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To achieve advances in plasma cutting, designers must balance component costs, cooling needs and reliability in extreme conditions

Thermal Dynamics introduced its North American-made inverter-based cutting power supply in 1987. The DynaPak 110 used 120-V primary power, provided a 20-amp output, cut metal up to 1/4 in., could be carried by hand, and sold for $1,290 (about $2,900 in today’s dollars). The unit revolutionized manual plasma cutting in fabrication shops of all sizes because it provided a previously unavailable combination of portability, performance, affordability and single-phase primary power input.

To understand how revolutionary this unit was, consider that before inverter technology, moving a plasma cutter around a shop often involved an engine block hoist, pallet jack or overhead crane. As for taking a plasma cutter into the field, forget about it. Today, plasma cutters are designed to be loaded into a job box and carried to the worksite.

Thermal Dynamics’ Cutmaster 40 plasma cutter weighs 22 lbs. and is rated to cut on 3/8-in. metal. With its compact size, multi-handle configuration and flexible input power, it is specifically designed for mobile fabricators.

All plasma cutting and welding equipment works by stepping down high-voltage, lower amperage AC line current to a voltage and amperage suitable for cutting or welding. Without going into too much technical detail (see here if so inclined), the equipment accomplishes this through a transformer and inductor that feature cores made from laminated steel plates and copper or aluminum windings. The sheer mass of “copper and iron” determines how much total power the unit can manage, but only if the frequency is fixed at 60 Hz.

Thermal Dynamics engineers knew that if they could double the frequency, they could cut the size of the cores or the number of windings in half; if they could quadruple the frequency, they could cut both in half. Thus, by boosting the frequency to tens of thousands of hertz, they could greatly reduce the mass of the transformer and inductor.

The component used to switch (“invert”) power at high frequencies is a specialized semiconductor called an insulated gate bipolar transistor, or IGBT, which is controlled by a microprocessor. As a result, transformers and inductors could go from being the size of a carry-on suitcase to the size of a baseball. This type of setup in a system is commonly referred to as “inverter-based” technology.

Another variation on faster power switching, called chopper technology, uses different components but accomplishes similar objectives. Choppers do not operate as quickly as inverters (around 5 kHz), but the design is simple and extremely rugged.

Cut evolution

Both inverter- and chopper-based technologies have seen significant advancements, but faster power switching and microcontroller advances are the reason power supplies will continue to get lighter and smaller.

ESAB’s Kris Scherm compares two 60-amp plasma cutting systems. The unit on the right, which uses more advanced technology, weighs 6 lbs. less, cuts thicker metal and features a more informative operator interface.

As an example, consider the evolution of the Thermal Dynamics’ Cutmaster family of plasma cutters. The Cutmaster 51, introduced in 2004, had a 40-amp output, maximum cut capacity of 5/8 in. and weighed 58 lbs. Now compare this to three modern technology offerings:

The Cutmaster 58, the direct successor, features the classic Cutmaster design with a roll cage but with an updated control board for gouging. It weighs 43 lbs. and the output has been increased to 60 amps to provide a rated pierce and cut capacity of 5/8 in. and maximum sever capacity of 1 in.

The Cutmaster 60i uses even more advanced power management technology. This is also a 60-amp unit, but it weighs only 37 lbs. and delivers a rated pierce and cut capacity of 3/4 in. and maximum sever capacity of 1 1/2 in. and offers excellent arc stretch for gouging. Operators can even see the increase in power output intensity in the brightness of the arc coming out the torch.

The Cutmaster 40, the newest plasma cutter, weighs just 22 lbs. but can deliver a 40-amp output for cutting 1/2-in. steel. It also accepts any input voltage between 90 V and 270 V, although the operator needs to connect the provided adapter plug for using 115 V primary. The system even automatically senses the input voltage and sets the maximum output accordingly.

Most state-of-the-art inverters for plasma cutting operate in the 20 kHz to 60 kHz range, which provides a sweet spot for a combination of benefits. From voice-of-customer research, ESAB knows that operators value the power-to-weight ratio (cutting amp output per pound of system weight) because they want portable units that take up less space. However, design engineers must balance factors related to component cost, cooling needs and reliability in extreme conditions. As a result, today’s products represent an ideal balance of physics, finance and functionality.

