[mconstrain]

Howdy,
I have a Hypertherm PowerMax 1000 which needs a 50:1 voltage divider. I built one using the typical resistor circuit, but the plasma cutter refuses to fire when using it. I tried three different circuits with resistances ranging from 5 kohms to 1 Mohm and no luck on any of them. I can read the voltage fine from a DVM while the unit is cutting, and I know the resistor circuits work by using a different power supply. But..... the arc refuses to transfer when the resistors are there. Do I need a diode, capacitor, or filter to make this work? I thought this circuit would be a piece of cake, but apparently its not as obvious as I thought. Any help would be great. Thanks

[jcc3inc]

Sir,

Where have you connected your divider? We connected our divider before the hi freq portion going to the torch. It was a 6K, 12 watt ww R in the plasma power supply, (connecter thru a shielded cable) in series with a 350 ohm R inside our THC control box at the control stqtion for the machine. This gave us 5.5V per 100V of torch voltage, and kept us clear of the high frequency starter voltage added to the DC plasma power supply voltage.

Possibly your are spoiling the starting aspect for the plasma?

Regards,
Jack C.

[mconstrain]

The Powermax 1000 has built-in connection points for measuring arc voltage and those are what I am using. This is a modern plasma cutter without high voltage starting, and its designed for CNC use. Amazingly though, they give very little insight into how to build/connect a voltage divider to those arc voltage terminals.

[Torchhead]

Something is (really) wrong. J15 and J16 in the 1000 are full tip volts. If your voltage divider is too low in value it will just burn up, not keep the machine from firing. Keep the total resistance above 10K and you will be okay. Not firing is from something else. It would take a short to keep the torch from going through the firing cycle. Since it's hot HV or HF start even a low impedance pre-filter would not effect it. At 50:1 the voltage is about 3VDC at normal cutting. You will have almost that much in noise in signal!.

I am really confused by the pictures. They way it's hooked up now it would never work. Another tidbit: The voltage is across workclamp and the tip electrode. Workclamp IS NOT GROUND. It may become that when you clamp it to the work, but you cannot assume that. You may be in for a rude surprise if you can make it fire. The resistors may arc to the metal plate you have. The reason Hypertherm is not going to give you detailed information about building a voltage divider is they don't want to be responsible for you killing yourself.

THERE ARE LETHAL VOLTAGES AND CURRENTS PRESENT. IF YOU DON'T KNOW WHAT YOU ARE DOING YOU CAN SUFFER SEVERE INJURY OR DEATH.

Tom CAUDLE
www.CandCNC.com

[mconstrain]

The large gold resistors were the first ones I used. They are not currently hooked up. The black heatshrink contains the current 1 Mohm and 20 kOhm resistors being used. Those circuits have to be built correctly, because they work fine when anything other than the plasma cutter is connected, ie, I connect my bench variable power supply and the output is always 1:50 the input. Here is the circuit I am using.

I think I am going to try a diode. There must be some spike on the negative terminal that is going back to ground on the Keyence controller reading the input.

And Tom, think about it..... If Hypertherm didn't want you building/connecting a voltage divider, why would they tell you exactly which pins to put terminals on producing 300 volts? Even in the manual, it is clear this is meant to be done by a user.

[jimcolt]

Can you show me a schematic of your complete crcuit...including the connections to the Hypertherm unit? Thousands of these systems are connected to voltage dividers for use with torch height control systems...and they operate just fine....there has to be something with your particular circuit that is affecting the output power of the system.

Jim Colt Hypertherm

[Torchhead]

Can you show me a schematic of your complete crcuit...including the connections to the Hypertherm unit? Thousands of these systems are connected to voltage dividers for use with torch height control systems...and they operate just fine....there has to be something with your particular circuit that is affecting the output power of the system.

Jim Colt Hypertherm

I can vouch for that. I have over 1000 units out with the same basic sensing circuit (THC SENSOR card). It's basically a simple resistor divider across J15 and J16. The total resistance we use is under 12K. There are other components downstream and some built in suppression for opertaion with HV and HF start plasmas. With the 1000 series the hookup takes about 10 minutes (arc xfer, tip volts and torch start). I have a 1000 G3 we use in our commercial cutting operations and for testing our CNC electronics. We have units on 600's; 900's and even a few Max100's (ugh!)

It would appear from the picture that one side of his circuit goes to chassis ground. If the wires are hooked up wrong to J15 and J16 then he is grounding the electrode to the table and if the workclamp is on the table ground it's a dead short.

I attempted to explain this in my previous post but the person obviously does not need my help. Perhaps he will listen to you.

