What to know before selecting a manual plasma cutter: Understanding size, power, components, cost

PRACTICAL WELDING TODAY® JULY/AUGUST 2000

July 26, 2001

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The first plasma arc cutting (PAC) systems, developed in the '60s, were 1,000-amp monsters designed to blast through 6-inch stainless steel.

The first plasma arc cutting (PAC) systems, developed in the '60s, were 1,000-amp monsters designed to blast through 6-inch stainless steel. Their mechanized torches were moved by X-Y cutting machines and powered by DC units the size of refrigerators. Surprisingly, the PAC industry evolved from high- to low-amp systems, water- to gas-cooled, and from gas- to air-cooled.

Today's hand-held air PAC systems are lightweight, portable, and relatively powerful for their size. They are used for cutting everything from thin-gauge metals to 1-inch plate. More traditional console PAC systems also are available to handle cutting tasks up to 2 inches and more. Hand-held PAC systems are now the fastest-growing segment of the PAC market because they offer a fast, efficient, and affordable way to cut.

This article offers an overview of manual PAC technology from the early days to the present, including an explanation of different power supplies, recommendations for selecting and sizing a system, and other functions and features to look for in a hand-held system.Regardless of the size, all PAC systems contain the same basic components, including a gas supply, DC power supply, and plasma torch. The torch requires a circuit to initiate an arc and a cooling system.

Gas Supply

Most older plasma systems used nitrogen as the plasma gas and air or CO2as the secondary gas, which required expensive bottles or bulk containers. Now, most hand-held systems use clean, dry shop air to cool the torch and provide the necessary plasma gas.

Shop air currently is the most affordable and versatile plasma gas. It is readily available and provides good cut edge quality on mild and stainless steel and aluminum. With the exception of special applications, such as thick stainless steel and aluminum cutting or plasma gouging, almost all hand-held systems today use air plasma. Several manufacturers even have developed air plasma systems with small, onboard air compressors.

Power Supplies

PAC power supplies are direct current electrode negative (DCEN). The process requires a constant source of DC and a high open circuit voltage (OCV) to initiate the arc (typically at least twice the operating voltage). The following is a summary of some basic differences among PAC power supply types.

DC Droopers. Early plasma systems included drooper power supplies, named for their drooping output power curves. These units provided a high OCV and relatively stable current and operating voltage. They used a fixed-output DC rectifier bridge consisting of a series of diodes to convert AC power from a transformer into usable DC for the cutting process.

These simple systems created a lot of power but wasted energy and had too much ripple in their output power. (Ripple is fluctuations in DC output that cause a rough cut and short part life.) To further regulate power output, multiple transformers could be used, each providing a higher level of output current.

Reactors. Reactor power supplies were the next step in power regulation. These used a reactor device to control the amount of AC voltage supplied to the bridge rectifier. The reactor consisted of a group of AC coils with a DC winding around it. The current in the DC winding controlled the amount of AC that passed through the reactor, which created an adjustable transformer that allowed variable DC output from the bridge.

SCRs. Silicon-controlled rectifiers (SCRs) are another type of continuously variable output power supply. SCRs convert three-phase AC power from a transformer directly to DC. They require huge capacitor banks and large transformers. SCRs are large and powerful and are used for high-amp PAC systems but are not well-suited to hand-held applications.

Switch-mode. Switch-mode power supplies use transistors to modulate DC power after the rectifier. Choppers are a type of switch-mode power supply that use power semiconductor devices such as isolated gate bipolar transistors (IGBTs), which take raw DC with ripple and chop it up, rapidly switching the power on and off to smooth the output characteristics. IGBTs can be fired much faster than old reactor-type power supplies. The result is a very smooth output power curve.

Inverters are another type of switch-mode power supply. They use devices such as transistors on the input side of the power train to raise the frequency of the AC into the transformer. Higher frequency input allows a much smaller transformer to be used. Because a smaller transformer is used, inverters are much lighter and more portable than conventional power supplies, making them ideal for hand-held applications.

