DillonCo said:
A little background for those who don't know: A plasma torch works (as I
understand it) by creating a high voltage arc between two electrodes to
ionize a gas. Once the gas is ionized it becomes more conductive, thus
allowing a low voltage (~105V) to pass through. A high current (~50A) is
used at that voltage to produce the heat needed.
My question is, how is it possible for the power supply to switch between
the two voltages? If the low voltage circuit was constantly connected to
the electrodes, what would keep the high voltage from running through it and
not the air? If the low voltage circuit isn't always conncted, how can
currents like that be swiched, and how can the supply tell when the gas is
properly ionized?
Any input would be appriciated.
Thank you.
This is a good question. One way to due this is to superimpose a (multiple)
high voltage pulse(s) in series with the low voltage supply. Arc power
supplies aren't really voltage sources, so that isn't necessarily the best
term to use, but anyway...
The high voltage pulse(s) generally doesn't need to be very long in
duration. A specially designed step up pulse transformer can be used to
generate this large voltage. The high voltage secondary of this transformer
is wired in series with the output of the low voltage/high current supply.
Since the pulse is of short duration the number of turns required on the
transformer secondary can be quite few for even a relatively small core
cross sectional area. As a result one can typically use quite large wire
for the secondary and the DC resistance is relatively small. Even so, this
method will introduce some finite and probably non-negligible resistance and
inductance into the circuit. This is unfortunate, but this impedance can be
switched out of the circuit by a set of suitable mechanical contacts (placed
in parallel with the pulse transformer secondary) such as a relatively high
voltage relay. This type of method is particularly convenient for
relatively low arc currents.
Alternatively since the output impedance of the high voltage supply is
normally quite high, you can in some cases place the two supplies in
parallel but with some high voltage switching device like a relay in series
with the low voltage supply. For arc starting the relay contacts would be
open and thus no current from the high voltage supply should flow through
the low voltage supply. Once the arc has become established by the high
voltage the relay contacts close thus effectively shorting out the high
voltage through the low voltage supply.
Other possible methods include ionizing the arc medium using RF radiation,
nuclear radiation, using separate high voltage electrodes placed in close
proximity to the low voltage main arc electrodes, mechanically short
circuiting the low voltage output electrodes together and then physically
taking them apart (possibly with extra inductance in series to produce extra
voltage at contact opening), or if the arc is high frequency AC and the
current isn't very high something perhaps like shown as the arc ignitor of
this schematic:
http://members.tripod.com/~wvsp/light1.html
The supply can tell when the main arc has been initiated since current will
be flowing through the low voltage/high current arc supply. This can be
sensed with a resistor or with other methods.