Wow. Exactly what happened with me, FWB block, variac smoke, and all. I've run out of my bag of FWB blocks now, so I built one on a sheet of PVC last night using some robust stud-diodes, and will test it this weekend. Will start off with the LC removed, then add the L part, then the C, and see what happens. I also added a 15 amp inline fuse to the variac as well.
JUST AN FYI TO THE TESLA LIST FOR ANYBODY USING THE 1256 Powerstats:
Recently I found that I had a short between the winding and the aluminum case, which was caught very quickly. First lesson learned was having an open cabinet with exposed components has allowed me to observes flashes or smoke very quickly that has saved various expensive components. Had I had the cabinet sides on the frame at the time, I'd never seen what I'm about to describe, and consequently would have destroyed a very expensive variac. Now, on to the story. Some models of the 1256 cores are supported by what appears to be green or black plastic (this is the appearance mind you) support blocks that slide onto the aluminum vertical threaded assembly rods positioned at four points around the periphery of the aluminum cage. These plastic blocks are in reality aluminum blocks with a thin layer of insulative material. I'm guessing at perhaps .010" to .020" thickness at best. What happened on my variac was the for some reason the insulation layer failed (no idea why or how), the aluminum support (which is in contact with the vertical threaded supports, which are grounded to the cabinet frame) eventually wore through the winding varnish (enamel, whatever), and shorted the conductor (winding) to ground. I was able to catch this immediately, and after a tear-down of the variac, found that some cleaning, dressing of the windings, and application of polyurethane over the damaged windings, was able to save the variac, and I placed it back in service. I added a strip of Gorilla tape to each block during the reassembly to ensure the other blocks did not do the same thing. Some of my 1256 Powerstates have phenolic support blocks, which I love, but were probably too expensive to machine, so I guess the manufacturer's switch to cast aluminum pieces with poor insulation was the cheaper alternative? My greatest fear right now is the question; Is this related to the failing FWB blocks (high voltage/frequency backing up into the power circuits)? Perhaps I need to review filters, but this is one of my weak spots in my knowledge of TC operations.
Terry
-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of David Rieben
Sent: Wednesday, December 13, 2017 5:18 PM
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Subject: Re: [TCML] **External Email** Re: ARSG bridge rectifier failures
Terry,
Actually, I have a single 400 VDC, 3900 uFd 'lytic run in parallel with the motor's input leads. Don't think it really "smooths" anything though, as the the motor's rotor, along with the G-10 disc and the flying tungsten electrodes provides plenty of inertial mass for 'filtering' the ripple.
Actually, I had been running my ARSG motor assembly trouble free for a good while, then I got a 'wild hair' and decided to change out my relatively small phenolic block FWB with one of those nicer looking metal cased units that were rated at an impressive 1 kV at 50 amps. (These units sure do seem fragile for such a monstrous rating, though)! Since grounding the outer case of the old smaller phenolic block exterior of the original FWB assembly wasn't an option, I never had that problem beforehand. However, once I installed the new metal block exterior FWB, it only seemed natural to ground the outer metal case (mains ground, NOT RF ground). The RSG motor still ran fine UNTIL the first moment I engaged the coil to produce sparks, then the FWB quickly failed as a short and actually caused my control variac to catch fire, seeing as I did not have this segment of the circuit adequately protected from over-current (only the main 100 amp throw breaker for the entire coil system)!! As soon as I replaced the FWB assmbly, SAME THING!
At this point I started scratching my head and finally figured that the only thing that I had done electrically different from my original setup was the grounding of the external metal casing of the new FWB unit. Once I removed the ground (and installed yet another new FWB unit), the problem was solved.
Fortunately, I ordered a bag of 10 of these FWB units. (I also placed better
20 amp overcurrent protection in the circuit to my motor, too). BTW, I run the supply line from the variac in my control panel to the coil's FWB rectifier and ARSG motor assembly via a metal clad (MC) 14/2 AWG cable, with the outer metal armour naturally grounded. My FWB assembly is mounted on small strip of copper as a heat sink and to a poly cutting board (and of course, I have left out grounding the outer metal casing of the FWB assembly).
