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Variac "knocking"

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Shaun

Mon Feb 18 2008, 04:09AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

So yesterday I was upgrading my SSTC to a fullbridge of IRFP260s, and I had everything soldered together and was in the testing phase.

Things were looking good as I started to turn up my variac, but right around 30V the arc length and intensity went way down (didn't vanish), and my variac began making a strange clicking or knocking noise, almost like a relay being activated. It clicks on every burst from the interrupter, increasing as I turn up the frequency, etc. It's almost like there's actually something moving inside, because I can actually feel the vibration from each click while holding the variac wheel.

No parts were damaged (variac fuse is intact), and I can still operate below this voltage, but I don't want to just turn up the variac for fear of blowing components. Has this happened to anyone else? Is it time for a new variac?

I was grounded to the mains ground, but switching to a ground rod failed to solve the problem. After that I also ran the AC mains thru my EMI filter between the variac and bridge rectifier. Still same problem. I'm at a loss here.
Please help.

P.S. I eventually decided to just "fu*k it" and plugged the rectifier directly into the wall. Interrupter frequency was about 1Hz, and I began to very slowly turn up the pulse-width. It was making hair-thin 5 and 6-inch sparks and then one of my FETs committed suicide.

Luckily I knew the dangers of plugging untested things into the wall and was wearing safety goggles accordingly. But DAMN, that thing EXPLODED!! The shower of sparks that flew out of it was like one of those pyrotechnic fountains. Except fountains don't have a deafening 60Hz hum. It blew the MUR diode and the TVS clear across the room, ripping the pins off the MUR! (the diodes were soldered directly to the MOSFET pins)

Bjørn

Mon Feb 18 2008, 04:41AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Have you tested your variac on some load that is known to be good? My first thought is that it is your SSTC that pulls too much current. I have several transformers that generate loud sounds and vibrations when shorted.

Shaun

Mon Feb 18 2008, 04:48AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

No, for some reason I didn't test my variac with a known good load. That will have to be tomorrow, need sleep now. The 8A fuse in the variac is fine, but peak currents could still have been well over that, I guess.

I am gonna start looking for a new[er] variac; this one must be about 20 years old. I'm also gonna look for one thats good for 15-20A.

Proud Mary

Mon Feb 18 2008, 10:02AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Yes, perhaps it's just magnetostriction due to a sudden load.

HV Enthusiast

Mon Feb 18 2008, 12:04PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Thats normal. What is happening is during each pulse (or rather after), there is a HUGE inrush of current to recharge your DC bus supply.

Its likely not good for the variac. I would either move to a large variac, or you can even use a choke to limit surge current (or rather smooth the current) during recharge.

Danielle

Mon Feb 18 2008, 08:00PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

I have experienced the same thing, and have attributed it to large inrush currents like EVR stated. I fixed it by not using the variac in situations where large inrush currents are expected. I know its not a solution, but what I did was go solid state on the variac, and it alowes for much higher currents if built properly. or build a large varible DC power supply like steve ward did.

HV Enthusiast

Mon Feb 18 2008, 09:25PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

The large boost controller like Steve's actually isn't going to help at all when it comes to inrush currents. Due to the topology, you are basically shorted right to the storage caps though the boost choke and the switch is in parallel so you can't control current so easily.

You really want o have some sort of input filtering to really limit the peak current at least into the caps.

Shaun

Tue Feb 19 2008, 12:21AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Hmm, a series inductor is starting to sound better and better. Do you guys think an MOT primary would do the trick?

Either way I won't be trying anything for a few days since looking at my exploded bridge depresses me and my CO2 laser is taking up most of my workbench right now.

HV Enthusiast

Tue Feb 19 2008, 03:15PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

A roll of 500ft 12AWG wire limits to roughly 25A. Turns out to be about 0.9 ohms at 10.5mH.

See here:

Shaun

Sun Apr 20 2008, 06:10PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Well it took me 2 months to get around to it but I finally rebuilt my H bridge. I built this one much better; I think what cause the failure before was the fact that all the FETs were on the same heatsink, with synthetic thermal grease not thermal pads.
Coincidentally the FET that actually exploded was the only one that tested continuity from drain to heatsink.
This time they have individual heatsinks and are built on a protoboard, instead of freeform soldered like before.

Anyway, I have so far tested it to 70VAC on the variac with no problems on the coil itself, however unless the pulse width is set very low I still get a mechanical "knock" from the variac on each shot.

My question is this: My bridge rectifier and DC buss capacitor are soldered to the protoboard, so can I still use a limiting inductor on the AC side of the rectifier?

Also, does anyone know what I can use for this inductor thats not unreasonably large or heavy? I'm trying to box this coil up so it actually looks half decent for once.

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