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Homemade Inverter Plasma Cutter

J

Jeremy Samuels

Jan 1, 1970
0
After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?
 
L

Luhan Monat

Jan 1, 1970
0
Jeremy said:
After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?

This does *not* look like a good 'beginners' project to me...
 
W

Winfield Hill

Jan 1, 1970
0
Luhan Monat wrote...
This does *not* look like a good 'beginners' project to me...

Another non-PFC RFI-blasting flame-throwing 5kW killer power supply...
 
J

John Crighton

Jan 1, 1970
0
After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?

Hello Jeremy,
as a fellow hobbyist, I do not want to dampen
your enthusiasm but there are no good books
that tell you how to build this sort of gear.
You will just have to keep your eyes and ears
open picking up snippets of information here
and there.

You can find out how other people have fared
by joining this Do It Yourself Welder group.
http://forums.diywelder.com/forum/index.php
Only a couple of members have had some
success but costly in burnt up expensive
semiconductor parts and time.

If you are really keen, see if you can buy
damaged goods at a rock bottom price
and use your skills repairing them. Much
cheaper that way. Somewhere something
must have fallen off a fork lift and got
bashed. Just keep asking and looking.

Since you say it is an educational project
you might be able to make improvements
that are two luxurious to be fitted to a
normal commercial item. For example
I mistakenly bought a cheap used SIP brand
MIG welder. It was rubbish in comparison
to regular welders but I have modified it
to work only on sheet metal car body
thickness material and it does a nice job.
It has an extra inductor, bank of large electrolytic
caps, extra switch to allow shielding gas
to flow before welding commenses. A better
motor drive with control box near operator
so I can adjust the wire speed while welding.

I learned a lot from that project.
Be warned, this hobby is addictive, one
welder or one cutter is never enough. :)

Have Fun,
John Crighton
Sydney
 
IF i not mistake the transforma is a " step-down transforma ". but are
you sure is 150V ? and at The torch is a AC or DC voltage ?. what i
know about the transforma is ---
primary power input WATT ( I X V ) = secondary power output WATT (
I X V )
 
C

colin

Jan 1, 1970
0
Jeremy Samuels said:
After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?

I worked on one of these, long time ago tho, they were amasing - fun to play
about with too, signed my name in thick sheet steel with the prototype lol.

It was very large - with power factor corection, had several stages of
conversion and at the time when mosfets/igbts up to the job wernt that easy
to get hold of.

I would sugest looking at phase controled, bridge driving smps ICs, rather
than trying to control with a pic. every time your software failed or you
pic crashed you would end up with lots of burnt bits of expensive plastic.
they will have curent control wich would mean you would not have to rely so
much on the design of the transformer to limit the current.

I sugest you get your soldering up to speed before you attempt something so
unforgiving.

Colin =^.^=
 
J

Jeremy Samuels

Jan 1, 1970
0
colin said:
I worked on one of these, long time ago tho, they were amasing - fun to play
about with too, signed my name in thick sheet steel with the prototype lol.

It was very large - with power factor corection, had several stages of
conversion and at the time when mosfets/igbts up to the job wernt that easy
to get hold of.

I would sugest looking at phase controled, bridge driving smps ICs, rather
than trying to control with a pic. every time your software failed or you
pic crashed you would end up with lots of burnt bits of expensive plastic.
they will have curent control wich would mean you would not have to rely so
much on the design of the transformer to limit the current.

Why not a PWM IC with overcurrent protection, controlled by the pic?
Use a DAC from the pic into the PWM. Control the solenoid with
pre/post flow, temperature fuse, PWM, LED's, and possibly current
feedback using the pic.

I've found a "hyperfast" rectifier used in a Lincoln plasma cutter
inverter. It's part RHRG30120, 30 amps 1200 volts, 65 ns recovery
time. Pretty cheap too, $3.
I sugest you get your soldering up to speed before you attempt something so
unforgiving.

I can solder fairly well, not surface mount but I've made a few perf
board circuits (ignition coil 555 drivers, etc.)
 