TOM CAUDLE
www.Candcnc.com

[jimcolt]

Hey Tom....the Max100 was a good system in its day....20+ years ago!

Jim

[mconstrain]

Jim, I guess if you don't use them, then a diode would do nothing for me either. Here is a picture of my circuit. The only thing connected to chassis ground is the cable shield. The Hypertherm is connected to an Animatics smart motor which is reading the adjusted tip voltage as an analog voltage input. A 5 volt power supply return line is grounded to that same point for the rest of the I/O, but nothing else.

One other thing I should mention.... This plasma cutter was already repaired once. When it was purchased brand new back last year, the power supply blew up as soon as we plugged it in. Hypertherm repaired it free and replaced the main board, but I have always wondered if there was some other problem with it, because the components that blew up were right next to J15 and J16. I would much rather believe the problem was with my wiring, but its a pretty simple circuit and if it was connected backwards or something, I would have 300 volts going to my I/O lines which would start blowing things apart which hasn't been the case when turning it on.

[jimcolt]

What are the values of the resistors R1 and R2? I will have one of our electronics engineers look at this, once we know the value of the resistors....if we think it would affect the performance or operation of the plasma system. We are not going to design you a special voltage divider....just tell you if the resistors you are using will affect operation.

We have a variety of voltage dividers that are designed for this application......they are quite a bit more complicated than just a couple of resistors...there is filtering and signal conditioning (to eliminate electrical noise which is present due to the arc attachment points on the plate)...and due to the fact that the output of the power supply is square wave at around 16khz.....with varying pulse width to control current. The voltage divider has to smooth out the noise and provide a stable analog signal if it is to be used for feedback for a torch height control system. Our voltage dividers also have isolation for use with ohmic contact between the shield and the plate...this is used for initial height sensing (IHS) to find the surface of the plate as well as for collision avoidance during a cut.

Jim Colt

[mconstrain]

Well, again, I have used a bunch of different resistors. The first design was
50 kOhm for R1 and 1kOhm for R2. The 2nd try used 5 kOhm for R1 and 100 ohms for R2. The final try used 1 MOhm for R1 and 20 kOhm for R2.

If Hypertherm sells voltage dividers, I would buy one straight away. I thought you only had them integrated on certain models of your plasma cutters. That is more the information I was looking for. I figured a crude voltage divider built with resistors should work just fine, but if the plasma cutter will require some filtering and/or isolation just to turn on, I would probably just buy one. I know enough about electronics to build one, but first I just wanted to prove the concept with this Animatics motor and add filters, etc from there. My initial height sensing is being done with a limit switch on the torch (ie. touch off from the cutting surface).

Thanks for your help Jim and if you could just tell me how to go about ordering one of those voltage dividers directly from Hypertherm, I will do it right away.

[mconstrain]

I tried the circuit without grounding the Powermax 1000 to the motor and it works just fine. The negative arc voltage lead off the plasma cutter definitely doesn't like being tied to the Smartmotor I/O. I know analog isolation on a voltage divider circuits is probably a good idea regardless, but is it considered bad practice to ground the negative arc voltage lead to anything and everything without isolation? Is the plasma cutter meant to operate even if the negative arc voltage lead is grounded correctly? The output signal itself is actually quite clean as far as I can tell. I haven't checked it on a scope yet, but the DVM is registering the arc volts very accurately relative to the height of the torch above the plate.

[Torchhead]

Hey Tom....the Max100 was a good system in its day....20+ years ago!

Jim

I struggled with one for two days to make it work with a PC based control and Digital THC. It would kill small animals at 50 ft with it's HF start. I think the fillings in my teeth worked loose! I'll say this: When you told it to fire it was going to start an arc SOMEWHERE.

[jimcolt]

Yes....it has the same high frequency circuit that we use in all of our large industrial systems......about 15 kvolts at about 2 megahertz. In the large industrial systems it is necessary for reliable starting in torches that have near perfect concentricity....the arc has no good starting point between the electrode and the nozzle due to the precision of the torch....which is necessary for the best combination of cut quality over the life of the consumables.

Fortunately, most oftodays air plasma systems use the (non high frequency) blowback starting process!

Jim

[jimcolt]

In a plasma system you never want to ground the negative side of the circuit...if that is what you were doing, then that would explain why the torch would not fire.

The electrode in a plasma system is always negative, the nozzle is positive (during the arc stating process) then is disconnected from the positive side of the circuit by a relay (pilot arc relay) as soon as the arc transfers to the plate....which is always positive.

