Monday, January 31, 2011

The Beast With No Name

Around 1995 or so, I received a call from a good friend, asking me if I might be interested in a very large and very heavy high voltage capacitor bank -- capacitors specifically intended for pulse discharge duty -- with its own charging and switching circuitry; in short a complete, turn-key pulsed discharge machine. I immediately said, "yes" and then wondered where I was going to put it since I was living in an apartment at the time.

The thing was immense, weighing just a bit shy of 2,000 pounds. It consisted of two 28" wide rack panel cabinets mounted on a steel skid with four groaning casters. The shorter of the two contained the high voltage power supply, ancillary power supplies for things like control voltages, and a bit of relay control "logic" to interact with operator input and interlocks while enforcing safety rules.

The larger of the two cabinets contained the pulse capacitors (six), a parallel plate current collector stack (nice low inductance design), six ignitrons (one on each cap) in coaxial housings, as well as a bunch of other hulking heavy duty high voltage equipment to trigger the ignitrons, including a nice glass hydrogen thyratron.

This thing was designed in the 1960s and built in the 1970s. It was moderately cutting edge stuff for its time. The company wanted to investigate industrial applications for pulsed power including well frac'ing, and magnetic metal forming. Fortunately, all of the designer's personal notes and sketches, as well as notes from later techs who worked on the machine to rehabilitate it in the early 1990s, were saved and given to me when I got the thing. That has been incredibly valuable.

What happened is that the company's focus had moved on and narrowed - they had made interesting strides in metrology. They were being purchased by another company already famous in the business. They needed to clean house, and SOMEHOW, that two thousand pound white elephant had to go. If I could move it, I could have it. It sat for a bit while I figured out where in the hell I was going to put it, and how. The owner of the company was awfully patient with me.

Well, I had a sort of professor emeritus status with the entity which at that time was right in the middle of morphing from The Dream Park Corporation (bankrupt) into The Diabolic Company. Said organization had just moved into an IMMENSE building which had previously held a K-Mart. I asked, and I was basically told, "any place that won't interfere with the haunted house operation, knock yourself out".

The short version is: I stashed it there.

I got some friends to help me move it.

And we dropped it. But that's a story not terribly relevant here and now.

The astonishing thing was, we didn't do it any harm - er, probably. There was a little sheet metal damage, but it had slid down and landed with a jolt, we didn't really drop it off the dock. The only thing affected was the ignitrons. You see, ignitrons are finicky beasts. They have their advantages (they can switch HUGE charge transfers) but one of their idiosyncrasies is that once they have been used in crowbar or capacitor discharge duty, it is crucial that they not be moved or jarred. When ignitrons ares used in those high-wear modes, the structure which supports the ignitor electrode over the pool of mercury becomes very fragile. And shifting or sloshing of that pool of mercury is likely to damage the ignitor, rending the tube inoperative. True story.

The way we were rolling that thing around the lab and bumping it over doorways to get to the dock, all six of those tubes were toast before we ever got around to dropping it off the dock. Of course, I didn't find that out until later. About the same time in fact that I found out replacement ignitrons cost $1,500. Each.

Not having a spare nine grand burning a hole in my pocket, I decided to use another means of switching. I got rid of the ignitrons to a friend who was collecting mercury anyway. Don't ask. One less hazardous materials disposal pickup (we have an awesome trash department) for me to deal with.

Meanwhile, I needed a way to move this beat around without a Class IV forklift. While I still had the shop and lab space at DiaboliCo's haunted house building, I took to rebuilding it. I removed the power supply and its cabinet, disassembled the ignitron housings, and plate stack, removed the cabinet, and eventually unstacked the six pulse caps. They weigh about 150 pounds each. I'm guessing. It I ever weighed them, I've lost it, though I ought to have the shipping weight in my folder. Anyway. Then I split the big steel skid the whole thing had been mounted on, with a torch. I added more steel, and two new casters, and I had two skids. Then everything was reassembled onto those. Mind you, with a few rare exceptions when I was in a hurry and people were around to help, I was doing most of this alone, all Leedskalnin-style, with levers, blocks, ropes, pulleys and the convenient architectures of the building.

After that, the machine followed me around, all 2,000 pounds of it, through two moves, and has been sitting in my workshop alongside my collection of other Large Heavy Objects, waiting for me to figure out which way to go with it. In the mean time, I've been learning about high speed and high current switching devices. Since I acquire the thing, I've learned a GREAT DEAL.

