Cut 40 circuit diagram, anyone got one, or ideas on my CNC issue?

[AdrianH]

Having bad day with the plasma CNC cutter, going through control chips like smarties!

Always on either start-up of arc or end of cut when there seems to be a sputter of the plasma cutting, like when the metal drops away and leaves a hole. I think the HF circuit is kicking in and I am getting hight volts into the control side some how?


I have tied all the earth’s together with good braided copper cable and these are a good connection, so the earth of the Plasma unit to the CNC frame and another off to the control box with PSU and CNC controller.

I then have the grounding clamp on the CNC frame and into the front of the plasma and a wire from the torch tip connected to to the grounding clamp.

I have done some continuity tests and discovered that the ground clamp is not connected to earth/chassis, so actually wondering if the CNC frame should be connected to ground, It should in my mind, but without finding a CUT 40 circuit or similar I can not be sure.

So just putting out to you knowledgeable lot to get ideas?

Adrian

 

[normspanners]

A diode pack gone open ? allowing the hf back into the dc side of things or even a buffer cap not absorbing the excess.??

 

[MattH]

Is any of your control side electronics Opto isolated?

 

[AdrianH]

A diode pack gone open ? allowing the hf back into the dc side of things or even a buffer cap not absorbing the excess.??

As far as I can tell, the plasma is working fine, now there could be an issue with the unit but, when I plug the hand torch in and cut manually all is fine, it my be there is insufficent filtering in the unit but I struggle without diagrams on these.

Is any of your control side electronics Opto isolated?

The computer side is all opto isolated, but it is always the stepper motor drivers IC's that fail. I should have said this sorry my bad.

I am wondering if earthing the plasma fame and motors is causing me an issue with the HF, it should not in my mind.

I could get some high voltage caps or surge suppressors? from the motor wires to ground and fit to the wires from the motor windings to the drivers and take these down to earth.

I am wondering how the better made/more expensive plasma torches that have the pilot arc work if they have a direct cable back to the neg terminal or clamp terminal. thinking of doing that and having a shorter cable to the workpiece.

Running out of IC's need to order more.

Adrian

 

[normspanners]

Could try a crowbar circuit with an overvoltage ic device, maybe it has one and its gone oc.
this type of thing http://www.onsemi.com/PowerSolutions/product.do?id=MC3423
they can be a small thing that looks like disk cap type but are a crow bar. I have seen them blown apart. in thunder storms etc

 

[normspanners]

Overvoltage devices can be divided into two categories: clamping and crowbar devices. Crowbar devices go to a short circuit when they trip. (The name “crowbar” comes from the concept of dropping a crowbar across a pair of power lines and shorting them out.) Clamping devices don’t go to a short circuit when they trip; however they do go to a low enough resistance to limit the voltage to a rated clamping voltage. In a perfect world, this voltage would be constant no matter what level of current is present. But, in reality, clamping devices (like MOVs) have a higher clamping voltage when the current is higher.

 

[AdrianH]

Possible Norm, there seems to be a sensing circuit that when the arc is started shuts down the HF, wondering if due to the extra wires that this is not working properly and the HF is always running, reading someone on the CNC forums talking about it, may have to join to get more information.

I could try running the box without covers and see if I can see the HF contacts internally and see if still running.

Another day I think.

Adrian

 

[AdrianH]

Norm missed your second post there, thinking of the MOV's, rather then the crow bar stuff, use crowbars protection in gas filled devises for Lighting protection on units and once gone that is it. The MOV's do age with use so I believe. Not sure if they do these in the 24 to 30 Volts range?

Adrian

Just looking on the web and yes they do low voltage stuff such as the 2V030 may be OK for a 24 Volt stepper driver, still a lot higher clamp voltage at 90 volts, but worth a try., or some end to end Zenner diodes say 28 Volts ones.

Adrian

 

[normspanners]

Sounds good, could be the hf is still running and when the cut comes to an end and the load comes off, the hf could then go through the roof with a big spike?

 

[MattH]

What kind of stepper drivers are you using?

 

[AdrianH]

The drivers are the cheap and cheerful TB6560AHQ 3.5 Amp units.

What gets me is it all worked for a while?

Adrian

 

[normspanners]

Had one of the small disc ones on a 3phase roller shutter door once, the door jammed and I goot up and freed it, then suddenly started to come down at full speed, possible half ton of door to start with but getting heavier all the time !!
The motor became a generator, it took out the 12amp mcb and the main 100amp one BUT protected the control gear, it was in pieces though, when I fished it out,it had blown it off the board ! you would not think a small ic like that could stand so much, and take out 100 A

 

[AdrianH]

When they go short, they are short, will get some ZZZZZZZZZ's and think about it fresh in the morning.

Adrian

 

[MattH]

So the drivers you are using have no Opto isolation!

All this talk of Mov's etc is locking the gate after the horse has bolted. Anything above 5v for a micro second on your control lines is going to kill those drivers.

There's a reason they are cheap and it's the same reason people generally don't use them for anything other than an MDF machine with threaded rod for screws.

I have a bunch of stepper drivers in a draw. Let me dig one out and send it to you for some testing. I'll post up some pics and info tomorrow.

 

[AdrianH]

The bords have opto,s fitted on the control side, it is the power side I believe picking up the high voltage. The 4 wires to the steppers.

Adrian

The control side is still flashing away.

 

[gerrymoore]

I had a problem with some of my stepper drivers (good quality versions) blowing the FET's...it was nothing to do with grounding and I have now removed all my grounds from the machine and wiring and it works perfectly. It was the Chinese psu I was using to supply the drivers. Bought a better quality one and - touch loads of wood and my head twice - no more probs....worth a look anyway.

 

[MattH]

I very much doubt you are blowing the power side on the drivers, they are designed to handle spikes of tens of thousands of volts in back EMF from the motor coils. 99-1 you are blowing the inputs.
Are you running limit switches / homing switches? Are all you signal cables shielded? Etc etc.

 

[MattH]

can you post a pic of you electronics enclosure to give us an idea of the overall layout?

 

[AdrianH]

I am not running any limit switches or home switches, the fault shows it self as one of the set of outputs not providing any voltage, AP and AM may have nothing, with BP and BM still provides output but because only one coil is being energized the motors only buzz. I am also using shielded cable with earth’s at the controller end to drive the motors., well up until the 4 wires going into the motors so there is around 150mm of un-screened at the ends.

The input from PC to is a shielded parallel cable or at least there is continuity between the metal parts either end.
The 24 Volt 14 amp power supply is a Farnell/RS electronics metal box industrial unit, so hope that is OK, no signs of spikes on the volt meter anyway, not that I would see them.

I am about to go out for an hour of two to a college but will post pictures.

Adrian

 

[MattH]

Just a thought but you say you have tied all your grounds together. Are you including the 0v DC rail of your driver/motor PSU. If you are then you might want to disconnect this and allow 0v to float.