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Re: NST phasing questions
Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
At 02:37 PM 5/26/2001 -0400, you wrote:
>I'd like to see this one answered definitively.
The Tesla list is not too good at getting things "answered definitively"
:-)) but here is MHO...
>I'm using line filters,
>but not backwards, as a recent discussion here suggested that the
>purpose of a line filter *IS* to protect the line, and not the
>equipment, though to me that doesn't seem logical (one faulty device
>pollutes the line, and everything else is unprotected). Recent
>discussions also said most EMI filters are symmetrical so that it
>wouldn't matter anyway, but mine (Sprague) are not - and not being such
>a wiz with electronics, I can't figure out from the schematic which side
>they are protecting - line or load, or whether the assymetry even
I have always used line filters as they are labeled. Line to line and load
to coil. There are all kinds of subtle things about line filters and how
they work and are designed but basically just having them there solves 90%
of the problems. The common generic "go anywhere" kind work well in any case.
>The more interesting question to me is what to ground where. Take a
>look at everything that needs a ground connection:
>1) Secondary base.
>2) Strike rail, if present.
>3) NST case/secondary midpoint.
>4) NST protection filter and/or safety gap.
>5) Line filter.
This is how "I" ground things. I use two ground systems. The AC line
ground with protects the variac and all the control equipment that "I"
touch. I use line filters on it's output toward the coil. I also have big
MOVs from that ground to each line to protect from spikes both at the
variac input and output. I carry only two wires (neutral and hot) to the
coil through a long cable (basically a chopped extension cord). There,
everything goes through line filters and MOVs again. Everything on that
side (where the coil is) is grounded to the RF ground. Motor case, line
filters, NST case, secondary base, safety gaps, etc. all go to the RF
ground. In the case of the NST, line filter, and motor that have grounded
cases, I use MOVs to insure the cases and the AC input wires are voltage
clamped so internal arcing will not occur if something goes bad (in an
rotary old gap I used to ocasionally arc the primary circuit to the motor
shaft but it never hurt anything other than fried bearings). I use a lot
of MOVs to clamp the voltages on things that could be damaged. Hot to
neutral, hot to ground, and neutral to ground.
>One web site called it a "deadly" mistake to ground the NST case to the
>RF ground. Right now, I have 1 & 2 above hooked to RF ground, and 2, 3
>& 4 hooked to a seperate spike in the ground (2&3 to one spike, 4 to
>another) - just a really big nail (10") in damp earth. Not that I
>recommend that - I'd like to hear what the experts recommend.
I would be a bit careful of using too many ground since they can get
voltages across them. But as long as they are short and to good rods
(nails maybe be a bit light but wet ground helps a lot) not too much can go
wrong. The most exciting situation comes when streamers hit the motor, AC
input wiring, and primary. I have never blown anything up from streamer
hits. I did a lot of fault modeling in that area. Basically, if you
ground everything that can be grounded to a good RF ground regardless of
where it is, all will be fine. MMC caps also seem to withstand streamer
hits fairly well but grounding really does not apply there. I have never
used strike rails myself but ground them where the secondary is grounded.
Hope this is some help.