Centricut Training Lessons

Troubleshooting PAC Systems - Starting Problems
by Dave Cook, Centricut Technical Services Director

Article originally appeared in
"Welding Design & Fabrication" - April 1999

WARNING! PAC system maintenance and repair should only be attempted by those skilled in electrical troubleshooting. PAC systems use high voltage and direct current (DC) electricity. Electric shock can injure or kill!

Symptom:
The torch fails to initiate the cutting arc when all other conditions for normal operation are correct: power supply energized, safety and parts-in-place interlocks satisfied, gas pre-flow at torch, and torch within transfer distance.

The sequence of operation for the typical PAC system is as follows: The start cut signal is sent to the power supply energizing the system. A solenoid opens, allowing gas flow to the torch. The pilot arc relay latches activating open circuit voltage (OCV) between the nozzle (+) and the electrode (-). A high frequency (HF) spark (see illustration below) is supplied to the torch by a high voltage generator with 3,000-10,000 AC volts output. (The high frequency generator usually includes a high voltage transformer, capacitors, spark gap assembly and coil.)

This high frequency spark ionizes the gas flowing through the torch making it electrically conductive. The ionized gas allows current flow between the electrode and nozzle. This current is limited to 20-40 amps DC by the pilot resistor. The arc blows out of the orifice and reattaches to the face of the nozzle forming a pilot arc (see illustration below). The pilot arc forms the electrical pathway to the workpiece.

If the torch is within transfer distance (usually .25" - .50" from the material), the pilot arc will transfer to the plate since the plate is connected to the positive pole of the DC circuit and is not limited by a resistor (see illustration below). A current sensing circuit senses arc transfer and shuts off the HF generator and opens the pilot arc relay.

In simple terms, the torch needs 3 ingredients to form a pilot arc: plasma gas, DC power, and AC high frequency. If any of the 3 is lacking, the torch will not fire, transfer or cut.

Troubleshooting:
When starting problems occur, the first step is to evaluate the pilot arc. Raise the torch several inches above the material so that the front of the torch can be seen clearly. In this position, signal the torch to fire. After several seconds of gas flow, a pilot arc should form at the torch. A "healthy" pilot arc will sustain itself in the air for several seconds. The arc should be a bright white-blue color and should project .25" to .50" from the end of the nozzle. The arc should not "spit and sputter" or sound raspy; it should be smooth and steady. Test the pilot arc a number of times in the air to see if the condition repeats.

Problem 1: Hard Starting
If the pilot arc spits and sputters but fires intermittently, the problem may be hard starting. Hard starting occurs when the HF is struggling to break down the high-pressure barrier between electrode and nozzle. This can be caused by insufficient HF or too much gas pressure. Here are 4 simple steps to remedy hard starting:

Check the gas pressure/flow:
The plasma gas pressure or flow setting should not exceed the factory recommended settings. Excess pressure in the plasma chamber makes it harder for the HF spark to jump the gap. The pilot arc is effectively blown out before it is fully established. High gas pressure accounts for the majority of hard starting problems and is often overlooked because it is so simple. (It is not uncommon for a zealous troubleshooter to replace every component in the pilot circuit only to find the air pressure had been turned too high.)

Clean and inspect the torch leads:
The leads are the hoses and wires that carry gas and power to the torch. Inspect them for visible damage or loose connections. The leads should be clean; if the shielding is covered with dirt, metal dust, and moisture, the high frequency energy can be dissipated. Blow off the leads with an air hose or wipe them down with a clean cloth. Coils in the leads should be eliminated because they can cause a large inductance. The leads should also be insulated from the cutting machine.

Inspect, clean, and re-gap the spark gap:
High frequency from the high voltage generator is usually sent to capacitors that discharge the electricity across a spark gap assembly. Over time the spark gap electrodes can deteriorate or become contaminated with metal dust and dirt. The electrodes should be cleaned and re-gapped to the manufacturer's specifications. (Usually around .015" to .030" depending on the system, the setting is usually listed in the instruction manual or on the power supply schematic.)

Check coolant resistivity:
If the plasma torch is water-cooled, check the resistivity of the coolant. Make sure the coolant meets the resistivity requirements of the manufacturer. For most systems it should be less than 10K ohms/cm or 10 micromhos.

Problem 2: Weak blue spark at torch
If there is a visible spark at the torch but it is a small blue spark that looks similar to the spark at the spark gap points, then the pilot arc has high frequency but no DC component. The most likely causes for loss of DC in the pilot arc are:
aDefective pilot arc relay: worn contacts or bad coil in relay
aDefective pilot resistor: pilot arc resistor is open

Problem 3: No spark visible at torch or at spark gap
aNo AC power to high voltage generator
aDefective high voltage generator
aDefective capacitors
aShorted or damaged spark gap assembly

Problem 4: No spark at torch, spark at spark gap
aSevere hard starting (refer to problem 1)
aShorted or open torch lead
aShorted or open connection in torch body

This article was published in
the April 1999 edition of
Welding Design & Fabrication.