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  1. #1
    Join Date
    Mar 2005
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    63

    plasma THC voltage

    Who can tell me:
    What is the relationship between arc voltage and cutting height on the THC?
    Thank you
    Dennis

  2. #2
    Join Date
    Jul 2005
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    2415
    There is a simple electronic equation E = I X R (voltage equals Current times Resistance). Resistance can also be the "Load Resistance". If you hold any one variable constant and change one of the others, the third will change to maintain the balance.

    Plasma is a constant current cutting process. When you set the current on the dial , the plasma unit will try to maintain that current (I). It's a balance relationship. Since the cut gap (arc length) represents the "load" (resistance), change of that distance (R) changes the other leg of the equation (E) in direct proportion.

    In simpler terms the gap voltage is proportional to the gap distance if the current is constant. It only takes about .010 inch of difference to make about 1 volt of difference in the gap voltage. It's like measuring the change in air pressure to determine altitude.

    Each plasma manufacturer has determined their specific voltage at a given cut gap. For some it's higher or lower than others but it will remain relatively the same for that given machine. The charts in the owners manual are based on a given torch, nozzle and cut current. If you use a value outside the chart (lower/greater current on a given tip) you find the voltage at a given gap is different.



    Plasma cutting air has the highest voltage. A tip sitting on the metal has the lowest. measure the gap voltage, eliminate the noise, and be able to move over the change of just a few volts (out of hundreds) and you can "servo" the Z to move the torch tip small amounts to make maintain cut gaps that have to stay within + - .020 while moving in X & Y at 200 IPM.

  3. #3
    Join Date
    Jan 2008
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    2247
    Torchead is right on track...although I will add a couple of comments:

    Think of the plasma arc as a resistor in an electronic circuit. The arc is made from the gas you are using (often air....but there are many industrial plasma's that use oxygen, nitrogen, argon/hydrogen mixes as well as many other gas combinations). When you change the amount of air (by increasing the air pressure) you change the resistance. When moisture (water from your compressor) gets in the air, the resistance changes. When the torch gets closer to the plate...the resistance changes. When the electrode and nozzle wear, the resistance changes. When you change cutting speeds, the resistance changes.

    The arc voltage height control actually measures the the voltage drop between the electrode and the plate....one end of the plasma arc is attached to the electrode, the other to the plate. This measured voltage is compared to a voltage set by the machine operator....the height control compares the measured voltage to the preset...then adjusts the z axis (up or down) to make the measured voltage the same as the preset voltage.
    So, at a known power level in Amps, and a known cut speed, with a known gas (air), and a new set of consumables....most plasma manufacturers publish a recommended arc voltage that will allow a certain torch to work distance to be maintained. The following will make that published voltage not work properly:

    1. Lower cut speed. If you cut slower than manufacturers specs...the THC will move the torch closer to the plate...too slow and it will crash into the plate. If you want to cut slower than the manufacturers specs....increase the arc voltage until the torch is at the physical height that provides the best cut quality.
    2. Faster cut speed. Faster speeds means the torch will rise too high off the plate. To cut faster, reduce the arc voltage until the height is best for your desired cut quality. Too high often puts some top slag on the plate, as well as increasing the angle on the cut edge.
    3. Water in air will change the height....although the severity of the water...and other contaminates such as oil (from the compressor) can create different effects. Usually water will cause the torch to dive.
    4. Electrode wear. Since the electrode naturally develops a pit at the arc attachment point...as it wears the arc gets longer....so the THC moves the torch closer to the plate to correct the voltage. It is common to have to increase arc voltage settings over the life of an electrode to maintain the correct torch to work distance. .010" pit depth will equal the need for approximately 2.5 to 3 arc volts increase.
    4. As the nozzle orifice wears..the energy density of the arc decreases, causing the torch to get closer to the plate. Increase the arc voltage to compensate and maintain the correct distance.
    5. Kerf crossing. At the end of the cut, when crossing the kerf into the lead-out....there will be a dive of the torch as the arc lengthens trying to find metal...and the THC tries to adjust. Many sophisticated industrial height controls have the ability to ignore kerf crossings.....and more sophisticated cutting software will add machine code automatically on kerf crossings that disables (freezes) the z axis momentarily to block the dive and possible collision. This also occurs when cutting holes...and the slug from the hole drops....the torch dives and tries to follow the slug.

    There's a lot more, but I'll stop now. Hopefully this gives a better understanding of the arc voltage / torch to work relationship.....and may help explain some issues that commonly occur!

