I used to wonder if the attraction to the earth's magnetic field was a
plausible explanation to the jumping cables. I came to the belief that it is
not, as the earth's field is not that strong at any given location. For
example, I have a mag mount that is so strong that I need to use two hands
and considerable force to apply it to the roof of my van, lest I cause body
damage. The magnet is not noticeably attracted to earth. A magnetic compass
needle is very light yet it's reaction to the earth's field is not
particularly violent. In fact, it will happily point to the barrel of my
rifle if I'm not careful.
As far as shielding, enclosures, etc. I see those as possibly enhancing
magnetic / mechanical forces caused by a high current pulse within the
shielded area. Aluminum makes a great magnet.
I used to do a classroom / lab demonstration where I would remove the rotor
from a 2hp 3 phase motor and place an aluminum can in the stator. Most
students (all adult engineering, electricians, test specialists, etc) would
guess that the stator's field would not have any effect on the aluminum can,
as it was non ferrous. I would then energize the stator and the can would
spin so fast that it would shoot out of the end of the stator. People were
then willing to understand that the aluminum bars cast within the iron of
the rotor, were the cause of magnetic action (through induction) and torque.
I say all that to get back to inductance and transformer theory: When an AC
current flows in a conductor (primary) it causes a changing magnetic field
which induces a potential in anything conductive (secondary), that is within
the effects of that field.
If there is an electrical circuit, current will then flow in the secondary.
In the case of the aluminum can, these were circulating currents. Now, the
secondary current is nearly 180 degrees out of phase with the current in the
primary, thus the magnetic field produced by the secondary current is also
out of phase with the primary's field. This leads to maximum mechanical
forces.
So, could a large current flow between two elements within a vacuum tube,
cause mechanical distortion of other fragile elements within that tube? I
believe so.
73, Jeff - K1LE - CT ><>
----- Original Message -----
From: "Conrad G0RUZ" <conrad@g0ruz.net>
To: " AMPS" <amps@contesting.com>
Sent: Friday, February 07, 2003 6:17 AM
Subject: RE: [Amps] Re: Voodoo "EMF' forces from grid current pulses
> This can be easily explained, the welding cables are subject to the earths
> magnetic field and any other localised magnetic fields, that's why they
jump
> about. The question is, would this happen in a shielded enclosure (amp
> chassis) and with the magnitude of current flowing in the grid during a
> fault condition? I am still sceptical. One thing's for sure, something
> happens, but what?
>
> 73
>
> Conrad G0RUZ
>
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
> Behalf Of Jeffrey Madore
> Sent: 07 February 2003 04:24
> To: 2; MorgusMagnificen@aol.com; AMPS
> Subject: Re: [Amps] Re: Voodoo "EMF' forces from grid current pulses
>
>
>
> >
> > >I strongly challenge this assertion. I know of no mechanism within the
> realm
> > >of classical EM field theory to account for this. I am assuming you are
> > >referring to the normal force that would accompany two current-carrying
> > >conductors which are in parallel proximity to each other.
> > >
> > Have you ever been to a welding shop and observed what happens to
> > arc-welder cables when an arc is struck? I have and I saw the cables
> > jump about on the floor.
>
> Yea verily, Rich!
>
> In my past life I did high current trip testing on 480 volt circuit
> breakers. 50,000 amps was not unusual but even much lower currents would
> make welding cables jump off of the floor.
>
> I remember experimenting with a fairly large 480/120 3 phase lighting
> transformer, with a dead short on the secondary and 208 volts applied to
the
> primary via a pretty large motor control contactor and momentary push
> button. It was a "hit the button while peaking around the corner" type of
> experiment. We were able to draw about 2000 amps momentarily on the
> secondary which really made a set of welding cables jump.
>
> In my experience, a single conductor doesn't need any other conductor near
> it to display mechanical force when high current is applied. The self
> inductance of the cable and resulting cemf produce sufficient magnetic
> reaction to make the cable jump if it is not secured. Very interesting to
> look at the Z of a 4/0 welding cable...
>
> I've often found that if our engineering theories couldn't explain what
> appeared to take place in reality, then we needed to look deeper into the
> matter.
>
> 73, Jeff - K1LE - CT
>
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