Re: 15kva 14.4 kV Transformer/MMC failure (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

Moderated and approved by: Gerry Reynolds <greynolds@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 23 Dec 2006 10:45:00 -0600
From: Dr. John W. Gudenas <comsciprof@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: 15kva 14.4 kV Transformer/MMC failure

Steve and DC
I have been using a completely rebuilt DRSSTC for the past year. It  
essentially has Steve Wards electronics with modifications I made.
Steve and I have had regular correspondence about this coil. The  
first failure was the MMC. I redesigned the MMC bank roughly for a  
20% voltage increase, but a 50% current increase.
This bank has not failed, however, on July 4th this past summer I was  
having a prolonged run outside. The coil was running at least an hour  
@ 140 v from the variac until a gust of wind directed the streamers
all over the place. The secondary arced and opened about 3/4 up and  
the IGBT's blew. After I killed power I felt the MMC and electrolytic  
supply and both were warm, but not hot.

Since then I repaired the secondary (I think Steve will get a look )  
replaced the IGBT's and the coil performs great. I did a demo for the  
science club at the university Physics lab and the coil
ran incredible. It is designed to produce streamers from a discharge  
point in the toroid center like a VTTC. This lets a small geometry  
coil streamers go up toward a ground area above (The hanging and  
grounded light fixtures in the Physics lab.) A newspaper reporter was  
there as well as faculty and about 45 students. The reporter had a  
great picture for the local newspaper  and I was told (I did not see  
as I was watching an LED in the coil base) that
when I cranked to 140 v on the variac the streamers made it to the  
light fixture that I measured afterwards to be about 48 inches away.  
This coil draws around 8 amps from the 120v main.

This was a 20 minute run at multiple power levels and all components  
were cool.  I don't have direct measurement data to support your  
conclusion, but I made the same inference that you did.
I only have the "build it bigger and stronger" experience that  
supports yours and Steve's notions. I experienced equivalent issues  
with my VTTC exploding high voltage mica caps (10x primary voltage)  
until
it became obvious that it was a current issue.  One can likely get  
away with a lot if you run your coil on short duty cycles. But the  
stress factor really goes up for long run time.

John W. Gudenas, Ph.D.
Professor of Computer Science


On Dec 22, 2006, at 10:59 PM, Tesla list wrote:

> Moderated and approved by: Gerry Reynolds <greynolds@xxxxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Fri, 22 Dec 2006 21:39:00 -0700
> From: resonance <resonance@xxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: 15kva 14.4 kV Transformer.. (fwd)
>
>
>
>
> I'm beginning to think that due to the higher efficiency of IGBT  
> switching
> (output sparks are much hotter) that the pri currents are higher  
> and much
> harsher on the MMCs than a typical spark gap switched cap bank.   
> Less losses
> equals more current in the pri and higher stress on the MMCs.
>
> I don't have hard data to support my theory --- just a hunch.
>
> Dr. Resonance
>
>
>>
>> Excellent DC!
>>
>> I find that my MMC eventually fails (before the IGBTs provided no
>> "special events" occur).  I have one DRSSTC that has gone through 2
>> MMC banks now, but still has the same IGBT set.  It consistently  
>> pumps
>> out 6 foot sparks at 1300W input.  If given the chance, id also try
>> running one of my coils for extended periods of time to find out  
>> where
>> the bottleneck in the design is.
>>
>> Steve Ward
>>

SNIP -------- SNIP ------------ SNIP