A

Andy

Jan 1, 1970
0
Jeremy said:
After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?

Andy writes:

Jeremy, I don't know squat about plasma cutting, but I would suggest
that you make you project in at least two steps.

The first would be to lashup the BIG POWER stuff, to see if it will
cut what needs to be cut when at maximum uncontrolled power.

The second might be to add the control components to accomplish the
level of control you want....

If you can't get the FIRST STEP working, there is no hope for the
second, and it will save you most of your development time....

If the FIRST STEP blasts a hole thru to another dimension, or creates
a wormhole, you are sure your idea can be made to work. Then it is
simply a matter a few six-packs and some well-thought-out circuits.....

Good luck, if you get it going, please tell us about it....

Andy
 
C

colin

Jan 1, 1970
0
Jeremy Samuels said:
Why not a PWM IC with overcurrent protection, controlled by the pic?
Use a DAC from the pic into the PWM. Control the solenoid with
pre/post flow, temperature fuse, PWM, LED's, and possibly current
feedback using the pic.

I've found a "hyperfast" rectifier used in a Lincoln plasma cutter
inverter. It's part RHRG30120, 30 amps 1200 volts, 65 ns recovery
time. Pretty cheap too, $3.

The phase shifting controler is efectivly PWM control, but instead of
varying the mark space ratio of each half cycle, it drives each side of the
bridge with 50% squarewaves and the PWM is efected by varying the phase, the
actual drive to the transformer sees a PWM squarewave, the advantage is that
simple drive transformers can be used to drive the upper devices wich is not
feasable with widely varing MS ratio, also tuning can be used to drasticaly
reduce switching transients as it alows switching at low volt/current.

probably stil best to let the controler manage the curent control loop, just
tell it what curent to limit it to either from a simple potentiometer of
from a DAC.

You wil probably go through quite a lot of power devices before you can get
it to work reliably.

Colin =^.^=
 
M

Mac

Jan 1, 1970
0
After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?

Jeremy,
How much do you know about H-bridges? Use courier or another fixed-width
font for the following ASCII art schematic:

V+
|
+-------+---------+
| |
\ S1 \ S3
\ \
| +----+ |
+------|load|-----+
| +----+ |
\ S2 \ S4
\ \
| |
+-------+---------+
|
GND

I'm not a power electronics guru, but here are some things to keep in mind:

S1-S4 must be in a well-defined state any time the rails are energized.
This may have implications for power sequencing.

I think there are only three states you will want: all switches off,
S1 and S4 only on, and S2 and S3 only on.

If S1 and S2 are ever on simultaneously, or S3 and S4, you will experience
rapid catastrophic failure of some sort.

Since switches don't turn off instantly, you may need to wait a while
(10's or 100's of nanoseconds? I'm not sure) during transitions after you
turn off a set of switches but before you turn on the other set.

You cannot directly drive IGBT's with a pic.

The only state which can be safely maintained for a long time is all
switches off.

Whatever circuit drives the upper part of the H-bridge will probably have
to be referenced to the emitter of the upper-side IGBT's. You may need to
use an optocoupler or something.

Neither side of the load is ground. Neither side is safe to touch. Neither
side can have the ground probe of an oscilloscope connected to it, unless
the oscilloscope ground is floating, in which case, the entire
oscilloscope should be regarded as unsafe to touch.

HTH.

--Mac
 
B

Bob Eldred

Jan 1, 1970
0
Jeremy Samuels said:
After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?

I agree with who ever said "not a good project for a beginner." You are in
for no end of nightmares trying to get a high frequency high power H-bridge
to work. The fact that you are asking about the transformer illustrates the
point. Transformer design is critical and H-bridges are notorius for causing
problems like "flux walking" that are guaranteed to destroy your switches
until you get a handle on the design and thoroughly understand it's nuances.
Even experienced engineers have trouble with these things until they learn
the art. I suggest you visit Harbor Freight and save your self a lot of
headaches. It will certainly be cheaper in the long run. I'm all for
education but you need to thoroughly study switching power supply design and
work with low power, easy to understand projects first. Secondly you will
need first rate test equipment like a digital strorage scope to capture
transient events that will plague any design. Good luck.
Bob
 
S

Scott Stephens

Jan 1, 1970
0
This has been done, google plasma cutter on the web &
rec.crafts.metalworking. Perhaps some welding news groups. The
Teslaphiles at pupman.com could also help. I've seen schematics out
there. You can e-mail me if you like, I'm somewhat interested.


