down! Thoughtful
> analysis, educated guesses, and half-baked opinions
> are all equally welcome.
>
> Regards,
>
> =====
> Gregory R. Hunter
>
> http://hot-streamer-dot-com/greg
>
> _
>