As I alluded to, I think we can leave the terrestrial magnetic field out of
the equation. A length of power cable in free air exhibits the property of
inductive reactance when there is AC current flow. This XL results when the
cables own magnetic field expands and collapses about the conductor itself.
The result is a CEMF which produces current that is out of phase with the
source current and thus bucks or impedes the flow of current from the
source. Hence, we have these two current components which are out of phase,
producing magnetic reaction and mechanical force.
XL on a power cable becomes a real issue as current and cable length
increase. It's optimal to have a low Z power source for the sake of voltage
regulation, but also for fast tripping of protective devices. The effect of
XL is greatly reduced by running the return conductor and ground conductor
in close proximity to each other.
I'm not saying to do this, but if you were to take a loop of #12 wire say
50 to 75 feet long, connect the ends to a 20 amp breaker and the ground bus
in your service panel. Then insert another 20 amp breaker along the wire for
control. A clampon ammeter set to peak lock is also helpful. When you close
the remote breaker you should observe movement in the #12 wire. Current will
peak in the range of 300 to 400 amps and one of the 20 amp breakers should
clear the fault in a little over a second.
Oh yes, safety glasses, gloves, all cotton clothing, and a C02 fire
extinguisher are all important when working with high current sources.
Jeff - K1LE - CT ><>
----- Original Message -----
From: <MorgusMagnificen@aol.com>
To: <amps@contesting.com>
Sent: Friday, February 07, 2003 7:19 PM
Subject: [Amps] Voodoo Magnetic Fields
> Let's calculate (not speculate) on the (Lorenz)force acting on a welding
> cable laying at ground 0, and one-meter long, carrying a current of
1000A.,
> due to its interaction with the geomagnetic field, which we will assume to
be
> vertical and of magnitude B=1Gauss (good estimates - just ask your
friendly
> geophysicist). Of course, we all recall that the formula for the force on
a
> straight conductor in a B-field is:
>
> F=BxLxI (MKS units)
>
> Using MKS units: F=.00001 x 1 x 1000=.01N.=.0022Lbs=.032Oz.
> Should really shake up that 4/0! Remember - this is the total force on
> 1METER of cable, with 1000AMPS flowing.
>
> Eric von Valtier K8LV
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