CNC MACHINE GROUNDING
We sometimes get calls about “grounding.” People want to know why they need to ground their system. Grounding of a CNC plasma machine is important for a few reasons. Some are related to safety and some to electrical noise interference.
Noise issues created by high frequency start plasma systems
Older technology air plasma systems, many imported low cost air plasma systems, and almost all industrial systems like Hypertherm’s MAXPRO200 or HyPerformance Plasma systems use a high voltage high frequency discharge at the beginning of every start cycle. A 15,000 volt, 3 megahertz discharge causes a small arc to transfer between the nozzle and electrode in the torch creating a path of ionized gas that eventually starts the plasma. While effective, a high frequency start can shake up a low voltage motor, encoder, and computer inputs and outputs. This is especially true of CNC machines that use a standard office computer or laptop as the machine control. The impact of a high frequency start can cause irreparable damage or, at best, destroy computer circuitry and drive electronics. This is why proper earth grounding, shielded computer and drive cables, and electronics with proper I/O input filtering are a must.
Noise issues created by inverter based plasma systems
Newer technology inverter based supplies like Hypertherm Powermax systems, can also create electrical noise interference. Fortunately, the problem isn’t as severe. Inverter power supplies develop a "pulse width modulated" output frequency in the range of 16 kHz to 60 kHz, depending on power supply and manufacturer design. Typically the impact of this technology, especially on stepper driven CNC machines is rough motion or strange error messages on whatever PC is used as the machine control. Usually a good earth driven ground and proper grounding of each electronic component of the machine to the earth ground will eliminate issues when this type of plasma is used.
Electrical grounding issues that affect height control
Poor work lead connections to the actual plate, a poor cutting machine frame, and cutting bed ground connections will also cause issues with your height control. This is because the height control on most CNC plasma machines is dependent on establishing a good arc voltage reading between the material being cut (positive) and the torch electrode (negative). Poor grounding and stray cutting table voltages can and often do affect the control of torch height on arc voltage based systems. For this reason, industrial grade CNC machines all use a specifically designed hardened, shielded, and isolated CNC designed to withstand electrical noise interference from any type of plasma. Despite this, industrial tables, if properly installed, all have a dedicated ground rod for safety and operational issues.
Owners of smaller, non-industrial CNC tables experiencing interference issues can most likely address the problem by adding a ground rod and an input line conditioner to your computer and drive electronics. This is easy to do by purchasing a common UPS (uninterruptible power supply) from most any office supply store. As mentioned earlier, entry level machines that use an office PC or laptop are far more susceptible to electrical noise and safety issues, so it is important to have a dedicated, properly designed grounding system.
On a semi-related note, people sometimes think the work lead from the plasma will meet their grounding requirements. This is not true. The plasma work lead should not be considered a "ground", rather it only serves as the return path for the cutting current.
As always, if you have any questions of problems, Hypertherm's North American Technical Service Team is happy to help. Contact the team via email at technical.service@hypertherm.com or by phone, if you're in the U.S. or Canada, at 800-643-9878.
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