I've been given one type of switch - which I've yet to fire, tho I am inching closer - and I've designed and built another type - which I've also yet to fire. I'm inching closer to that event too.

Stupid mill.

While the mill has been down I've been working on the parts of the long-stalled railgun project. The parts that don't require the mill, that is. That would including the pulser, or as the previous owner called it, "The Banger" (as far as I am concerned, it still doesn't have the great name it should, yet). From what he told me of their insane escapades, I bet every one of them needed hearing aids. Exploding wires are louder than gun shots, kids.

The first thing I did was excavate the machine itself from a mound of boxes. The new effort provoked a quick round of shop-cleaning this weekend. The second thing I did was remove all of the ancillary parts that were necessary for triggering the ignitrons, about 50 pounds of transformer, capacitors, rectifier, and blah-blah oh yes and a nice thyratron.

I ought to describe how the thing worked before I got my grubby mitts on it and tore half its guts out. It'll be illustrative of how things often go in the pulsed power world. Say you've got a whacking huge amount of energy you want to apply to load with a whacking huge switch. Trouble is, whacking huge switches by their very nature, tend to require a whacking huge something to activate them. Might be a huge pulse of electricity. Or perhaps some compressed air. Explosives are often used. Not kidding. So you come up with this pretty substantial trigger pulse to trigger your whacking huge switch. That pretty substantial trigger energy has to be switched by a substantial switch. The substantial switch needs at least an adequate switch, and so on, finally ending at the big red, jolly, candy-like button on your control panel.

So, this is how this particular machine was originally designed to work:

1. The operator sets the voltage at which they want the machine to fire on a dial (a Simpson relay meter).

2. The operator places the work in the fixture, which is already connected to the machine's output bus.

3. The operator presses the 'Start' button, and I would presume, gets the hell away from there.

4. The machine charges, slowly I assume, and when the meter needle reaches the aforementioned setpoint, the machine discharges in the following sequence:

5. because the Simpson meter contacts aren't good for any current at all, when they make contact, they are used to control another, ordinary industrial control relay.

6. that relay's contacts apply a voltage to a control grid in a pentode

7. the pentode turns on and dumps the charge of a small high voltage capacitor through the primary of a pulse transformer

8. the secondary of the pulse transformer delivers a small high voltage pulse to the

9. hydrogen thyratron located in the capacitor cabinet, which turns on and connects one side of six beefy high voltage capacitors to ground, dumping their charge through the ignitor electrodes of six ignitrons. Inside each ignitron, 100 joules of energy vaporizes a small amount of the mercury in the pool into the vacuum of the tube. The mercury vapor connects the anode and the cathode, turning on the ignitron tube until there is no more current to keep the vapor hot. Functionally, it behaves a lot like an SCR.

Seems a bit complicated, don't it? But that's the way it is. I'm going to try to do this with more sophisticated (and more importantly, obtainable) components, but there is still (and will always be) a long series of steps between you and the big drama. That's another good point worth raising - isolation. You don't want any kind of freak fault to allow the main store energy to get out into places it doesn't belong, such as remote control pendants. That would be bad.

Oh yeah, it came with a crude remote control pendant. I'm already working on a more sexy remote control, uh, "pendant". Well, panel anyway. Okay really, it's more of a box. With a panel that mounts inside the box. Like a miniature rack, sort of. Only with storage inside the lid. I'll try not to let it get any bigger. Also, it's long term project, and won't be done before the pulser-with-no-name is in operation.

Speaking of names, I could use one. It's an idiosyncrasy of mine - I like to name things. If you've got any clever ideas, leave me a comment or something.


jodys said...

I'm watching your progress on this and enjoying the hell out of it. It is nice for me that I'm able to follow a lot of the technical details. Mostly, though, I enjoy the tone with which you write--its entertaining!

Robert said...

I can think of a name or two; none safe to use in polite company, though.

Gomez said...

Jody: thanks! I tend to write in a very conversational tone. In fact, I find it hard not to, but I can if I work at it.

I've also loosened up a bit since I realized my goal with this blog is NOT to "teach" anything to anyone.

Robert: ain't no polite company around here! But it would need to be something I can _use_. If it's really bad, well... probably not. :)

Gomez said...

There are still no pictures here dammit. Man. I need more time.