[yamen moner]

When my Cnc plasma cutting pierces the arc sometimes working good and sometimes no, good main the arc on and off time working at right time, but sometimes ( off ) not working right and late to end arc, machine go to the next Pierce and arc still on , l change relay in THC card with new relay but still arc on right and arc off is late, I tried to connect the arc tourch direct with the lgk power source without THC card also still the problem arc on right but arc off late, how can I fix it please

[rodw]

You need to provide a lot more info.
What Plasma? Machine Torch? Pilot Arc Start or HF start?
What Controller?
What THC.

[coherent]

Lots more info needed. Include software to the list above. Possibly a software issue. Check your code before running it first and see if all the correct on/off code is included and your pierce delays etc are correct before you start trouble shooting hardware.

[shad71]

I tried to connect the arc tourch direct with the lgk power source without THC card also still the problem arc on right but arc off late

This delay has a some LGK plasma cutters. Delay depend from the capacity of the elctrolythic capacitor in the plasma start input. Usually we find this cap and change to lower value (was for example 470 uF - change to 22 uF). Capacitor is on the main control board - just trace a start signal from connector on the cutter face panel to this PCB and find this cap (near this cap usually there are a rectification bridge - 4 diodes)