Re: Cap Safety Gap (fwd)
---------- Forwarded message ----------
Date: Wed, 17 Dec 1997 13:49:00 EST
From: Esondrmn <Esondrmn-at-aol-dot-com>
Subject: Re: Cap Safety Gap
to: Ed & List
It doesn't require a very high value resistance to spoil the Q enough to
prevent HF ringing. We prefer a value of 250 Ohms. A good plasma
technologist whom I work with on a regular basis at UW-Madison recommends
any value between 20 and 200 Ohms. When using NST's be sure to have this
value in series with each bushing. Also good idea to run it through about
60 turns air core wound on a 1 1/2" dia PVC tube -- this helps clip and
block the VHF signals that can really damage a xmfr. We don't use a cap
safety gap at all but our caps are very conservatively rated at 50 kV pulse
duty and don't get near breakdown until around 110 kV. We have never, ever
lost a cap in our commercial units that have been in service (in some
cases) for over 20 years. It is very important to control the total
discharge gap opening -- usually less than 0.200 with NST's and less than
0.500 with pole xmfr operated devices in the 12.4-14.4 kV range.
Hope this information is of assistance.
So, I would guess, my current 600 ohms in each H.V. feed leg should be an
adequate amount of resistance - yes?
The total gap distance in my rotary set up is probably .020" for each fixed
electrode plus three static gaps of .028" each for a total of about .125". On
an asynch rotary, this probably doesn't mean much without taking into account
the operating bps or dwell time - correct? Are you saying then, that there is
a minimum speed setting for the rotary? I have been operating in the 200 to
400 bps range. Low speeds don't seem to cause any problem. At higher speeds,
I get much better coil performance - for any given set of input parameters.
If the speed gets too high, the cap safety gap fires, just as if I get too
high on the primary transformer input current. I guess either way is just
another method for inputting higher average power levels into the primary of