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Re: Cap Failure
10/13/96
You wrote:
snip
>During the next run, power was adjusted to about 80% (about 112
>volts) on the 15 KV 120 MA neon tansformer source, with good coil
>output, when after about 20 seconds one end of the capacitor was
>suddenly and explosively blown off with no prior warning. A loud "pop"
>and a yellowish colored flash were heard and seen, and capacitor oil
>was ejected in the immediate area.
After taking a lot of heat publically and privately for starting the
prior thread on Capacitor Explosion, I make the following observations.
I also had the exact same cap and was running it at even lower power
when the exact scenario occured.
>As Ed Sonderman had previously described, these capacitors consist of
>a number of identical rolled capacitors interconnected by soldered
>netal straps. The metal looks to be tin(?), and these straps are
>soldered to the end foil of the individual capacitor rolls as well as
>the capacitor end terminals. The individual cap rolls look to be about
>4" in diameter and about 3-4" long, and are connected in series.
>The PVC area around the plastic endcap and terminal was fractured, and
>the endcap blown off with enough force that it apparently pulled off
>the soldered connection between the metal strap going from the end
>terminal to the end foil of the outermost capacitor roll. The last
>roll was physically ejected from the capacitor housing, but was still
>electrically attached via strapping to the next capacitor roll.
>
This is an exact duplication of my cap explosion, only the end roll was
also blown out and parted from the others.
>Under closer inspection, it was not obvious exactly what caused the
>cap to blow. However, this appears to be the third cap in the earlier
>group purchase that has failed in this manner, in this case at a
>relatively low power level (2.5 KVA). Don will post some pictures of
>the cap so that others can see the construction technique and failure
>mode.
Well boys and girls, I believe I know why they blow. Run out to you
garage, shop or lab and look closely at the printing on the cap casing.
Yep, the pvc casing is only rated at 10 kvac. I may assure you that
the inner rolls are intimately in contact with the inside of the
casing. When I closely examined my casing there were several small
black carbonized tracks in the casing. Under magnification they appear
to be arc tracks through the pvc wall. As pointed out earlier, these
caps are sealed with the specifications of a bomb. And, with our
resonances and kickback I'm sure the 10 kvac case rating is handily
exceeded.
>He's also going to contact Condenser Products to get a replacement,
>since it should be covered within the 1 year warranty period.
Good luck.
>Thanks to previous posts on this conference, we were aware of the
>possibility of explosive failure, and had taken precautions to orient
>the cap so that it would do no damage if it blew (other than make an
>oily mess :^)). It appears that there may have been a problem with
>some of the CP capacitors in this lot, since I've not heard of any
>other similar CP failures in a while, and none outside of this lot...
snip
Follow up on cap explosion.
I refused replacement of another defective cap from CP. All my cap
rolls were intact and tested out. I resolder the straps and placed
them in a 6" acrylic form. I filled it with mineral oil and pumped it
down while on end (idea from R. Hull's post). I drilled several detent
holes in the 1/4" acrylic end plates (Malcolm's suggestion). I ran it
intermittently for almost two weeks at increasing voltages.
So far, I have observed no arcing through the clear acrylic form and
the cap preforms excellently.
Law suit? This was some hysteria created on this list by Scott Meyers
and the president of CP. I never mentioned it even though I'm a J.D.
Go back and read all the posts.
This was a great learning experience for me and others I'm sure. Better
yet, it generated a fantastic TC safety faq for this list. I'm sure it
will prevent future injury and even death for current and future
coilers.
R. Hull, may we expect your video on safety, wiring, construction and
control cabinet?
RWW