Re: Rotary gap grounding

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Ed,

"I" think about what might happen.  Think through things like:

1.  Motor fails and puts hot AC line voltage on the motor case.

2.  Motor case is directly hit by an arc.

3.  Rotor arcs over or fails and puts primary voltage (both AC and RF) on 
the motor case.

For case 1, any old AC line ground will work to fault the AC hot line to 
ground.

For case 2,  We have high voltage (350kV) but low current (maybe an amp) at 
say 250kHz.  If the case is floating, it "will" arc to the AC wiring.  The 
wiring has pretty low impedance and even an amp at 250kHz should not do 
great harm.  However, one amp at 250kHz with unlimited voltage behind it is 
just not a normal AC ground wiring thing.  A nice RF ground would stop it 
right there with no problem.  So a grounded case to the RF ground is called 
for.

For case 3,  20kV 250kHz at like 1000 amps!!  Ok, you DON'T want that on 
the AC line or the wennie AC grounding!  1000 amps with 20kV behind it IS 
going to blow something up.  Now we're talking good heavy RF grounding for 
the motor case.

"I" ground all the stuff near the coil to good heavy RF grounds.  I don't 
have AC grounding near the coil since it is no match for a good RF 
ground.  I only carry neutral and hot line voltage to the coil.  I do 
ground the variac and control side equipment to the AC line ground and 
often to another RF ground since it sometimes "tingles" when I touch 
it.  The AC line ground has pretty high impedance to RF frequencies and is 
certainly not meant to discharge a primary cap in 1uS.  But a nice heavy RF 
ground can!

I also have MOVs (big ones) all over the control side AC wiring to clamp 
high voltages.  They prevent any AC wiring form going to really high 
voltages no mater what (even if they blow the crap out of something, they 
still lock the voltages down).

So think about what could go wrong, and design for it ;-))

Cheers,

         Terry





At 11:06 PM 12/1/2002 -0500, you wrote:

>We have discussed this in the past, but I do not recall what the popular
>opinion was.  For those with rotary gaps, do you ground the motor case?  If
>so, to what ground?  The items that are likely to get hit with a strike from
>the toroid, we usually tie to the main RF ground.  This doesn't seem like a
>real good choice for the motor case ground since the RF ground is often at
>least several hundred volts above actual ground.  I base this idea on the
>observation that you can draw sparks off the main RF ground wire when the
>coil is running.  I doubt the motor case / field winding set up was designed
>with more than a few hundred volts insulation in mind.  My existing gap motor
>
>is mounted in a wooden box and is under the coil and out of the way.  That
>motor case is floating.  The new rotary I am working on will be in a more
>likely area to get hit with strikes.  I planned to also build a wooden box to
>
>enclose it and will add a metal strike shield on top if necessary - but am
>still undecided as to what to do with the motor case.
>
>Ed Sonderman