Scott

--
**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!
http://home.comcast.net/~scottxs/

POLITICS, n.
A strife of interests masquerading as a contest of principles.
The conduct of public affairs for private advantage. - Ambrose Bierce

There is no giant behind the devastation of the world—only a shriveled
creature with the wizened
face of a child who is out to blow up the kitchen because he cannot
steal his cookies and eat them, too. - Ayn Rand
**********************************
 
A

Alan Turner

Jan 1, 1970
0
Hi Jeremy,

After seeing that cheap harbor freight plasma cutter, I was thinking
how hard is it to make an inverter plasma cutter? So, I've decided to
build one. I don't expect anything close to a real plasma cutter, this
is mainly an "educational" project.


The torch, of course, I would buy (I'm not that crazy). But the power
supply is just a 5-20 khz squarewave H bridge into a high frequency
transformer. Output voltage is about 150 volts. The secondary of the
transformer is loosly coupled to the primary, as the secondary voltage
drops considerably while cutting. Secondary current is around 30 amps.


For the controller, I was considering using a PIC microcontroller (my
programming is better than my soldering). Current adjustment would be
nice, but initially I will just have an adjustable 5-20 khz squarewave
output. The mains will be rectified and smoothed with a electrolytic.
An H-bridge IGBT module (600 volts, 25 amps) will be pulsed from the
PIC. My question is, what type of transformer should I use? Is there
anything else I should consider? Any good books?

Getting a project like this to work is harder than it looks.

I'll say right now:

* I AM NOT AN EXPERT. Don't take my advice very seriously - I'm just
hoping to point you in the direction of reasonable looking
documentation.

* SAFETY FIRST. Be very careful with high voltage supplies. Keep one
hand in your pocket :). Be particularly careful with charged
capacitors. It's possible to get a nasty shock even with the power off!
Ensure that you are not the only one around while you work on the thing.

I attempted to build a high current H-bridge as part of my undergrad
thesis. My report (and some photos) are available here:

http://www.biccard.com/alan/thesis

You might consider reading the application notes I have referenced in the
bibligraphy - particularly those from International Rectifier.

The inductance of the leads holding the bridge together can result in some
nasty transients when you turn the H-bridge switches on or off fast. These
transients can destroy the switching devices and/or the gate drivers.

You will need to be extremely careful with layout. Use high power modules
instead of paralleled discretes where possible.

Use google to search sci.electronics.design for some good posts on snubber
networks by Win Hill. Read them all twice.

You will need a fast storage oscilloscope and a variable DC supply to
drive the bridge with while you test it.

When something fails, there will be a big bang and it will be very hard to
determine what failed first :). The best approach I know of is to test
the bridge VERY cautiously. I'd use a procedure something like the one
outlined below.

1. Use a light resistive load to test the bridge for the first time.

2. Keep the bridge supply voltage at zero, and look at the gate drives to
the IGBTs. Check that bridge phasing is correct, that the gates are not
being overdriven. Look for overshoot at the edges of the pulses (due
to wiring inductance oscillating with the gate capacitance). FETs and
IGBTs can be easily damaged by exceeding Vgs(max) or Vge(max). Be aware
that your oscilloscope probe has capacitance of its own.

2. Slowly increase the bridge supply voltage. Look at the
drain-source/emitter-collector voltages at the edges of the square pulses.
Zoom right in to the rising and falling edges. Look carefully for
very short spikes which would exceed the maximum Vds / Vce. Exceeding Vds
even very briefly can write off FETs. Dunno what the situation is with
IGBTs.

