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Plasma cutting sequence

I

Ignoramus27153

Jan 1, 1970
0
Could anyone say what is the proper plasma cutting sequence? How about
this:

when trigger is pulled:

- open air valve for a certain amount of time (like 0.25 sec)

- Then start current

* When arc voltage is above certain value, turn on HF, turn off HF if
voltage drops below that

When trigger is released:

- stop power
- keep air valve open for a period of time, like 5s

does it make sense?

I am programming my microcontroller, which I mostly did for stick and
tig (proper relays are closed when they should be), but I have no idea
what to put in for plasma.

thanks

i
 
C

Carla Fong

Jan 1, 1970
0
Ignoramus27153 said:
Could anyone say what is the proper plasma cutting sequence? How about
this:

when trigger is pulled:

- open air valve for a certain amount of time (like 0.25 sec)

- Then start current

* When arc voltage is above certain value, turn on HF, turn off HF if
voltage drops below that

When trigger is released:

- stop power
- keep air valve open for a period of time, like 5s

does it make sense?

I am programming my microcontroller, which I mostly did for stick and
tig (proper relays are closed when they should be), but I have no idea
what to put in for plasma.

thanks

i


I've never measured what our machine (Hypertherm) does, but that
certainly sounds like what the sequence is.

I think the post cut air purge is 8 seconds on ours... Probably just
enough time to cool the torch components. Likely not a critical value.
 
I

Ignoramus27153

Jan 1, 1970
0
I've never measured what our machine (Hypertherm) does, but that
certainly sounds like what the sequence is.

Thank you.

It's OK, the numbers could be fudged with, I just need to get the
sequence right. I already programmed this particular sequence (with
delays and voltage thresholds as parameters)
I think the post cut air purge is 8 seconds on ours... Probably just
enough time to cool the torch components. Likely not a critical value.

Sounds good, I will change my value from 5 to 10 then.

A ignorant question, do plasma cutters need only air? (no second gas
going through another line)?

i
 
W

woodworker88

Jan 1, 1970
0
The purpose of the post cut purge is to cool the torch components
significantly. Since in a plasma torch the arc is contained within the
nozzle if you shut off the air, you will have a puddle of copper
electrode in the tip. I think mine depends on the time the torch has
been on, but I have heard it go on for as long as 30 seconds. Makes
sense, the hotter the torch, the longer it should run.

Plasma cutters need only a compressed gas to plow the hot metal out of
the cut. Obviously the most economical is clean, DRY compressed air
but professional plasma cutting often uses dry nitrogen instead (no
oxidation when cutting Aluminum or Stainless). I've never heard of
anyone plasma cutting with shielding gasses (argon,etc).
 
I

Ignoramus28229

Jan 1, 1970
0
The purpose of the post cut purge is to cool the torch components
significantly. Since in a plasma torch the arc is contained within the
nozzle if you shut off the air, you will have a puddle of copper
electrode in the tip. I think mine depends on the time the torch has
been on, but I have heard it go on for as long as 30 seconds. Makes
sense, the hotter the torch, the longer it should run.

Got it. Will increase the limit. Thanks.
Plasma cutters need only a compressed gas to plow the hot metal out of
the cut. Obviously the most economical is clean, DRY compressed air
but professional plasma cutting often uses dry nitrogen instead (no
oxidation when cutting Aluminum or Stainless). I've never heard of
anyone plasma cutting with shielding gasses (argon,etc).

Thanks, that's nice. I do have a compressed air dryer, actually.
 
P

Pete C.

Jan 1, 1970
0
Ignoramus28229 said:
Got it. Will increase the limit. Thanks.


Thanks, that's nice. I do have a compressed air dryer, actually.

Suggest you check out the plasmacutting group on Yahoo groups. It has a
couple techs from plasma cutter manufacturers as regulars that could
give you a lot of info.

Pete C.
 
I

Ignoramus28229

Jan 1, 1970
0
Suggest you check out the plasmacutting group on Yahoo groups. It has a
couple techs from plasma cutter manufacturers as regulars that could
give you a lot of info.

Thanks. I am also a member of homemade welders yahoo group, but it is
pretty quiet. I joined the plasmacutting group and already asked my
question.

Thank you!
 
S

Steve Sousa

Jan 1, 1970
0
Ignoramus27153 said:
Could anyone say what is the proper plasma cutting sequence? How about
this:

Are you using the same welding machine you talk about here for plasma
cutting?
If so, what did it take to make the change?

Thank you.

Best Regards

Steve Sousa
 
I

Ignoramus28229

Jan 1, 1970
0
Are you using the same welding machine you talk about here for
plasma cutting? If so, what did it take to make the change?

Yes, the same machine. Its transformer has six secondaries, two per
each leg of three phase. If I connect these secondaries in parallel
(two on each leg would be paralleled), delta connected, I get 66
VAC. That's for welding.

If I connect these secondaries in series (that is, two in series on
each leg), Wye connected, I get 220VAC on each leg. That's what I
would use for plasma cutting.

