Ed Huntress wrote:

"William J. Beaty" wrote in message
om...

"Tim Williams" wrote in message

...

"William J. Beaty" wrote in message
e.com...

I wonder where this 300V (or 1350V from Jim Lux' page) actually
comes from?

Nowhere. Tell me, how is stick welding performed?

Um... You don't know? And you just IGNORE the breakdown
equation without comment? OK, I'd like to hear your reasoning
for why that "Paschen rule" isn't important. We can send your
discovery to Jim Lux and he can add it to the High Voltage Handbook.




The rules and handbooks you're talking about are things with which I'm not
familiar, but it sounds like this is being made more complicated than it is.

The dielectric strength of air is around 3 x 10^6 V/m. That means that you
can create a spark with something like 90V at a gap of 0.0012 inches.

That's within the gap range of a typical EDM. Gaps run from perhaps 0.0002
in. for fine-finishing to maybe 0.005 in. or a little more for roughing.

It doesn't take a very high voltage to initiate a spark if you have a
sensitive servo mechanism to maintain a close gap.

Ed Huntress



Bill Beaty is correct. The minimum breakdown voltage for air is at STP
is around 300 volts. Dielectric breakdown of air is a complex process.
Free electrons in the gap must be sufficiently accelerated by the
electrical field so that they can create additional electrons when they
collide with neutral air molecules. Furthermore, the rate that new free
electrons are created must be greater than the rate that existing free
electrons are being lost through recombination - the average lifetime of
a free electron is only about 11 nanoseconds in air at STP. If the rate
of free electron creation exceeds the rate of loss, then "avalanche
breakdown" of the air occurs and a spark jumps the gap.

If you are to the right of the minimum point on the Paschen Curve,
decreasing the gap size will continue to decrease the breakdown voltage
in a fairly linear manner since there is still sufficient distance for
electric field to accelerate electrons in the gap sufficiently to
trigger avalanche breakdown. However, once you reach the Paschen minimum
(about 320 volts for a 1 mil air gap for air at STP), any further
decreases in gap distance will require a HIGHER electric field to break
down the air. Because the gap is so short, a higher E-field is needed so
that electrons can reach the velocity needed to trigger avalanche
breakdown before disappearing into the positive electrode. The commonly
accepted Paschen minimum sparkover voltage for air is 320 volts at a
distance of about 1 mil. It can be more or less for other gases.

However, much of the above is moot since most EDM'ing is done under a
dielectric fluid using actual mechanical contact and high pulse currents
to "blast" away the small areas that make contact as the working
tool/wire is slowly advanced into the work piece.

-- Bert --
--
--------------------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
Lichtenberg Figures (electrical discharges in acrylic), & Scarce OOP
Technical Books. Stoneridge Engineering -- http://www.teslamania.com
--------------------------------------------------------------------