Early inverter power supplies were limited by low output current and complicated design and poor reliability. When problems occurred, sophisticated techniques and troubleshooting were required to solve them.

Today's inverters are more reliable, robust, and powerful. Most manual PAC systems now use inverter or switch-mode technology. These sophisticated, electronically or microprocessor-controlled devices are better able to tolerate variations in line voltage, take more abuse in the field, and deliver better cutting performance while consuming less power

.

Torches

All plasma torches contain the same basic elements, including:

1. an electrode to carry the negative charge from the power supply.

2. a gas distributor, or swirl ring, to spin the plasma gas into a stable, swirling vortex.

3. a nozzle to constrict and focus the plasma jet.

The torch is primarily a holder for the consumable parts. Torch improvements have been aimed at optimizing the torch and consumable designs to improve cooling, enhance starting characteristics, and increase cutting capacity. Improvements also have been made in material selection for consumables and torches to improve durability, such as using high-temperature durable plastics in place of ceramics.

Ergonomics have improved with features such as trigger torches, better handle designs, and options for torch angle or adjustable torch heads. Safety improvements include parts-in-place (PIP) circuits and switches or triggers to prevent the torch from firing without the parts properly installed and the operator ready.

Most hand-held systems on the market today use one of two methods to initiate the plasma arc. The tried-and-true method is a high-frequency (HF) starting circuit built into the power supply. This system uses a high-voltage transformer (similar to a bug zapper), capacitors, and spark-gap assembly to generate a high-voltage spark at the torch.

The spark ionizes the plasma gas, enabling current to flow across the air gap between the nozzle and electrode. The resulting arc is called the pilot arc. High-frequency starting systems are simple, relatively dependable, and require no moving parts in the torch. However, they do need periodic maintenance to prevent hard-starting problems. Another potential problem is that high frequency radiates from the system, creating electrical noise that may interfere with sensitive electronic equipment.

Contact start torches use a moving electrode or nozzle to create the initial spark that enables the pilot arc. When the torch is fired, the electrode and nozzle are in contact in a dead short, or short circuit. But as the gas enters the plasma chamber, it blows the electrode back (or the nozzle forward), creating a spark. This process is similar to the spark created when a household electrical plug is pulled quickly from a receptacle.

Contact start torches produce much less electrical noise than HF systems. These also are instant-on torches, which reduce cycle time because of the lack of preflow.

Cost of Operation

Many variables contribute to the overall cost of operation for PAC, including labor, power, duty cycle, gas, shop air maintenance, consumables, consumables life, speed of cut, amount of cleanup, or secondary operations required.

The two most important factors to consider when purchasing new equipment are consumable cost and consumable life. Because the part life of different systems varies, consumable cost alone is not the best measure of a system's cost of operation.

Consumable cost, or the total consumable cost divided by the consumable life in hours of arc-on time per hour, is the most useful measurement. For example, if the cost of a nozzle is $4, the cost of the electrode is $6, and together a set lasts 2.5 arc hours, then the cost per hour, or CPH, is ($4 + $6)/2.5 = $4.

Just the nozzle and electrode are used for this calculation because the other consumable parts are designed to last much longer. To calculate CPH for all torch components, a weighted average should be used based on usage ratios. Typically, shields, swirl rings, and caps outlast nozzles and electrodes in a minimum 20-to-1 ratio.

Before purchasing a new hand-held system, a company should do its homework by:

• Knowing what kind of power supply technology is used and understanding the system's capacity.

• Not skimping on power by purchasing enough power to do the job with ease.

• Calculating the cost of operation.

• Looking for an ergonomic and safe torch design.

• Testing the sales claims with a real-world cut test.

• Making sure the system is backed up by a good warranty.

• Adding technical support after the warranty expires.



David Cook

Contributing Writer

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