The purpose of a transformer on the power supply is isolation from the mains. It is a safety feature to prevent electrocution (along with fuses). You can run directly of mains power, but the risk of death rises.

Why do you want the output end of the plasma cutter at 240 volts. Plasma is a function of current. The extra volts don't gain you anything. Please explain???

I think maintaining 240v is just a limitation of running directly off the mains power. You are maintaining the mains voltage, to get the Amps, in lieu of using a Power Transformer. Cheaper, lighter, risky. I could see a use for that, If I was cracking a safe, and I needed portable equipment.

High Voltage (10,000) is necessary to Arc Start, but it doesn't do the cutting. The Amps do the work.

You are on your own from here. Please post your results. This is interesting.

 

From the information I have found about the specifics of plasma cutter power supplies is that they have a 240 - 400 volt open circuit voltage (from torch electrode to the positive work clamp) and the short circuit current depends on the thickness of metal being cut, and I chose 40 amps should be do able

EDIT: and once the power supply is done and tested to work correctly I will begin developing the high voltage start circuit

 

While I said that you don't gain anything from higher volts in Plasma; that it is the Amps that does the work.

I need to hedge my answer.

Without knowing their circuits, there is an exception. That exception is if they are doing Capacitor Discharge. There is a formula for Joules stored in a capacitor that is affected by voltage.

To calculate Joules: SQUARE the VOLTAGE, MULTIPLY that AMOUNT by the CAPACITY in MICROFARADS, and DIVIDE this
AMOUNT by 2,000,000. This is the energy stored in the capacitor.

EX:
If I charge 1800 cap to 210 volts (210 * 210 * 1800 / 2,000,000) = 40 Joules.
If I charge 1800 cap to 450 volts (450 * 450 * 1800 / 2,000,000) = 182 Joules """FATAL""" .


So if the Plasma Cutter circuits are using Capacitor Discharge, the increased voltage will have a drastic effect on current delivered.

 

I'm not sure why the open circuit voltage is so high, I always thought it was just for ease of arc ignition, even though a separate high voltage circuit starts it, so I'm really not sure, I might start off with a lower open circuit voltage and see how it effects operation

 

They need the higher working voltage to achieve a long enough arc to reach through the material they are cutting.

The general rule is 70 amps per inch of thickness when cutting mild steel but to stretch an arc out that far on a supersonic jet of super heated plasmatized air takes a few hundred volts.

On top of that more voltage equals more cutting jet power. Believe me from personal experience using a 50 amp 350 VOC plasma cutter cuts way better than a 70 amp 170 VOC plasma cutter.


I would think someone who calls themselves Plasmahunt3r would know this sort of basic plasma cutter stuff.

Now I think both of you are in way over your heads on this project.

 

Check out these sites for HV Arc Start Circuits. They are both doing the same thing, abiet, for different goals:

http://www.casano.com/projects/hfstart/index.html

The Ignition Circuit at this site accomplishes the same HV Arc Start, but is using Capacitor Discharge for his MPD Thruster:

http://www.myelectricengine.com/

 

Will that high voltage start circuit burn up any of the power supply circuit when I kick it on?

 

Both The Welder Circuit and the MPD Thruster is DC. The HV is AC. AC and DC can coexist on the same line.

What they are doing, is using a custom built Air Core Transformer, to induce High Voltage AC into the High Current DC side.

The transfomer secondary has the High Current DC (the DC output from you inverter that you are building). DC passes through the secondary coil, but is an open circuit, because of the gap between the Negative and Positive at the workpiece.

When the High Voltage turns on, It induces High Voltage AC on the secondary, which causes a spark across the gap, which closes the High Current DC circuit. The Spark is the switch that closes the DC circuit.

I have built this, and in practice, I could only get a max 1/4 inch spark. I redesigned the circuit to use Blocking Capacitors instead of a special transformer coil, to get a 1 inch spark. My redesign is the Plasma Arc circuit schematic I gave you on the second page of this thread.

I also found, in practice, is to add a choke coil on the DC line, between the Rectifiers and the Secondary. The Choke allows DC to pass, but blocks AC. The purpose of this Choke Coil is to protect your Bridge Rectifier from the High Voltage. Even though I had 1000V 50A bridge, the 10,000+ High Voltage would burn up my Full Wave Bridge Rectifier. Just 20 turns, 1/2 inch diameter, of 12Ga solid wire, protected by bridge from the HV Spike. This tip on Choke Coil just saved you 10 bucks.

 

Thanks for the tip on the choke coil, on the secondary, that's what I was worried about, and I was also wondering if the high voltage spike could induce power back through the switching transformer onto the igbts causing them to fail too

 

Thanks for the tip on the choke coil, on the secondary, that's what I was worried about, and I was also wondering if the high voltage spike could induce power back through the switching transformer onto the igbts causing them to fail too

Did you manage to finish the project, or at least get started? Id be interested to work on it together

 

Did you manage to finish the project, or at least get started? Id be interested to work on it together

Since this thread is over 6 years old and the TS hasn't been seen since 2014, I doubt you will get an answer.

 

Did you manage to finish the project, or at least get started? Id be interested to work on it together

Like crutschow said this is an old thread. But when it was started things were very different that today. Today for very little money you can get the basic board, one that is used in many of the off the shelf inverters for less money than just the components needed. You will still need to add some components but the basics are there and it's a proven well known design. Take your pick form Ebay, some even come with a digital read out -

https://www.ebay.com/sch/i.html?_fr...ine+Wave+Inverter+SPWM+Board+EGS002+&_sacat=0

 

Like crutschow said this is an old thread. But when it was started things were very different that today. Today for very little money you can get the basic board, one that is used in many of the off the shelf inverters for less money than just the components needed. You will still need to add some components but the basics are there and it's a proven well known design. Take your pick form Ebay, some even come with a digital read out -

https://www.ebay.com/sch/i.html?_fr...ine+Wave+Inverter+SPWM+Board+EGS002+&_sacat=0

Thanks

 

So i am working on a homemade plasma cutter i thought about taking the simple route of rewinding microwave transformers like i have done with the welder i build in the past but, i decided i want to learn how to design and build an inverter power supply(switch mode) and i need some help as i cannot find any good information on designing one for my specific need.

This power supply will be required to output at least 240v DC (open circuit voltage) and a short circuit current of at least 40 amps for a start, although i would like to implement variable current later on, and even higher current

My main questions are: how do i design and create the high frequency transformer? and how do i achieve the outputs required? what determines the output voltage and the output current? i understand that switch mode power supplies are PWM controlled, so does the transformer determine the output voltage and the PWM determine the output current?

Hopefully someone here will be able to help me

An inverter transformer that can provide both 240 volts OC and 40 amps into a low resistance is a high power package. Based on some other plasma cutters it could indeed be a 4000 to 5000 watt device. The good news is that if you use a higher frequency the transformer will not be as heavy as a 60hZ unit. The inverter should be a driven type so that the power can easily be controlled, meaning you will not be making just a simple oscillator inverter.

 

My take on this is that the thread starter attempted but failed to reach its stated objective.
Of course, with all his initial bluster and arrogance, he would not have come back to say: “you know Tcmtech, you were right. This project was way over my head.”