Valid arc lost & top limit is reached while THC moving up

snowgoer540 wrote:

If you click "Ignore ArcOK" and run a test cut, what happens?

I've been away from the machine yesterday.
Tested today with ArcOk "Valid Arc Lost". 
Tested with "Ignore ArcOK" it cuts fine with no errors in 5 cuts
But I have some irregular slats that it needs THC
 

Yes I'm in Mode 1.

Sorry I doesn't understand how it works hal scope very well here is a photo:

 

1 - I have tripled checked the Z axis scale and it's corret.
The commanded movement equals the distance movement of Z axis.

2 - Checked the connection of ArcOK and it's well tighten.
The ArcOK signal is a dry contact from Hypertherm cutter.
I have one cable in 24VDC and the other in the Input-15 of 7I76E.
From the cuts I think it cuts aproximadately the same amount when it shows the error. 
 
3 - Changed to "setp db_arc-ok.delay 10" with THC enabled 




EDIT: I've noticed that when Ignored ArcOK the voltage drops too much from 63 V.

I'm doing more test with Ignore ArcOK ON. It happened more frequently now the torch shuts off with almost the same lenght of cut but shows no error and continues to the end of the cut with the torch off (no stopping)

This is starting to sound awfully like intermittent behaviour due to oxidisation of the dry contacts  inside the plasma cutter because the high impedance circuit from the 7i76e input to ground (check the mesa manual). This results in a current less than the minimum spec passing through the contacts causing the oxidisation.

There are two ways to fix this, one is to use a pull down resistor between the 7i76e input and field power 0 volts to ensure the minimum current is exceeded. (What I do)
The other way is to add a capacitor to create a "spark" each time it operates to burn off the oxide.

Some others are vehmently opposed  to my views and I am not interested in creating an argument. All I can say I have personal experience of the frustration this causes. I am assuming that Hypertherm has used the same relay they have used in all their previous plasma cutters which is rated between 100 mA and 10 amps.

My recommendation is to use a HS10 100 Ohm 10 Watt resistor as the pull down. This will result in about double the minimum current. Expect a few cycles to burn off the oxide before reliable operation is restored.

If others propose  a different method or resistor value, just ask them if thy have any first hand experience with this particular issue before adopting their suggestions.