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Re: Power Arcing
>>From bert.hickman-at-aquila-dot-comSun Sep 29 21:56:11 1996
>Date: Sun, 29 Sep 1996 08:11:14 -0700
>From: Bert Hickman <bert.hickman-at-aquila-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Power Arcing
>All,
>I was running my coil at somewhat higher power today (off three 15 KV 60
>MA neons instead of two), when suddenly my vacuum gap motor stopped
>running. After tearing it apart, I discovered that a field winding wire
>had arced over to the 3rd-wire grounded motor frame, melting and
>openning up the #22 AWG wire. From the looks of it, a high voltage arc
>had initially bridged the gap, and was followed by a heavier flow of
>120V power which blew open the wire. An easy repair was made via solder
>and shrink tubing, and the unit put back into service. There's no EMI
>filtering of any kind on the lines going to the motor, but this unit had
>functioned just fine for 5 months.
>The coil was again powered up, and the same thing happenned again! The
>cord to the unit is the standard 3-wire one which came with the
>Shop-Vac, and is about 6 feet long. It looks like high voltage is being
>picked up or induced into the cord somehow. Before powering up (or
>blowing up) a different motor, I need to determine the source and
>correct it. My first guess is that its common-mode RF pickup from the
>near-field radiation coming from the coil. I am planning a couple of
>potential changes, such as running power through shielded power cords or
>flexible conduit, and adding EMI filters.
>I'm also concerned about the RF energy coupling into the aluminum
>siding, window frames, and guttering, and plan to make sure that these
>are adequately grounded to AC ground. It looks like any length of wire
>or cable in the room with the coil will also pick up a goodly amount of
>induced RF from the coil AND the sparks.
>What are some of the techniques used by the more experienced coilers to
>control these?? I'm concerned about boosting power (especially with a
>pig) until I get this situation under control! Thanks in advance!
>Safe coilin' to ya!
>-- Bert --
Bert,
Been there, done that, but nobody had T-shirts available. DO NOT
connect any metallic objects in your lab that could get hit directly
to AC GROUND!!! You'll be inducing surges into your house wiring and
possibly blowing up appliances, wall oulets, overhead fans etc,
anywhere, unpredictably, and this will worsen as you raise your TC
power levels.
Make a dedicated earth ground with ground rod(s) for your strike
screens, aluminum windows etc. In my firing room (which is now temporaily
shut down because I'm in the process of moving), I had suspended 2 inch
mesh chicken wire, 2 inches out away from the drywall and open stud
walls containing ac wiring, outlets, etc. Below the ceiling light
fixtures I ran lengths of #18 insulated PVC covered 600 volt wire
(the insulation doesn't matter, really!), and strung these 1 foot
below all of my lights in straight rows. All these wires and the #4
ga. bare copper wire that I ran along the bottom edges of all the chicken
wire walls were grounded together at a single point where my #0000
welding cable ran through the wall to a single 10 foot galvanized
steel ground rod on the outside of the wall. From this common
ground point I had a 15 foot length of #0000 also running across the
floor to the base of my MTC system. All AC cabling from my control
console (rotary motor, gap blower vacuum cleaner motor, heavy AC
lines to power transformer) run to the base of the TC tower through a
multi conductor shielded cable. This shield is grounded at the base
of the MTC coil tower to the system common ground point where the
heavy welding cable supplying ground is connected. The shield of the
control power cable is left floating at the control console end. I
have an RFI filter installed in the power cable as it reaches the TC
tower base just befor it goes through my 10mH control reactor and the
dry pig transformer. This emi filter, the core of the reactor, the
core of the pig, the frame of the rotary break and the base
connection of the secondary ALL tie to this common point. There is
no connection to AC ground to be found in the base of MTC. Back at
the control console the only thing tied to AC ground is this cabinet
and the case of the variac. In the base of MTC there are a pair of two conductor
AC power wires where they come out of the large multi-power/control
cable, these power the rotary break motor, and the quench/cooling
blower. I slid 3/4 inch flattened copper braid (after opening it up)
over these lines and covered them with heatshrink. They connect at
their junction with my main multi conductor incoming power/control
cable to the common ground point. At the motors themselves the braid
is stopped about 1.25 inches shy of the motor frames and left
unconnected. I have no rfi filter in either rotary motor or blower
motor line, although I think I will add them for good measure back at
the control cabinet in the near future.
With this setup I could hammer the strike control screens (walls) and wires
all day long at 7 kVA input power and had no further trouble with
surges being introduced into the house wiring. Those strike wires
suspended under my room ceiling lighting appeared to be 100%
effective in catching all streamers that otherwise threatened to hit
the lamps and sockets and thusly get into the house wiring.
I was pretty lucky with just a single 10 foot ground rod at these
power levels, but I think my attention to ground routing within the
lab and around the coil had a lot to do with my success. In the new
lab I plan to do the same thing, only better, and with more rods in
the ground outside.
This certainly works for me.
Regards,
rwstephens