The voltage reference that you need to monitor for arc voltage height control systems is the voltage during steady state cutting as measured between the negative (electrode) and the positive (plate). At a fixed torch travel speed...the DC voltage measured in this manner will increase as the torch to work distance increases, and will decrease as the torch to work distance decreases.

I keep hearing references to "tip voltage".....from Hypertherm's perspective we have no idea what this is...as our torches do not have a "tip"! The primary consumable parts are the nozzle, the electrode, and the shield (which is electrically floating for nozzle protection from double arcing to the plate).

The signal conditioning and filtering that I suggested in the earlier post are not for the benefit of the plasma system....but will likely be necessary if you are designing an arc voltage height control system to filter out all kinds of electrical noise that you will see when monitoring the plasma DC voltage output. Hypertherm has been designing and manufacturing arc voltage torch height controls since 1975....and does have quite a bit of experience in this area!

Most of the voltage divider boards that we offer are designed for use with our high end industrial plasma systems and height controls....they probably have quite a bit more functionality than you need for your application. Our newest air plasma system, the Powermax45 is available with a voltage divider output for torch height control systems...this divider is built into the main control board on the power supply.

Jim Colt

[mconstrain]

Alright, I guess I am confused then..... How would you go about measuring a voltage difference without having some sort of reference leg? You always have to have a reference of some sort when measuring voltage. Now I suppose you wouldn't necessarily have to ground the negative lead to measure a potential difference, but very few analog inputs work that way. Most want the negative lead grounded and compare the grounded lead to the positive one. I realize its a bad idea to ground to the table, but that is not what I am doing. I am only grounding the negative lead to the I/O ground on the motor.

[jimcolt]

You can easily do this experiment....connect a DVM (digital volt meter) to the terminals J15 and J16 in the Hypertherm Powermax system. Set the meter on a DC scale with a range of up to 300 volts. Fire the plasma torch and cut at a fixed speed....raise the torch....read the voltage on the meter, then lower the torch, read the voltage on the meter. You will notice the voltage rises as the arc length gets longer and lowers as the arc length gets shorter.

In a torch height control....the circuitry monitors this voltage and adjusts the torch height by driving the z axis motor up or down to maintain the correct voltage, consequently you can use this to control torch height. The voltage divider is used to isolate and filter this signal from the plasma, bringing it down to a useable level that can be used by a comparator circuit that compares the actual arc voltage to a known voltage......and then adjusts the torch height to compensate for plate that is not perfectly flat. Conventional plasmas need to maintain the correct torch to work distance within plus or minus .020"....or within about 5 arc volts. With High definition class plasmas, torch heigh must be maintained to plus or minus .004" of the ideal height, or within 1 arc volt.

I don't understand why you want to ground anything from the plasma directly to the drive motor.....

Jim

[mconstrain]

Err..... Jim, I don't think we are understanding each other. I understand how a THC works and as stated before, I have already connected a DVM many times. When you connect a DVM, there is a black lead and a red lead. When you connect the black lead to J16, you are connecting the logic ground on meter to the negative lead on the plasma cutter. This is "grounding" the logic on the meter to the plasma cutter.

I am doing the same thing with my Smartmotor. I am grounding the black lead off J16 to the logic ground on the Smartmotor I/O. You HAVE to ground the negative lead to the logic reference measure anything. DVM, PLC, or Smartmotor. It doesn't matter. You have to ground the negative lead to the logic ground to measure any potential difference between J15 and J16. This is not optional. What I want to know is why the plasma cutter will accept a logic ground on the DVM, but not a logic ground on the Smartmotor.

[jimcolt]

OK...I guess I misunderstood you. I'm just concerned with the fact that the negative output from the plasma can be as much as 300 volts DC different than the frame of the cutting machine and the plate you are cutting. Not knowing exactly what you have in mind for THC design.....I will not try to influence your design! Just be careful!

Jim

[Torchhead]

Quit using the word "grounding" for anything that is not referenced to actual Earth ground, then you will quit confusing yourself. Use circuit "common". The negative side of the battery in a car is DC common. It's not grounded to anything but itself. Putting a meter with an internal battery on two terminals creates NO GROUND REFERENCE. It simply measures the potential across two points. They can be hundreds or thousands of volts above or below another circuit (or earth ground) and all you will see is the differential voltage.

The table NEEDS to be grounded (earth ground) to keep you from frying yourself with the plasma. As soon as you clip the workclamp to the table it establishes the voltage level of the workclamp. If the table is ungrounded then the table will become "hot" in reference to real earth ground. Touch a normal grounded piece of equipment and a hot table and you will (or your relatives can) verify the potential difference.