I do this reconnection using contactors, three of them connect for low
voltage, and two for high voltage.

This partis already working (contactors are mounted and doing all
that reconnection).

i
 
L

Lloyd E. Sponenburgh

Jan 1, 1970
0
Ignoramus28229 said:
Yes, the same machine. Its transformer has six secondaries, two per
each leg of three phase. If I connect these secondaries in parallel
(two on each leg would be paralleled), delta connected, I get 66
VAC. That's for welding.

If I connect these secondaries in series (that is, two in series on
each leg), Wye connected, I get 220VAC on each leg. That's what I
would use for plasma cutting.

I hope the insulation resistance on those secondaries is high enough for the
higher voltage series connection.
220v isn't very high, but the inter-winding insulation might not be designed
to take that under continuous duty, since the transformer was designed for
66V operation.

LLoyd
 
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Ignoramus28229

Jan 1, 1970
0
I hope the insulation resistance on those secondaries is high enough for the
higher voltage series connection.
220v isn't very high, but the inter-winding insulation might not be designed
to take that under continuous duty, since the transformer was designed for
66V operation.

Remember that voltage from one turn of a winding to the next turn is
pretty small (and does not change when I reconnect the
transformer). These windings seem to be fairly far apart.

See a picture here:

http://igor.chudov.com/projects/Welding/11-New-Rectifier/welding-xfmr.jpg


i
 
L

Lloyd E. Sponenburgh

Jan 1, 1970
0
Ignoramus28229 said:
Remember that voltage from one turn of a winding to the next turn is
pretty small (and does not change when I reconnect the
transformer). These windings seem to be fairly far apart.

I wasn't refering to the inter-turn insulation; inter-winding. If the
windings are separated from one-another and any other point that might be
called "common" by enough insulation, then it'll work. I wasn't nay-saying,
just raising an issue.

My first real job was in the test lab of Florida Transitron in DeLeon Spring
Florida. My job was to "hi-pot" all the prototypes, and to set up burn-in
rigs for temperature-rise and core-loss determinations.

LLoyd
 
W

Wayne Cook

Jan 1, 1970
0
Could anyone say what is the proper plasma cutting sequence? How about
this:

when trigger is pulled:

- open air valve for a certain amount of time (like 0.25 sec)

- Then start current

* When arc voltage is above certain value, turn on HF, turn off HF if
voltage drops below that

When trigger is released:

- stop power
- keep air valve open for a period of time, like 5s

does it make sense?

I am programming my microcontroller, which I mostly did for stick and
tig (proper relays are closed when they should be), but I have no idea
what to put in for plasma.

I know you've had pretty good luck so far on your conversions and I
commend you on what you've achieved so far. However my experience
tells me that what you're about to try is going to be fraught with
disappointments.

Plasma cutting is much more complex than most people think. But I'll
try to list the basics real quick (I really don't have time to get
into details at the moment, we are having to take turns watching my
wife at the nursing home at the moment).

To start with you have the air coming on, the high frequency engages
and initiates the pilot arc (this is something you've not taken into
account). The pilot arc is a low amperage arc from the electrode to
the nozzle but it's fed from the main cutting arc (this is where I had
trouble fitting the new torch to my old PCM-100 plasma cutter). If the
pilot arc is to hot then it'll fry the consumables, to low and it's
not long enough to initiate the main arc. The main arc is initiated
when the pilot arc crosses to the work piece. This is the main cutting
arc.

Things to consider are to much amperage or voltage, to sudden a shut
off, to sudden a ramp up, and many other little details play hell with
the consumables on a plasma cutter. This is what makes the more modern
machines so much better than the old ones. They have better control of
the whole process which is what makes the cuts cleaner, the
consumables last longer, and the overall process much nicer.

Good luck.
 
I

Ignoramus28229

Jan 1, 1970
0
I know you've had pretty good luck so far on your conversions and I
commend you on what you've achieved so far. However my experience
tells me that what you're about to try is going to be fraught with
disappointments.

Plasma cutting is much more complex than most people think. But I'll
try to list the basics real quick (I really don't have time to get
into details at the moment, we are having to take turns watching my
wife at the nursing home at the moment).

To start with you have the air coming on, the high frequency engages
and initiates the pilot arc (this is something you've not taken into
account). The pilot arc is a low amperage arc from the electrode to
the nozzle but it's fed from the main cutting arc (this is where I had
trouble fitting the new torch to my old PCM-100 plasma cutter). If the
pilot arc is to hot then it'll fry the consumables, to low and it's
not long enough to initiate the main arc. The main arc is initiated
when the pilot arc crosses to the work piece. This is the main cutting
arc.

Things to consider are to much amperage or voltage, to sudden a shut
off, to sudden a ramp up, and many other little details play hell with
the consumables on a plasma cutter. This is what makes the more modern
machines so much better than the old ones. They have better control of
the whole process which is what makes the cuts cleaner, the
consumables last longer, and the overall process much nicer.

