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Re: pigs and other novice questions



Tesla List wrote:

> Original Poster: Bobbaust-at-aol-dot-com
>
> Ok, so I'm a newbie.  By reading your chatter for a few days I have surmised
> the following:
>
> 1. Some of you guys are using pole pigs in reverse as an input xfmr for your
> TC (see, I have the language down already).
> 2. You are running them on 240 vac but using current limiting devices in
> series (I assume to keep from sucking the guts out of your fuse box).
> 3. You are looking to achieve an input current of around 10 to 50 amps, which
> I'm guessing is less than 10% of what you'd see if it were direct connected
> to 240 vac.
>
> So here's my question: Why don't you input 120 vac instead of 240, and have
> less voltage to drop?  Or better yet, use that welder's secondary to drive
> the pig, since the welder puts out current-limited voltages in the range of
> 20-50 vac?  Or use big fat SCR's to chop the input waveform?  (Or do you need
> nice clean sine waves?)  Am I missing something?

If you were to input 120 VAC you would only get 1/2 the output voltage. On most
pigs used on TCs, that would be 7200 VAC. Not bad, but it's harder to get spark
gaps to work right etc. at that level. 20-50 VAC would probably not work at
all.
240 VAC also gives us more room for expansion, Watts = Volts * Amps. The
current
limiting devices are needed because the pole pigs have no current limiting
built
in, other then the windings themselves melting. The reason people use 10-50
Amps
is that they need to limit the current to what the pig can take. Not to mention
the rest of the coil system. The high power levels will destroy small coils.

For example, my pigs are rated for 10KVA. They could handle more then that for
short runs, the things are built like tanks. But that's the rating. That
equates
roughly to 10,000 Watts. 10,000/240 = 42 Amps. That is the maximum rated
current
on the primary. Because the pig has no internal way to limit the current,
it will
just draw all you can deliver before the breakers pop. The reason is that
the HV
side sees a dead short when the spark gap fires. Since the arc is low restance
and the voltage is high, the amps get high too. Assume 10 ohms for the arc
restance, E = I * R so I = E/R if E = 14,400 (nominal pig output voltage)
and R =
10, I = 1440. I don't know anyone with 1440 Amps available. Not to mention the
windings would never stand up to that. So we limit the current so that the pig
can't try to draw more then we want it to.

SCRs usually don't like inductive loads. Most SCRs I've seen would fry
attempting
to drive a transformer. Some coilers have tried to use light dimmers as a cheap
variac, those use SCRs to control the output, and most of those units fail.
There
are some that will work, but I think they use something else to control the AC
wave.

I'm still new to pigs, so I may have some of this wrong. I'm going to post
anyway
so someone can correct me if I'm wrong. I just read Terry's reply to you,
and it
looks like I may have half a clue here... That's the basic idea as I know
it. The
numbers I used in the math may not be exactly right either. :)