Re: ballast questions

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

Original poster: FIFTYGUY-at-aol-dot-com 

In a message dated 7/9/04 11:03:21 AM Eastern Daylight Time, tesla-at-pupman-dot-com
writes:

 > 2. I don't see any practical way
 >  to make an air gap in the core without cutting the top off. How important
is
 >   it? The core weighs around 80 lbs. I want to ballast the pig to around
 >  20A. With a core this size, would saturation be a problem?

     Since I just got a 10 kVA pig myself (Thanks, Doc Resonance!), I could
tell you about what I've had to learn so far:
     The specific type of core material and its cross-section is what's
important regarding saturation for a given number of Amp-Turns . Saturation 
is a Bad
Thing, and better too much metal than too little to avoid the problem. Who
cares if your ballast weighs 100 lbs instead of 50 if it gives a greater 
safety
margin? OTOH, asking "is an 80-lb core enough to not saturate" is silly
because it doesn't describe the core geometry.
     The specific type of core material will be really difficult to ascertain
when salvaging. However, it's not that important if the inductor is overbuilt.
     The ferrous cross-section is important to not saturate by being run past
a certain flux density. The flux density depends on how many turns you wind
and the current through them. So saturation is a runaway condition.
     Per Dr. Resonance's suggestion, I hooked the HV side of the pig up to my
Jacob's Ladder (luckily I had just completed a nice 4' tall one with a 6" dia
glass tube and 1/2" copper pipe. At 50A of 240V into the pig with inductive
ballast alone, I need to find a 7' tall by 16" diameter tube to contain the
arc...). Put an ammeter on the pig's LV (primary) input, and use the Jacob's
ladder to load the thing as you change your balloting.
     I started with 4 ohms of resistive ballast in series with the inductor.
Presumably you want to end up with a minimum amount of resistive ballasting as
it is a big waste of energy. I was using some nice dynamic breaking resistors
off a 40HP DC drive, probably rated around 2KW. After 5 minutes of steady
running at over 35A pig input they were approaching orange hot 
(interestingly, the
current did not change appreciably). As I modified my ballast inductor for
better performance, I removed resistance (with taps) until I was using the
inductor alone.
     I built the inductor with 10ga THHN stranded wire. 5 minutes operation at
50A gets it past warm, but not dangerously hot. I'm going to either use 8ga
THHN or magnet wire, or both. How much wire you needs depends on the
cross-section of the core you're using - bigger core, more length per turn. 
I've been
removing turns as I made the inductor more efficient, so for my setup I used
about 140' of 10ga THHN.
     The interesting thing is that as it has evolved, my inductor is
approaching the design Ritchie Burnett had engineered. See his Web page. I 
think the
"slide choke" advocated by Dr. Resonance is easier to manufacture and make
running adjustments on, but I'm guessing it's not as efficient as the 
shell-core
design.
     For the core, I butchered a 25kVA three-phase SCR Drive Isolation
transformer. Plenty of metal there, obviously more than enough. Core was made
entirely of "I's" of various lengths (but all 3.5" wide).
     My core has evolved to a center cross-section of 3.5" by 2.5" wide, for
an area of 8.75 sq in. Almost identical to Ritchie's. I plan to add another
1/4" thick for the heck of it. BTW, as the core cross-section approaches a 
circle
the efficiency improves, as it requires less winding length per turn which
means lower resistance.
     If you use just a center core, the air gap is effectively pretty darn
long (from one end of the core all the way around to the other). As you 
shorten
the air gap (by shimming the top "I" of Ritchie's inductor closer, or by
lowering a slide-choke's core into its coil), the total reluctance of the 
magnetic
path decreases. If you take the ferrous core completely out it becomes an
air-core, which won't saturate, but it won't have enough inductance to do 
practical
pig ballasting. My inductor is like an "E" (built of "I" stacks, not
overlapped laminations at the moment), with no "I" top. If I take the "E" 
side limbs
off and place them under the center core, the current let through goes from 
30A
to 50A.
     So far, it doesn't seem to be saturating at 50A, as the core only gets
warm at the edges of the ends where the cores meet the air.
     Advice:
     Use 10ga THHN minimum. For over 40A, 8ga magnet wire would be better.
Maybe bifilar wound with 12ga much easier! Undoubtedly there's a lot of 
wire to
salvage from that welder? Start with 200 turns and reduce during testing.
     Use one "E" section from your welder, whichever has the largest center
core cross-section. If any other "E" or "I" sections bridge either of the two
air gaps at the top of the "E" you are using, then remove them or cut them to
open the gap. You can always close them later.
     8 sq inches center core cross-section seems like a good minimum.
     If you bridge the "E" with an "I" to complete the magnet path, make sure
the "I" laminations face the "E" edgewise. Don't lay the "I" with the flat
sides facing the "E" - you'll be shorting out the mag path with the one "I"
lamination facing the "E", which makes things get hot. Ask me how I learned 
what
should have been obvious...
     I used a 4" piece of PVC as a bobbin. I warmed it with a propane torch
and flattened it into a square-ish cross-section. After winding (4 layers 7"
long) I dropped the core into it. It did start to soften and melt at the 
end the
touched the improperly aligned lamination stack.
     However, it made for a relatively thick bobbin which wasted space. The
less unused space in the "E" windows, the more efficient the inductor 
(magnetic
path minimized). Suggest locating the winding at the bottom of the "E" center
(away from the open end), as the tops of the "E" limbs get warm at the air gap.
     For final design, I'm going to split the inductor winding so each leg of
the 240V goes through one half. Not so much for anticipated equalization, but
to help protect the control box and house wiring by effectively using the
ballast's inherent filtering property to block RF noise. Just make sure to 
keep
track of polarities!
     At 50A, I'm dubious of my 20A variacs not saturating. Sure, I can adjust
the ballast alone to change the pig current and achieve about the same effect
on the TC as primary voltage adjustment, but it would be nice to get the extra
20% higher voltage the variacs would produce. So I'm thinking about using the
ballast adjustment after all, but running through a "boost" transformer to
get the extra voltage. This works out to about a 2.5 kVA fixed 
autotransformer,
which is easier and cheaper to scrounge/make/install than 50A variacs.
     BTW, 260uF of PFC's don't seem to affect the pig's primary current, or
the qualities of the Jacob's Ladder arc. The PFC loop does appear to have 10A
through it, however.
     Remember to keep the ammeter away (3' is good) from the ballast inductor
when doing current measurements!

-Phil LaBudde