    Jim Colt

  4. #4
    Join Date
    Feb 2006
    Posts
    133
    Thanks Jim, I always look forward to your replies!
    By the way..... I realize that it is a competitors machine but would you have any ideas on my previous cut quality post? It happens more frequently on heaver plate, Have not seen it on 1/4" and below.
    Hope I'm not hijacking a thread here!
    Thanks, Steve

  5. #5
    Join Date
    Mar 2005
    Posts
    63
    Thank you both for the explanation of the relationship of arc voltage and THC.
    So in a nutshell, I'll need to experiment more to fine tune my machine to determine where I need to set the voltage differently for say, cutting circles or short inner cutouts verses longer cuts or profiles.
    Can I assume that the amperage and the THC setting can remain the same in the Sheetcam setting for the entire part and use the voltage level to adjust for actual torch heigth and feed speed change?
    Being new to the cnc plasma I'm finding it considerably more complicated to master than my cnc router.
    Thank you for your help.
    Dennis

  6. #6
    Join Date
    Jan 2008
    Posts
    2247
    Dennis,

    Always set the voltage last...as that will determine the height. The height controls cut edge angularity and dross levels.

    Plasma has far more parameters to adjust as compared to most other processes. Interestingly....the High Definition systems on many industrial grade cutting machines are far easier to set-up. On most...the operator just chooses the type of material and the thickness...everything thing else is automated. Those systems are expensive, far more complicated (as many as 6 different gas settings)...yet produce better quality parts faster and less expensive than with an air plasma.

    Jim

  7. #7
    Join Date
    Jul 2005
    Posts
    2415
    The MP1000_THC tip volts setting can be adjusted on the fly (as it cuts) from the:
    UP / DOWN Buttons on the front panel
    UP DOWN Buttons in MACH
    UP/DWN buttons on the optional Hand Controller.

    There is also a customizable table where you can preset material type/thickness and matching tip volts and bring it up and select from the list (before the job runs) . You can modify the settings in the list with the edit button. It keeps your notes for you.

    It would not be difficult for MACH to send new tip volts based on speeds and feeds to the MP1000 BUT since it varies on other factors to be perfect it would almost have ot have a "teach" feature! If you can furnish me a chart of what they should be for a given feedrate and material then I can have it send the value as it cuts from MACH. Honestly with conventional plasma you have a job on your hands to get all of the numbers!

    It's been my experience that just slowing down the Feedrate on smaller holes and changing the lead-in to perpendicular helps. You just group the inside holes together (layer in SheetCAM) and define the feedrate different. If you do need to use a different tip volts then you may need to burn the small holes as a different job and set up the target tip volts prior to cutting OR just manually adjust when it goes to cut the holes in the separate layer. There is a limit to what conventional plasma can do. It's only accurate to a point and if you try to cut holes smaller than about 3/4" dia on material thicker than 16 ga you start to get holes that are not perfectly round. Where I need perfect holes I "peck" the center of the hole with just a pierce point and drill the holes with twist drills. that still has an XY positional error of about .010 or greater.

    The needed parameters for a specific tip and material (like pierce height, initial cut height, default feedrate, dwell at the end of cut, etc are all in the tool definitions in SheetCAM. You can setup as many tools (and or POSTS) as you want. I have different tools defined for different tip sizes. You can also override certain parameters when you define the cut process in SheetCAM.

    I have multiple posts that modify the REFDISTANCE (inside the post file) so the auto touch-off frequency can be changed. In the default post the touch-off is only done every 20 inches of XY travel so where you are doing close pierces, it does not have to waste the time to ref before every pierce. On thin material where it tends to warp easy I set that to "0" so using that Post it touches off every pierce. Since MACH knows the position of Z as long as the material stays flat and level it will pierce at the right height.

    You are correct. Cutting with plasma is a lot more complex than a router. Even doing the artwork is different. We do a lot of both.

    You can spend a lot of time messing with settings to try and get the perfect cut. There are so many variables that it will shift on you. You can set everything from tip volts to plunge rate either in MACH or SheetCAM. You just need to find the best average and use that to cut with.

    There are THC Corrections settings that prevent the Z from going too low or too high; there are THC Rate settings that change the dynamics of the THC feedback loop (how quick/fast the Z responds). Change those parameters are your own risk and keep the old settings so you can get back!

    TOM CAUDLE
    www.CandCNC.com

  8. #8
    Join Date
    May 2007
    Posts
    250

    Smile arc voltage and cutting height

    I like this post . It increases my understanding.

  9. #9
    Join Date
    Sep 2015
    Posts
    29

    Re: plasma THC voltage

    I know this post is old, but rather than starting elsewhere.....


