How accurate is the 50:1 voltage divider

[xnaron]

I am using my Proma SD to read the voltage on my 8" test cuts. Powermax 65

Material is 0.115" mild steel as measured on calipers. This puts it around 11 gauge. So I tried around 130 ipm at 45amps and lots of dross. When I bumped it up to 180ipm pretty much no dross.

45amp shielded consumables as machine torch running at 45 inch. 0.060 pierce and cut. I know the chart says 0.15" pierce but I am running with no z control for the test cuts.

Here is a video of the cut showing the voltage reading on the Proma SD (remember no z stepper connected). Mid 80s to 90 v reading.



Here is the 50:1 voltage divider settings.

180ipm cuts are the clean ones on the left

The cut chart says I should be around 130 volts. I am going to say that the proma is probably reading it too low (see this behavior on my chinese herocut plasma cutter but don't have a cut chart for it so didn't know what was normal. I am just throwing this out there to see what voltages others are reading on their THC (proma and any other) with respect to the cut charts. Is your voltage close to the charts?

thanks,
Brendin

P.S. I did try the calibration of the voltage on the proma setting 2v = 100v (found a youtube video on it). Apparently it didn't work?

[adbuch]

Hi Brendin,
Interpolating between 10 ga. (0.1345") and 12 ga. 0.1046") would put the cut speed for your 0.115" thick material at around 198 ipm (from the Hypertherm cut chart, production settings), so your 180 ipm would be in the "ball park".

You could measure your voltage divider to see how accurate it is (with the machine powered off). You would need a dc voltage source and an accurate volt meter (dvm). Apply a known dc voltage across the entire resister divider network and measure your divided voltage across the 50:1 tap.

Increasing your cut height will increase the arc voltage, so you could also increase your cut height a little bit at a time and measure the divided arc voltage as you have previously been doing. Another variable is the accuracy of your proma device. So there are lots of variables here.

You may also get some ideas from Jim Colt's video on the subject.
David

https://www.youtube.com/watch?v=V4gAEg4RKIQ

[robertspark]

the voltage is unimportant.... it is just a number as all thcs will have some variance. (before some smart person says something.... yes you need a voltage number it just you need to find the number that works for your thc! on the proma I found my voltages readings were low too)

as long as is cuts at the right height and the edge is fine, the number on the thc is just a target.

some THCs can use sample and hold.... so they will sample the voltage at say 0.5 to 1 sec after the start of the cut and then track that voltage number (the neuron can do this, and the price too if set that way)


I am not sure about the voltage divider in your machine, but in my pmx45 it uses a 100k and a 2k resistor to get 50:1...

technically.... this is not a 50:1 voltage divider.... yes the ratio is 50:1 but you need to consider ohm's law....

say we have 100v across the voltage divider... at 50:1 you would think it would give you 2v out.... but that is not the case...

100v / 102000 ohm's = 0.00098039216 amps

so across the 2k resistance we have 2000 ohm's X 0.00098039216 amps = 1.96078431373 volts (I just copy and paste from the calc app... I'm not typing it out!)

which is about a 2% error...... but the resistors are not precision spec... so there will be some variation in them too....

also the thc may have its own internal parallel impedance too which will alter this reading.... then go and plok another plasma cutter on the thc 50:1 input and you will get some other reading....

the price thc has two 50:1 inputs.... one for the 45xp and another for all other plasma cutters as apparently with the 45xp hypertherm have changed the impedance of the voltage divider ....so price have compensated for this through their design.

[xnaron]

That makes sense guys. @robertspark how close does the MiniTHC come to the chart voltages on your 45?

[acourtjester]

One thing you really need to do is be sure you are cutting at the correct height without the THC control and cut for a long enough time for the arc voltage to stabilize, You video is way to short. As Robert said what works on your table is what you need to find and use, there are many posts here about this and they all come down to that, you may need to create your own chart.

[xnaron]

One thing you really need to do is be sure you are cutting at the correct height without the THC control and cut for a long enough time for the arc voltage to stabilize, You video is way to short. As Robert said what works on your table is what you need to find and use, there are many posts here about this and they all come down to that, you may need to create your own chart.

Yes I agree. I did swap out my Herocut for a used Powermax 65 I got a really good deal on from a Hypertherm Dealer. I did bump my test cut up to 8" but then my speed also was almost doubled to 180ipm. I suppose I can turn down the amperage and slow the cut down and also bump the cut length up. Maybe cutting at a lower amperage and slower will also yield a better cut?

I did make a test cut today with height sensing but no thc. It worked out ok but lots of opportunities for improvement. I am going to try and get my hands on a MiniTHC. The price THC guys are non-responsive and it is listed as out of stock. I assume a small business and perhaps some issues have come up preventing sales.

[robertspark]

. I assume a small business and perhaps some issues have come up preventing sales.

covid19

[Rodw]

I had a similar experience with a since sold Everlast machine. What I found was the divider on these cheap machines altered the voltage sensing circuit so that a 50:1 divider was seen as 75:1 and a 16:1 divider was seen as 24:1. eg. out by 50%. I resolved the issue by wiring 2 x 48 volt power supplies in parallel and feeding a known 96 volts through the divider so I could measure a known standard.

So for a Proma to see 90 volts is entirely consistent with my observations. 130 volts divided by 1.5 = 86.7 volts.

Since I upgraded to a Thermal Dynamics cutter I've never had any such issues....

[SeanP]

Yes no need to worry about what the book says, check what you are getting at with no thc on say cutting a 6'' cut and set to that reading.
My old pm45 would be 10v less that the book says, even using that old torch on the new XP45 (which does work no problem) it's still 10v less than book, yet the XP45 with it's duramax lock torch in near enough dead on to book settings, work that one out

[Rodw]

Yes no need to worry about what the book says, check what you are getting at with no thc on say cutting a 6'' cut and set to that reading.
My old pm45 would be 10v less that the book says, even using that old torch on the new XP45 (which does work no problem) it's still 10v less than book, yet the XP45 with it's duramax lock torch in near enough dead on to book settings, work that one out

Thats actually fair comment. I compared notes on cutting charts I had come up with with a manufacturer who rebadges Everlasts and we were about 10 volts out between my machine torch and his hand torch. He said the machine torch torch I used pulled the volts down by about 10 volts based on his experience.

[acourtjester]

xnaron you are getting into the finer tuning of your table, and with the feedback here this post will be a help to others in the future. Your getting to the point of creating, gathering, downloading files for cutting and making sparks. For you bottle openers you may look at the posts here about soaking in Muriatic acid and water for a quick clean up, it will clean off light dross and mill scale.