'Pig Power!'

Last Sunday, I met up with Mike Tucknott and Steve Crawshaw (and a friend of his) to do a bit of disturbing the peace with some pig power!

I took my coil round to Mike's parents' garage. We ran it in its standard configuration, i.e. using my 1kW NST, to check it was working and in tune before connecting it up to Mike's pig. Between the two of them, Mike and his coiling buddy Brian Le Page, they have a well sorted system for controlling the pig; a big variac, a welder ( with variable current settings) and a load of neatly wired up switches and connectors.

We knew that the stand for my coil makes the coil a little too tall for the average sized garage, and this one had metal beams right above the toroid, so we decided to remove the bottom board (and feet) and rest it on the legs. This gave us approx 4 inches more clearance. The gap, the new blower box (built for the occation, the day before) and the cap were now resting on some carpet on the concrete floor. The NST was replaced with the power feed wires from the pig, which was sitting out of strike distance. Everything was checked out OK and ready for action. With Mike at the controls we let her rip!

Performance:


(more pictures in photo gallery)

With the welder set on maximum current limiting the output was noticeably better than with the NST. We then cranked up the current a bit and tried again. It got better, for a while, until Mike shut it down a bit sharpish, saying something about the cap's burning! 'Oh no! Not my MMC!'.

The MMC was not on fire, fortunately, but what we had neglected to think about was that the ends of the individual caps were now very close to a concrete floor, which tends to conduct very well at high frequencies. Judging by the black singe marks on the carpet there must have been a terrific amount of corona leaking off the cap wires! We placed the cap up on a sheet of 1/4 inch plastic, then turned up the current some more, and continued to have fun.

About 2 minutes later something popped in the cap, with a flash and a nasty smell.


(Click on image for close-up, 61KB image)

The corner cap had failed in spectacular fashion! It burst in the middle of one side and burnt two neighbouring caps, and the internal pressure also blew the contents out one end and into another cap!

We can estimate the voltage output of the pig as 12kV, and each MMC string had 22 caps (0.047uF @ 1500VDC/450VAC) in series. I used the 450VAC rating, with my NST at 10kV (RMS) as a guide to the string size. After looking at other coilers' MMCs I realised that mine was rated a little bit on the conservative side; it should have been bullet-proof!

I suspect that when the cap was resting close to the concrete and blasting corona from its ends, it either overheated the cap or weakened it in some other way. I'll replace all four damaged caps (and any suspect resistors too) and keep in on a higher support next time.