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Re: Ballast Resistance For Pig Coils (fwd)
---------- Forwarded message ----------
Date: Sat, 27 Oct 2007 11:57:12 -0500
From: David Dean <deano@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Ballast Resistance For Pig Coils (fwd)
Hi Adam
On Friday 26 October 2007 10:35:43 pm you wrote:
> ---------- Forwarded message ----------
> Date: Fri, 26 Oct 2007 17:41:37 -0700 (PDT)
> From: Yurtle Turtle <yurtle_t@xxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: Ballast Resistance For Pig Coils
>
> What size resistance are you folks using inline with
> ytour ballast for their pig powered coils to prevent
> "thumping"? I decided to tune up my coil this evening
> in preparation for Halloween. I recently started using
> two slide inductors instead of my welder. This evening
> all was running well, as I slowly cranked up the amps.
> As I got to around 25 amps, the actuator motor died on
> my Powerstat stack. I replaced the fuse, and it blew
> again. I decided to set the voltage, then remove the
> fuse, to keep from risking damage to the stepper
> controller board. As I continued to run, I heard
> several hard thumps from my powerstats. I quickly
> turned up the breakrate, which solved the problem.
> However, I noticed that the line in to the Powerstat
> actuator had flashed over to the chasis. Apparently I
> am getting some nasty spikes that I hadn't experienced
> with the welder.
>
You can get some nasty spikes with just the right ballast combination.
> I know folks have advocated using resistance in line
> with their ballast. Is there a way to calculate what
> resistance is effective at certain configurations, or
> is it more trial and error?
One way to look at it is as if the entire load were resistive. Then decide
what percent you want to waste.
For example:
Say you are running a 240 Volt single phase system. Say also your target
current is 50 Amps. Voltage and current are in phase so 240 * 50 =12KW.
E=I/R so 240/50=4.8Ohms
So you resolve to give up 10 percent for safety, that is 0.48 Ohms.
5 percent 0.24 Ohms
Lets go with 10 percent. P= I^2*R, so 50^2*0.48=1.2KW
So you would need a resistor that could dissipate at least 1.2KW and a
resistance of about half an Ohm. Of course when the TC is hooked up the
current seen through the resistor will be in spikes and so the resistor
should be made with greater dissipation capacity or fan cooled.
An easy way to make a suitable resistor would be to get a heat strip from an
electric furnace. The individual strips are usually rated approx. 5KW on a
240 single phase system and consist of a frame with a length of nichrome wire
coiled up into a spring like thing and stretched through the framework
through insulating bushings. The wire is about 12.5 Ohms so if you take it
out and cut it into five pieces of 2.5 Ohms each, and hook the five pieces in
parallell, you will get a 0.5 Ohm resistor capable of dissipating 5KW with a
small fan.
I made one several years ago. I used some aluminum flat 1/4" X 1" X 8" with
some holes drilled for mounting a mechanical lug on one end of each bar, and
holes drilled and tapped for screws to hold the wire. If I had it to do over,
I would use all mechanical lugs and just clamp the end of each piece of wire
under its own lug. Or straighten each end of each wire just enough to crimp
on an uninsulated ring terminal to go under the screw, because sometimes the
end of the wire wants to brake if you bend it first straight and the try to
bend it around the screw. I ended up using washers and just laying the last
turn over and so then the washer acted as a clamp.
Another option is a restring kit. It is just the new heating wire and a couple
of ends for the furnace wires to hook up to. If you used a new heating wire
it might not be so prone to break. I just used junk I had lying around.
later
deano
>
> The reason I'm concerned is I don't want to blow up my
> Powerstat actuator or my VFD.
>
> I have EMI filters between my incoming power and my
> pig/ballast, but maybe I should consider something for
> voltage spikes. While a spark gap for 120/240 sounds
> silly, and hard to set, the 120 volt input to my
> actuator jumped 1/8" to the grounded chasis. Without
> having access to some kind of power monitor, I
> wouldn't know if MOV's would last for years or
> seconds.
>
> http://www.hot-streamer.com/adam/bigass_coil/schematic.pdf
>
> thanks
> Adam
>
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