3. Look out for capacitive coupling from the drain/emitter to the gate. It
is possible for this capacitance to turn on the switching devices briefly
at inappropriate times (possibly with catastrophic results). For this
reason, do not to use gate resistors which are too large.

There was an interesting discussion on this group about the purpose of
gate resistors for fast switching of power FETs. There seems to be some
disagreement about why we put them there. Maybe they appease the Gods :>

I'm sure people more knowledgeable than I will post good advice on this
topic. I think the best advice I ever read on line-powered switching PSUs
was in the Art of Electronics (by Win Hill, Paul Horowitz)- "Buy it."

Best of luck - and let us know how it goes!

Alan
 
T

timmmmayyy!

Jan 1, 1970
0
Thought about doing that.
I would use a smps to convert the mains voltage.
I would use a ferrite transformer of the ETD 49 type or a bigger design of
the etd cores.
 
J

Jeremy Samuels

Jan 1, 1970
0
I attempted to build a high current H-bridge as part of my undergrad
thesis. My report (and some photos) are available here:

http://www.biccard.com/alan/thesis

You might consider reading the application notes I have referenced in the
bibligraphy - particularly those from International Rectifier.

With the exception of using MOSFET's, your H bridge is nearly exactly
what I had in mind. I have a half bridge IGBT module, but obtaining
another would be difficult. I think using the same IGBT would be
ideal, so I'll either have to buy some IGBT modules or use a half
bridge.

The inductance of the leads holding the bridge together can result in some
nasty transients when you turn the H-bridge switches on or off fast. These
transients can destroy the switching devices and/or the gate drivers.


I can see issues with wiring distance vs. cooling being an issue.
Would you recomend using insulated or bare wire? Insulated wire might
add some capacitance, which might increase transients as well as affect
the maximum frequency.

You will need to be extremely careful with layout. Use high power modules
instead of paralleled discretes where possible.

Use straight wires whenever possible? Straight wires=less inductance?
Use google to search sci.electronics.design for some good posts on snubber
networks by Win Hill. Read them all twice.

You will need a fast storage oscilloscope and a variable DC supply to
drive the bridge with while you test it.

DC supply can be a PC power supply. An addition power supply can be
added in series/parallel, if I insulated the output voltage from ground
(but keep the case grounded). A combination of two power supplies with
-12,-5,5, and 12 volts would be precise enought.
When something fails, there will be a big bang and it will be very hard to
determine what failed first :). The best approach I know of is to test
the bridge VERY cautiously. I'd use a procedure something like the one
outlined below.

1. Use a light resistive load to test the bridge for the first time.

2. Keep the bridge supply voltage at zero, and look at the gate drives to
the IGBTs. Check that bridge phasing is correct, that the gates are not
being overdriven. Look for overshoot at the edges of the pulses (due
to wiring inductance oscillating with the gate capacitance). FETs and
IGBTs can be easily damaged by exceeding Vgs(max) or Vge(max). Be aware
that your oscilloscope probe has capacitance of its own.

2. Slowly increase the bridge supply voltage. Look at the
drain-source/emitter-collector voltages at the edges of the square pulses.
Zoom right in to the rising and falling edges. Look carefully for
very short spikes which would exceed the maximum Vds / Vce. Exceeding Vds
even very briefly can write off FETs. Dunno what the situation is with
IGBTs.

With my understanding, IGBT's are also sensitive to gate over voltage.
3. Look out for capacitive coupling from the drain/emitter to the gate. It
is possible for this capacitance to turn on the switching devices briefly
at inappropriate times (possibly with catastrophic results). For this
reason, do not to use gate resistors which are too large.

There was an interesting discussion on this group about the purpose of
gate resistors for fast switching of power FETs. There seems to be some
disagreement about why we put them there. Maybe they appease the Gods :>

I'm sure people more knowledgeable than I will post good advice on this
topic. I think the best advice I ever read on line-powered switching PSUs
was in the Art of Electronics (by Win Hill, Paul Horowitz)- "Buy it."

Best of luck - and let us know how it goes!

Thank you for the advice, very informative.
 
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