Maker Pro
Maker Pro

Plasma Torch Power Supply

D

DillonCo

Jan 1, 1970
0
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.
 
A

Aaron Pasteris

Jan 1, 1970
0
The ignition arc is created using a tranformer in series with the "low"
voltage supply.
The transformer is driven at a high frequency and high voltage. The
combination of high frequency and voltage allow it to iuonize the air gap
and start the arc.

The 'low" voltage power supply is bypassed with a filter capacitor so a high
fequency signal does not pass through the supply.

Some units are 'lift arc' starting where the torch is placed in contact with
the work then withdrawn to start the arc. This type requires no high voltage
to start.

Aaron
 
F

Fritz Schlunder

Jan 1, 1970
0
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.
 
F

Fritz Schlunder

Jan 1, 1970
0
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


Oh yeah. I forgot to mention another important method of generating the
high voltage needed to initiate electric arcs. Resonance between an
inductor and capacitor can be used to generate very large voltages quite
easily. Take a look at figure 7 of this datasheet:

http://www.semiconductors.philips.com/acrobat/datasheets/UBA2021_3.pdf

Such an arrangment is quite novel in its simplicity and functionality since
it can also be used to provide the arc ballasting function required for
stable arc operation as well as the very high voltages needed for arc
initiation.

The datasheet for that Philips device is quite skimpy but this application
note gives a bit more information:

http://www.semiconductors.philips.com/acrobat/applicationnotes/AN98099.pdf
 
J

John Crighton

Jan 1, 1970
0
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.

Hello Dillon,
there is an explanation available here
http://www.hypertherm.com/technology/long_life.htm
http://www.hypertherm.com/technology/plasma_history.htm
Regards,
John Crighton
Sydney
 
D

DillonCo

Jan 1, 1970
0
Aaron Pasteris said:
The ignition arc is created using a tranformer in series with the "low"
voltage supply.
The transformer is driven at a high frequency and high voltage. The
combination of high frequency and voltage allow it to iuonize the air gap
and start the arc.

The 'low" voltage power supply is bypassed with a filter capacitor so a high
fequency signal does not pass through the supply.

I'm having trouble imagining how the transformer in used in series. My
impression is this (ascii art):

G__O___mmmmmm____
------ |_____
______________wwwwww____| |
+ | *
R = *
-________|____________________|
|
G

G is Ground
* the arc gap
= is a capacitor
O is an oscilator (switches between connected and not conected)


Is this right?
 
J

John Crighton

Jan 1, 1970
0
I'm having trouble imagining how the transformer in used in series. My
impression is this (ascii art):

G__O___mmmmmm____
------ |_____
______________wwwwww____| |
+ | *
R = *
-________|____________________|
|
G

G is Ground
* the arc gap
= is a capacitor
O is an oscilator (switches between connected and not conected)


Is this right?

Hello Dillon,
you are getting close.

If you want to find out how an arc starter for tig welding
works, have a look here.
http://www.geocities.com/arcstarter/welder/arcstarter/hf.html

Look here to see the manual of my old Hi-Freq arc starter
http://content.lincolnelectric.com/pdfs/products/navigator/OBSO_IMS/IM238B.pdf
the circuit is on page 8
You can see some pictures of the innards
of this old arc starter here
http://users.tpg.com.au/john_c/

Here are some more pictures and circuit of an old arc starter
http://www.geocities.com/ftpspace1/welding/millerHF15/hf15.htm

These are just plain old arc starters for tig welders.
I have not seen a detailed circuit diagram for the
arc starter in a modern plasma torch but those diagrams
above may give you a few clues.

Regards,
John Crighton
Sydney
 
D

DillonCo

Jan 1, 1970
0
These are just plain old arc starters for tig welders.
I have not seen a detailed circuit diagram for the
arc starter in a modern plasma torch but those diagrams
above may give you a few clues.

Wow, those are great! Just two more questions:
1) TIG welders draw consederably less current than plasma torches, right?
2) What is the purpose of the spark gaps? There were two on every drawing,
and it seems like they could have been combined into one. Why do they keep
them seperate?
 
J

John Crighton

Jan 1, 1970
0
Wow, those are great! Just two more questions:
1) TIG welders draw consederably less current than plasma torches, right?

We may be at cross purposes here, Dillon.
The plasma torch is used for cutting sheet metal and
thick plate steel. It does a better job than an oxy torch on
thin material.
http://www.hypertherm.com/technology/long_life.htm

The power supply for a normal tig welder is not suitable for a
plasma cutting torch. They are different. The plasma cutting torch
power supply operates at a higher voltage when it is cutting
compared to a tig welder power supply when it is welding.
You can not really compare the two supplies they are as different
as apples and oranges.
Have a look here and compare the specifications of the supplies.
http://www.usaweld.com/product_page/tig_welder/tig201.html
http://www.usaweld.com/product_page/plasma_cutters/300.html
2) What is the purpose of the spark gaps? There were two on every drawing,
and it seems like they could have been combined into one. Why do they keep
them seperate?

The old arc starters that I pointed out to you, work similarly
to a Tesla Coil and old fashioned spark transmitter.
http://www.vistech.net/users/w1fji/spark.html

Two gaps work better than one gap. Tesla coil enthusiast who
want the most from their projects sometimes use several gaps.
For a little arc such as required by a tig welders arc starter, one
gap can be used. These gaps are expensive to buy from a
welding shop as a spare part. (hundred dollars Aust)
Two gaps are better as I said. Gaps are a whole science on
their own among the tesla enthusiasts.
If you are interested in learning about home made spark gaps
and Tesla coils here is a good place to start.
http://www.pupman.com
There is a search box and FAQ.

To save you time searching the pupman site
here is a site that explains the theory.
http://www.apc.net/bturner/coils.htm

The welding arc starter just uses a 1:1 or 1:2 air cored
transformer not 1: several hunded turns as in the tesla
coil. The arc starters secondary winding is very heavy
to take the welding current. Just enough RF high voltage
to jump 1/4 inch in air is just fine. With argon flowing
in the tig torch the arc will jump further, about an inch
from the tungsten welding electrode to the work piece.

If you are interested in building your own welding gear,
have a little read of the archived messages from other
hobbyists.
http://www.diywelder.com

Regards,
John Crighton
Sydney
 
D

DillonCo

Jan 1, 1970
0
I'm actually building neither a TIG welder nor plasma torch, but something
that could benefit from their power source ideas (high voltage, then high
current). However, this information gave me some ideas on how to get this
done. Thanks for all your help!
 
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