    I have a hypertherm PM65 and proma thc. (1:50 voltage divider)

    Hypertherm manual suggests 130v, 65a when cutting 3/8" plate.

    When I do this my torch to work gap climbs to about 1" - obviously leading to bad cuts.

    In reality I am having better luck with a 90v setting on the THC and the other variables staying the same. 90 v keeps me at about ~0.120" cut height

    Any idea why my voltage is so different than what the manual suggests? (this is with new consumables)


    For 3/8" steel plate at 65 Amps, with the formula V=IR

    90V = 65A (R)

    R=1.38

    ..... Can I assume a resistance of 1.38 will get me the correct cut height for other thicknesses of steel?


    say for instance 1/4" plate cutting at 45 Amps:

    V = IR
    V = 45A(1.38)

    V = 62 volts


    then I would set my THC @ 62 volts to maintain a cut height of about ~0.120" ?

  10. #10
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    Jul 2005
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    2415

    Re: plasma THC voltage

    Ohms law is too simplistic to use to run any calculations. It's far easier to just turn off the THC in MACH and go a cut at the correct height for the torch you are using and watch the voltage. If you have the THC mounted close to the plasma (which is required if you use the 50:1 ratio because of noise) then that may mean running back and forth. Whatever the voltage that is shown while y9u make a proper cut IS the proper voltage. IF its not right then something is wrong with the voltage divider at the plasma or with the THC itself. You can check the actual divided volts on the 65 CPC connector or the wires coming from it. It should the actual torch volts divided by 50. A hypertherm fire in the air (open circuit voltage) is close to 165. At the pins (5 & 6) it should be 3.2 to 3.3 volts DC. If is no correct then the reading will not be correct. Check it again with the THC disconnected from 5 & 6 and if its still off it's in the plasma. If is correct then it's either from the THC or external noise affecting the circuit.

    Bottom line is if the volts are wrong (low) then it will try to raise the torch to raise the voltage. The THC just tries to move the torch to match whatever you tell it you want it to hit as it cuts.

    TOMcaudle
    www.CandCNC

  11. #11
    Join Date
    Jan 2008
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    2247

    Re: plasma THC voltage

    I'll add....the numbers in the Hypertherm manual are correct, but assume: that you have the correct consumables in the torch, they are new, you are running at the exact specs listed in the cut chart for the power level, material thickness and type of material. If you change speed, change physical height, change amperage, change cut air pressure....the voltage will change. The arc is a resistor......changing any of the parameters will change its resistance, and the voltage reading from electrode to workpiece.

    You can read actual raw arc voltage on the main power board inside the Powermax65 on screw terminals J28 (-) and J26 (+). If you get a reading on this......at the same time the divided voltage output will be 1/50th of the raw voltage (if you are set at 50:1)..

    As Tom said.....the physical height is what matters. If your height control system is out of calibration....then set the voltage so the physical cut height is correct according to the Hypertherm cut charts. As the consumables wear....you will need to occasionally increase voltage to maintain the correct height through the end of the consumable life.

    Jim Colt Hypertherm

  12. #12
    Join Date
    Sep 2015
    Posts
    29

    Re: plasma THC voltage

    Thanks for the prompt reply's Tom and Jim

    I will test pins 5 & 6 to verify 3.2-3.3 voltage range, and also j28- and j26+ if I need to dig deeper. - did notice the hypertherm voltage divider board actually has different settings like 1:10, 1:50, ect but I didn't touch those

    My leads from the plasma to the THC are about 12" long, and mounted on the side of the machine.


    THC does work at 90v, but 90 volts just seems so far away from the recommended 130 volts. I think its worth investigating further.

    I will report my findings

  13. #13
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    Jan 2008
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    2247

    Re: plasma THC voltage

    The voltage divider board is set at the factory default of 50:1. You can also set it at 20:1, 21.1:1, 30:1 and 40:1.

    Jim Colt




    Quote Originally Posted by snowdaze View Post
    Thanks for the prompt reply's Tom and Jim

    I will test pins 5 & 6 to verify 3.2-3.3 voltage range, and also j28- and j26+ if I need to dig deeper. - did notice the hypertherm voltage divider board actually has different settings like 1:10, 1:50, ect but I didn't touch those

    My leads from the plasma to the THC are about 12" long, and mounted on the side of the machine.


    THC does work at 90v, but 90 volts just seems so far away from the recommended 130 volts. I think its worth investigating further.