Because inverter-based power supplies manipulate the AC primary current to create cutting power, they are incredibly tolerant of low-voltage situations and line voltage fluctuations. Instead of the standard nameplate voltage and a narrow window (e.g., 230 V ±10 percent), they may have an entire voltage range, such as 115 V to 240 V ±10 percent or 208 V to 480 V ±10 percent. This makes them especially suited for running off generator power, which can be notorious for voltage fluctuations.

Plasma power supply controls and consumables have been developed to optimize gouging. The inset photo compares consumables for gouging (left) and cutting (right).

Inverters also draw less primary power, so they are less likely to trip circuit breakers and can better tolerate an under-sized extension cord that creates voltage drop. The garage or small shop fabricator will especially appreciate being able to get by with a 50-amp/230-V breaker yet enjoy 60 amps of cutting power.

Arc stretch

Fabricators can use plasma gouging to remove bad welds and torn metal, as well as scarf gouge away rivet heads and frozen bolts and nuts. Plasma gouging can be more precise than carbon arc gouging, as well as more convenient for shops that already own plasma equipment. Plasma gouging may look similar to cutting, but it places different demands on the system.

For plasma cutting, operators hold the torch at a consistent 1/16 in. to 1/8 in. above the plate. Conversely, gouging requires quite a bit of torch manipulation (as does trying to cut in tight corners or cutting near maximum thickness). As such, these applications require more “arc stretch,” which is how far the torch can move away from the plate, or through the thickness of the plate, before the arc extinguishes.

Basically what happens when a machine “breaks arc” is that it can no longer manage the voltage requirements (voltage increases proportionately with arc length). Some modern plasma systems provide notably better arc stretch than others, as their power management and controls have been designed with gouging in mind. In fact, many plasma systems have a gouging-specific operating mode to provide good arc stretch, faster arc restarts and manage gas pressure and arc force for gouging.

Gouging requires a “softer” arc – meaning it is less forceful and constricted – because the objective is to remove a wider swath of metal, not blast through it. The benefit of an inverter and microprocessor-controlled machine is that engineers can tailor electrical characteristics to match application needs without sacrificing performance. As a result, operators can switch between modes for cutting, gouging or expanded metal (grates) with the turn of a knob.

It’s worth noting that gouging-specific plasma consumables have also evolved. Starting about a decade ago, Thermal Dynamics developed four different sets of tips, each with a different orifice size to produce a particular gouging profile, from a deep, finger-like gouge to a wide, shallow bowl.

Much of the “magic” in plasma cutting happens within the torch, such as with Thermal Dynamics’ Black series electrode design that lasts up 60 percent longer and can be used in any SL60QD 1Torch system with up to 60 amps of output current.

Under control

Plasma cutters with software controls also help to convey more information to operators, notably for the faults common with all plasma cutters, such as low gas pressure or a “parts in place” fault to indicate a loose shield cup. On plasma cutters from 15 years ago, faults caused one or more of the LEDs to flash and change color, and the operator would then need to consult the manual to “decode” the indicator.

On plasma cutters from five years ago, the digital readout displays an alphanumeric code (e.g., E004 = loose shield cup), but operators still need to consult the manual to decode the fault. On the newest plasma cutters, the digital readout displays information that operators can intuitively figure out, such as PIP for parts in place or LoP for a low gas pressure fault. The human machine interface becomes more communicative and helpful, leading to more productivity and a better experience.

What’s next for plasma cutters? The next-generation of plasma cutters will get a platform upgrade with newer inverters and microprocessors, further reducing size, increasing portability and offering more location flexibility. Also, look for continued advancements in operator interfaces that further simplify set up and operation, as well as convey more information.

Lastly, remember that all the arc physics come together in the plasma torch and its components. What can seem like a simple enhancement, such as a new electrode design that extends consumables life, takes thousands of hours of R&D effort, not to mention manufacturing investments. The good news is that plasma equipment manufacturers are constantly looking for the next performance improvement.

Thermal Dynamics

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