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Re: Large Transformer Question (fwd)
---------- Forwarded message ----------
Date: Thu, 12 Feb 1998 16:33:23 -0500
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Cc: seraphim-at-WPI.EDU
Subject: Re: Large Transformer Question
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: 'Tesla List' <tesla-at-pupman-dot-com>
> Subject: Large Transformer Question
> Date: Wednesday, February 11, 1998 10:02 PM
>
>
> ----------
> From: Joshua Resnick [SMTP:seraphim-at-WPI.EDU]
> Sent: Wednesday, February 11, 1998 9:51 AM
> To: Tesla List
> Subject: Large Transformer Question
>
> List,
> I have two H&R 5Kv xfmrs being used in series for my primary. I
> have not turned my coil on yet but they work great (cap testing)
> without tapping the connection between the two secondaries to ground.
> Some have said they have connected this to ground and others said they
> have not. I was wondering what advantage , if any, comes from grounding
> this center connection.
This reduces the voltage stress. If you do not ground the cores and the
inner windings to a good ground, then there will at times be a voltage of
several KV between the cores and the inner windings. You can reduce
this to ZERO volts by grounding. A small price to pay for so much
extra insurance!!
> Also my capacitor is .1uf poly / foil stack. Being new to cap building
> I used only one layer of foil. The cap is built with two continuous
> strips interleaved with polyethylene.
I am confused. How can 'only one layer of foil' have 'two continuous
strips interleaved with polyethelene.
I am wondering if maybe your cap isn't really a cap after all.
Do an ohmmeter check and see if you have continuity. If you do,
then you don't have a cap, but a dead short with some poly for
good looks :(
What thickness of poly are you using? It is also possible that you had
a poly breakdown and therefore a foil short.
NEVER NEVER NEVER fire your poly caps "dry". Surface conduction
can cause a 15KV spark to snake along several inches of surface
even though it might only be capable of 'jumping' a fraction of
an inch. You NEED the oil!!!
> When I loaded and gapped the
> system to test my cap briefly before attempting to pot it I noticed what
> I felt to be amperage effects on the foil coming from the top of the
> stack. My roommate (college) believes that the melted foil was from
> flaming arcs from the simple gap we had setup that had touched the foil.
> The whole strip didn't melt, only a portion on the edge.
"Melted foil"???? "Flaming arcs"???? "from the simple gap setup that
had touched the foil"????
Sheeeesh! You guys believe in living dangerously!!
Melted foil is NOT a normal thing. Are you saying your gap somehow
arc-flamed to the cap? We have a safety problem here. You should
be very careful. Don't take unnecessary chances with your life.
Use an ohmmeter to check your cap before introducing the oil.
Bring system power up slowly, and with only a few gaps at first.
Once you have verified that your cap is working, then you can
bring up the power levels.... otherwise you may have your cap
destroy itself unnecessarily.
> Anyway I
> decided to run bussing strips up each side of the stack to help
> distribute the current more evenly. Has anybody had experience with
> over-current problems with caps?
I have used heavy duty aluminum foil many times for my caps. I
have never had the stuff melt on me. When doing post-mortems on
old units removed from service due to age or failure, I have often
seen that the foil develops minute holes over time. The thinner the
foil, the more readily this occurs. With a good heavy duty foil
this is not anything to be overly worried about.
> Last question - I cut some microwave xfrms apart in a band saw and
> removed the secondaries. I measured them out around 350 mH, an
> acceptable Tesla-Coil choke. Have people used them before? What current
> / voltage limitations do they have?
>
If you short out the secondaries (remove the connection to the core),
then microwave transformers make good series chokes for the PRIMARY
of other transformers. You are talking about just using the removed
secondaries as a choke on the HV side, I take it, as an air-core
choke. My gut feeling is that the insulation will fail between
layers. But you have nothing to lose experimenting. Just be careful,
OK? We want you guys around with us a bit longer!
> Thanks in advance,
> Josh
> ____
> /\/\ ((____)) /\/\/\/\ Joshua Resnick
> / | | \ \ http://www.glubco-dot-com/weaponry
> / | | \/\/\ \ __ http://ece.wpi.edu/~seraphim
> / ____| |____ \ - One Room - One Dream -
> |||||||||||||| \ - One Million Volts -
>
>
Hope this helps.
Fr. Tom McGahee