Bud,
The wire diameter used is just the generally accepted limits for magnet wire
and I used an old posting at
http://www.pupman.com/listarchives/1998/April/msg00222.html from Father Tom
Gahee, (back in the good old days) for my mini pig project, while for the
ballast I used covered electric wire and noted the manufacturer's limit.
As for core size, when I built a mini-pig I researched various sources and
found several formulas,
all of which varied slightly, with a lot dependant on knowing the cores
magnetic specifications, which of course I didn't, and most of us won't
either.
So I came up with an average (based on others) of taking the square root of
the power and multiplying that by 0.14, which is what I also used for my
ballast cores.
The cross section area mainly affects saturation level, once that happens
the ballast no longer restricts the current to anywhere near the same
degree, so you need to avoid that occurring at all costs. The core size is
also related to thermal mass, so another reason that size is important. If
the core wasn't lossy in theroy it shouldn't get warm of course.
The formula gives me just under 12 sq ins for a 7 kW ballast, although at
the time I think 5 or 6kw was the design figure I aimed for, (I've just got
greedy since then).
So a bigger cross section is better for thermal mass, but critical for
avoiding saturation. If we assumed a cross section needed to avoid
saturation was 12 sq ins, a lossy 14 sq ins core could actually overheat
more than a 12 sq ins lower loss core though - and welders tend to have
cheap lossy cores, as opposed to say a pig's lower loss type of core.
So provided the core is sufficient size to stop saturation, then I would aim
for a low loss core from a source other than a welder as my first
preference.
Easy to say, but harder to acquire in the real world, so for a welder maybe
aim for an older big one rather than a newer smaller unit to play safe, but
if you want lighter in weight and very little over heating / voltage drop,
(mine never even gets warm) aim for a proper transformer core and wind your
own.
Bear in mind my thinking of going for low loss, is all based on living in
the UK where we can only 'get' 450ma or so 11.5kv pigs (and even they are
like golden Hen's teeth!), but in the US with 14.5kv (? kW) pigs being much
easier to acquire, a voltage drop of 0.5kv is really not that important of
course.
You don't need many turns, but you will need to make sure the core's winding
space is sufficient for what you do need.
I think in the US a lot just use a big coil of wire with an old iron mass in
the centre, so much depends on just how portable you need to be, and how
much loss you can afford to accept.
While a coil of old wire with bits of old iron bed-frame in the middle maybe
fine in the garden at home, it's not acceptable when turning up at an
occasional public event, well not in the UK <grin>.
Regards
Phil Tuck
www.hvtesla.com
-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Buds tape mail
Sent: 31 July 2015 13:22
To: Tesla Coil Mailing List
Subject: Re: [TCML] Variable Ballast
Hi Phil and others, I have been reading thru your web site and Richie's
very thoroughly. I have built a 12" X 51" secondary #18 wire and have
finished a 56" toroid and will have this thing ready to go very soon. I
have a rotary spark gap (copy of Ed's in Rochester) and a 14kv pole pig. I
have been reading extensively about primary inductive ballast. I do have
two 225 amp buzz boxes I could use in parallel as ballast like others have
done. I understand the principle series on the primary side limiting the
power to the pig.
I saw how and why you wound your own ballast. My question is
this........can we just use a very large welding transformer........say 400
amp unit as the ballast while shorting the secondary or should we just bite
the bullet and wind a huge ballast from an older transformer with a huge
iron core? I have my own machine shop.......... www.tape-inc.com
..........and can do almost anything once I know what it is I need to do. I
think most of us do not know the how and whys of ballast design.....you know
the core size min/max and wire size min/max for a pig of this size.
Bud Mohrman
-----Original Message-----
From: Phil
Sent: Friday, July 31, 2015 5:16 AM
To: 'Yurtle Turtle' ; 'Tesla Coil Mailing List'
Subject: Re: [TCML] Variable Ballast
Jon,
The modern cheap import ones you see knocking about are generally not up to
the job for a pole pig, without getting pretty hot during use. I melted and
shorted the windings of one a while back while running @ 10.5kv / 400mA -
(hardly pig output) simply because I left it running for several minutes
(they normally have a thermal cutout to disconnect the mains input during
normal use)
Their duty cycle is very low sometimes, especially as Yurtle mentions, if
Alu' conductor is used; also the resulting voltage drop from lossy windings
gets multiplied up by the pig, so you could potentially be losing 0.5kv at
the worst case.
Winding your own ballast is a better option.
Regards
Phil Tuck
www.hvtesla.com
-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Yurtle Turtle via
Tesla
Sent: 30 July 2015 20:52
To: GENE GARRAGE; Tesla Coil Mailing List; Tesla Coil Mailing List
Subject: Re: [TCML] Variable Ballast
I got a 225 amp Lincoln welder at a garage sale.It was old enough to have
copper windings. I chose to short the welding leads internally, and
eliminate the cables.
From: GENE GARRAGE via Tesla <tesla@xxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Wednesday, July 29, 2015 6:53 PM
Subject: Re: [TCML] Variable Ballast
From: Jon Danniken <danniken@xxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Thursday, July 2, 2015 2:28 PM
Subject: Re: [TCML] Variable Ballast
On 07/01/2015 11:14 AM, Miles Mauldin wrote:
> Do I need to find an old welder, or will an inexpensive new one work
> just as well. Read many posts about shorting the secondary and using
> the primary in line with a pole pig. Thoughts, ideas or suggestions
> all wanted for a variable ballast.
You can get an old 220V arc welder for cheap on craigslist if you are
patient, less cheap if you are not patient. As a bonus, you also get a
welder.
Jon
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