Good luck.

Thanks Wayne. Yes, I have to consider the low amperage pilot arc
indeed. I will try to rewrite my procedure to account for it. In any
case, I think, it makes sense to get welding right (stick welding is
already working, but for TIG I would use the controller). So I have
quite a while before I can try plasma. As for modern plasma cutters,
mine is controlled by a microcontroller, so I can program it to do
anything I want (knowing what to do is a problem). I wish that I could
have a local friend with a plasma cutter to see how they work.

I will post more on this.

i
 
J

Jim Thompson

Jan 1, 1970
0
[snip]
(I really don't have time to get
into details at the moment, we are having to take turns watching my
wife at the nursing home at the moment).
[snip]

My sympathies. Just did that with a son... it's really tough.

...Jim Thompson
 
W

Wayne Cook

Jan 1, 1970
0
[snip]
(I really don't have time to get
into details at the moment, we are having to take turns watching my
wife at the nursing home at the moment).
[snip]

My sympathies. Just did that with a son... it's really tough.

Thanks.
 
M

Martin H. Eastburn

Jan 1, 1970
0
I'd consider a ramp up and hold ... then ramp down (when trigger released)...

The ramps prevent surge currents that might bite the electronics and might punch
holes through the metal...

I'd go to some of the sites - Hypertherm is a good site - see tech docs for how things work.

Martin

Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
NRA LOH & Endowment Member
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member
http://lufkinced.com/
 
K

Kevin Y

Jan 1, 1970
0
There's an guy on ebay every once awhile selling plans to convent your
welder to a plasma cutter. There like $3 plus shipping maybe there
something useful in his plans I don't know. I thought I'd throw that out
there.

His torch is made from PVC pipe which bring a smile to face every time I see
his item for sale.
 
I

Ignoramus28229

Jan 1, 1970
0
Plasma cutting is much more complex than most people think. But I'll
try to list the basics real quick (I really don't have time to get
into details at the moment, we are having to take turns watching my
wife at the nursing home at the moment).

Oh, and I must say that I am sorry about that.

i
 
K

kidkv

Jan 1, 1970
0
Ignoramus28229 said:
Oh, and I must say that I am sorry about that.

i
PLASMA GAS SELECTION CHART
Gas Recommended For Advantages Disadvantages
Air

* Carbon Steel
* Stainless Steel



* Clean Fast Cut on C.S.
* Affordable
* Convenient



* Short Electrode Life
* Nitriding on Cut Surface
* Oxidation on S.S./ AL

Nitrogen

* Stainless Steel
* Aluminum
* Carbon Steel



* Excellent Cut on S.S./AL
* Excellent Electrode Life
* Affordable



* Nitriding on Cut Surface

Argon/Hydrogen

* Stainless Steel
* Aluminum



* Excellent Cut Quality and Speed on Thicker Material (> 1/2 in.)
* Less Smoke/Fumes



* Expensive
* Not for C.S.

Oxygen

* Carbon Steel



* Clean Cut
* No Surface Nitriding
* Fast on C.S.



* Short Electrode Life
* Oxidation on S.S./AL
There are various types of torch cooling systems. Low-amp torches (150
amps or less) can be cooled by channeling a secondary gas through the
torch. Higher-powered torches use water and require a cooling system
with a reservoir, pump and heat exchanger. It's very important to use
de-ionized water for cooling, since the coolant (water) may contact
both negative and positive potentials inside the torch.
Theory: Sequence of Operation

When an operator gives a start cut signal, the system energizes and a
prepurge of gas lasting a few seconds will flow through the torch. This
ensures that proper gas flow is available before an arc is struck. The
initial arc, which ionizes a portion of the plasma gas, is generated by
a high-voltage spark between the nozzle and the electrode. Current
flows through the ionized gas (plasma) to the nozzle.

The gas flow pushes the arc out of the orifice where it reattaches to
the outside of the nozzle, forming a J-shaped pilot arc. In some
systems, the pilot arc is controlled by a timing circuit. Other systems
have an arc transfer sensor to detect the current change (when the
cutting arc takes over) and switch off the pilot circuit. Some systems
also have an automatic restart pilot, which is useful when cutting
grating or expanded metal. With automatic restart, the torch can cycle
back and forth between "pilot" and "cut" modes as long as the start
signal is present.

The pilot arc forms a "pathway" to the metal. If the torch is close
enough to the workpiece, then the arc will transfer from the electrode,
through the nozzle, to the metal. Once this transferred arc is
established, it will continue as long as there is metal to transfer to.
The constricted plasma jet concentrates the energy of the arc on a
small area of the workpiece, heating it to melting temperature and
blasting the molten material out of the cut.

When a stop signal is given (or the start-cut signal is shut off), the
cutting arc stops and the gas continues to flow for a few seconds to
cool the torch and torch parts. It also shields the electrode,
preventing outside air from reacting with the electrode element as it
cools.
hay i hope this helps out some:))
 
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