As you and James were discussing, and since my motor as well as the FWB rectifier unit are mounted directly underneath my primary coil support assembly, there must be a ton of high RF voltage that is induced into the low voltage wiring and frying the sensitive silicon of the unit, and all that I can figure is that grounding the outer casing gives these induced voltages a short path to dissipate their energy. One has to wonder where these induced high voltages are dissipated without the ground, though? Well, I have on occasion noted sparking inside my control panel, too and have tried to add additional filtering of the low voltage mains inside the control panel cabinet to combat this :^o - may have answered my own question.
Merry Christmas to you and yours as well, David
----- Original Message -----
From: "Terry Oxandale" <Toxandale@xxxxxxx>
To: "'Tesla Coil Mailing List'" <tesla@xxxxxxxxxx>
Sent: Wednesday, December 13, 2017 2:55 PM
Subject: Re: [TCML] **External Email** Re: ARSG bridge rectifier failures
> David,
>
> Just curious if you've got an Cap/Inductor filter on the output of your
> bridge? I don't think I have had any problems prior to adding this to the
> output of the bridge, but couldn't imagine that the ripple filter would be
> a cause to the problems, especially if everything runs fine without the
> high discharge performance.
>
> Terry
>
> -----Original Message-----
> From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of David Rieben
> Sent: Tuesday, December 12, 2017 5:25 AM
> To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
> Subject: **External Email** Re: [TCML] ARSG bridge rectifier failures
>
> Terry,
>
> I had pretty much the same problem with my big coil. I, too, am using a DC
> motor designed for a treadmill, that is rated at like 130 VDC for the
> drive of my ARSG. I was using those 1 kV, 50 amp rated FWB 'blocks' for
> the rectification of the 120 VAC mains, through a 120/140 volt, 10 amp
> rated variac for speed control. When I grounded the outed metallic casing
> of the FWB rectifier in question, it would almost immediately get fried
> once I even started to produce a spark output from my coil! Fortunately, I
> had ordered a bag of 10 of these FWB units, and I discovered that when I
> removed the ground from the outer casing and just allowed it to 'float',
> the problem was solved.
>
> Maybe you could explore this solution for your setup?
>
> David Rieben
>
>
> ----- Original Message -----
> From: "Terry Oxandale" <toxandale@xxxxxxxxxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Sunday, December 10, 2017 9:22 PM
> Subject: Re: [TCML] ARSG bridge rectifier failures
>
>
>>
>>
>> I'm having an issue with the bridge rectifiers I'm using to power a
>> variable speed DC motor for the rotating spark gap. When the coil is not
>> energized (but motor is spinning), the diode stays cool, and appears to
>> be
>> reliable. As soon as I energized the coil sufficiently to operate with
>> breakout, the rectifier gets a little warm, but appears to remain
>> reliable. With more coil performance, I notice more heating of the
>> rectifier. It appears, after some testing, that the lifespan of the
>> rectifier is inversely proportional to the power put into the coil. I
>> have
>> a voltmeter and ammeter to monitor the power to the DC motor, and there
>> is
>> nothing to indicate a problem (but these are analog meters, and may not
>> be
>> sensitive enough to see everything I'm looking for).
>> So far, the only variable is the coil's output (performance), so that
>> leads me to believe something is back-feeding or being induced into the
>> motor's power cord and back to the rectifier. The motor only draws about
>> 1
>> or 2 amps through 90 VDC at speed, yet the rectifier is rated at 30 amps
>> and 600 volts. I don't run the motor at the full 90 VDC because the coil
>> operates best at a lower RPM, and hence a lower voltage to the motor. The
>> rectifier is fed through a small variac, whose output then passes through
>> the rectifier. The rectifier does have an LC filter to dampen the full
>> wave output of the rectifier. I don't know if it's the cheap Chinese Ebay
>> bridge rectifiers or not, but am considering building one out of some
>> robust diodes that I've had for years.
>> Any thoughts please?
>>
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>>
>
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