You may not be able to see them but power anomalies that originate within your automated control system are costing you expensive downtime. Your automated production equipment has a low tolerance for poor power quality, especially if it is controlled by a PLC(s).
Improper grounding can account for up to 40 percent of power-related problems including costly damage and downtime. In addition, if not adequately suppressed, surges (a high voltage spike or impulse of very short duration) can account for another 40 percent.
Surges can be produced by lightning or utility companies switching feeders or capacitor banks. The most common surges are produced within your facility by equipment cycling on or off (or speeding up or slowing down). These surges, lasting only a short period of time (microseconds), are injected onto power and data circuits, causing equipment damage and safety hazards. Problems like these can be avoided by implementing a single-point grounding system that follows the National Electrical Code (NEC) for installation and by the use of properly selected and designed surge suppressors.
Most people do not think about power quality issues like surges until they have a storm; however, the factory environment itself can be an automated control system’s worst enemy. The automotive assembly line depicted in Figure 1 is an example of an environment that has a constant flux of surges. Production equipment, such as robotic welders and conveyor system motors, is constantly cycling on and off, causing a significant back current (a common type of surge) to be induced into the power system.
A variable frequency drive (VFD) itself generates disruptive harmonics within its internal switch-mode power supply and drive circuits that reflect back into the branch circuits. Even state-of-the-art systems designed to isolate the VFD from the distribution are plagued by electromotive force (EMF) induction surges. These transients interrupt data transmissions between the PLC and its I/O as well as plant communications.
Electronic control components and data protocols are prone to damage from the very same motors that are under their control. They are perpetrators of problems that will potentially cause your system to be out of sync, produce defective products, or stop your production line from running.
Most systems incorporate AC protection on the front end of the control panel but fail to install it on other surge paths. PLC input devices (proximity sensors, photo-eyes, intelligent vision systems, etc.) offer surges a back-door path to the PLC. Other connections — such as remote I/O, Fieldbus, RS-485/232, Ethernet, ControlNet, and DeviceNet — are also paths for surges to the PLC.
A surge introduced by any I/O connected component can damage control equipment or cause system failure. Each connection among PLCs, input devices, and each AC motor system requires protection. Designing a system that has only front-end surge protection is like trying to secure your home by locking the front door while leaving other doors and windows open.
To establish a robust system, adequate surge protection of all connections is paramount. Improving power quality for your sensitive electronic equipment will decrease the number of seemingly inexplicable glitches in your production line. You can decrease downtime and production defects by investing in the appropriate surge protection.
A PLC operating in an environment of continuous surges will suffer from data corruption and premature failure. With the proper combination of surge protection and filtering, the PLC’s power sources can be effectively protected from upset or damage from these sources. Figure 2shows a control cabinet and incoming power that requires AC surge protection. All suppression products should be mounted as close as possible to the equipment it is designed to protect.
In addition, a single-point grounding system, where all connections to the ground come to a single point in the facility before referencing the earth, is essential for the installation. This is accomplished by connecting all grounds back to the original neutral-ground bond in the building or the secondary neutral-ground bond of an associated step-down or isolation transformer. This is accomplished by connecting all grounds back to the original neutral-ground bond in the building or the secondary neutral-ground bond of an associated step-down or isolation transformer.
With all of these considerations, it is beneficial to select a surge protection partner with appropriate expertise in power and grounding issues. A qualified supplier will offer a plan for power quality of the complete system rather than pieces of the whole.
This article was contributed by Transtector, Hayden, ID. For more information, visit here .