    I will report my findings

  14. #14
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    Sep 2015
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    29

    Re: plasma THC voltage

    I uplugged the THC, multimeter set to AC volts - testing while cutting I got 0.48v red lead on pin 6, black lead on pin 5

    65 Amps, 3/8" material, cut speed out of the manual. about 0.1" cut height, the ground cable is connected directly to material being cut.

    The cut quality couldn't be better, so I'm wondering whats up with the .48v




  15. #15
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    Jan 2008
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    Re: plasma THC voltage

    Try that again....this time with your meter on DC volts. .1" is too high...and may be the cause of the dross on the bottom of the cut. 3/8" at 65 amps will be dross free if the height is right (book says .06") and the speed is right. Jim Colt Hypertherm

    Quote Originally Posted by snowdaze View Post
    I uplugged the THC, multimeter set to AC volts - testing while cutting I got 0.48v red lead on pin 6, black lead on pin 5

    65 Amps, 3/8" material, cut speed out of the manual. about 0.1" cut height, the ground cable is connected directly to material being cut.

    The cut quality couldn't be better, so I'm wondering whats up with the .48v




  16. #16
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    Sep 2015
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    29

    Re: plasma THC voltage

    Dropped the cut height to 0.06", much less dross and very narrow cut looks good.

    This time on DC mode .. I got a reading of 1.84 volts

  17. #17
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    Jan 2008
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    Re: plasma THC voltage

    Sounds like the voltage divider is at around 70:1, not 50:1. If you are cutting at .06" height at 65 amps on 3/8" material the actual arc voltage will be real close to 130 volts. I guarantee that if you put a meter on the raw arc voltage that you will read close to 130.

    You either have a bad voltage divider card in the Hypertherm......or something in the Height control circuit is affecting the height control output reading. Can you disconnect all connections between the plasma and the height control, do a cut at a fixed .06" height at the book cut speed on 3/8"....measure the voltage from pin 5 to 6 again?....it should read roughly 2.6 volts.

    Usually if a voltage divider fails it has no output.

    Jim Colt Hypertherm


    Quote Originally Posted by snowdaze View Post
    Dropped the cut height to 0.06", much less dross and very narrow cut looks good.

    This time on DC mode .. I got a reading of 1.84 volts

  18. #18
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    Jul 2005
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    Re: plasma THC voltage

    1.84 X 50 = 92 volts. As Jim said either the input on the THC is dragging the voltage down or the divider is not putting out the right voltage. At cut height = .060 and correct amp setting on the Hypertherm and correct feedrate you should see 2.6VDC
    You cannot use any other divider setting with that THC. Its 50:1 or Full Arc Volts.

  19. #19
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    Sep 2015
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    29

    Re: plasma THC voltage

    I took the case off the machine and carefully did the same cut to see whats really going on. everything is set at book values.

    test 1: THC unplugged and not hooked up, took voltage reading right from the hypertherm divider board. I got 2.40v - 2.38v (it was on 50:1)

    test 2: THC unhooked / unplugged, took voltage reading at end of wires that I attach to THC device... I got 2.40v roughly again.

    test 3: THC not plugged into wall, but pin5/6 wire attached to THC (on the DV +,- center section of the proma thc for 50:1).... I got 2.40v at piece height/initial fire, and then the voltage dropped to ~1.84v while cutting at 0.06" height

    Some thing seems fishy here.


    hooked up exactly like this and i touch the multimeter prongs to the metal where the wire connects


  20. #20
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    Sep 2003
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    15

    Re: plasma THC voltage

    We just acquired a small 4x8 table with Mach3 - Gecko 540 and PM45 from a garage operation.

    The system was "badly" set up, with neither limits, home switches, nor shields anywhere (we come from larger controls such as Biesse and Thermwood / camsoft and it took a week or so for us to get acquainted with the system). All switches in order now.
    we then addressed the Proma 150 which was not working when we got the system.
    We finally got it to work as it should after a couple of days, but I feel it needs refining....
    we tend to cut 16 ga at about 150 ipm on 30A with a pierce set at .015 and cut at .06.

    without THC, no issue.. with THC set at 111 V, Z DRO is at +/- .120 ...
    I am not clear as to what should be the driving factor ; should we aim to maintain targeted "ideal" arc voltage regardless of Z height or should we adjust voltage to maintain constant cut height (in this case .06") regardless of changes in work elevation ?

    Also, I understand the proma should be located near the PM45 due to noise issues. is 2' to 3' too far ?

    The system uses sheetcam. The post selected is a Mach3 with THC. but I am not sure what this entails if anything regarding THC control ... are there better posts to use as far as triggering THC on and off automatically based upon geometry and cutting conditions ?

    Thanks in advance

    Pierre
    Pierre

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