COOPERATIVE RESEARCH DETERMINES POWER QUALITY EFFECTS ON MACHINE TOOLS.

NIST scientists have been working with the Electric Power Research Institute (EPRI) to investigate the need for power quality ride-through standards on machine tools. Such standards define the level of power quality events a piece of equipment must be able to tolerate (ride-through) before it automatically shuts down. EPRI standards have been developed and adopted in a number of areas, particularly within the semiconductor equipment industry. Last summer, EPRI engineers and NIST staff measured the performance of a machine tool and several machine tool performance measurement devices (e.g., lasers and encoders) in the NIST Sound Building during synthesized power quality events. This research resulted in close collaboration between NIST and EPRI for conference presentations, participation in power quality workshops, and standards development for the performance assessment of machine tools. The resulting power quality analysis technique and solutions are being considered for a number of machine tools across the United States, with the potential for great cost savings in time and materials.

Since purchasing a particular machining center, NIST has experienced unacceptable machine behavior during local power events that have damaged in-process parts. Malfunctions occur when typical local power events unexpectedly shut down the machine's controller, resulting in the machine spindle falling into the part due to gravity. So far, two frames and one base for the joint U.S. Charters of Freedom Project with the National Archives and Records Administration have been damaged with much loss of time and materials. This machine behavior is not reproducible using the emergency stop function, during a controlled shutdown, or when halting the power using the main breaker. In correspondence with the machine tool manufacturer and controller provider, the local machine distributor was not able to resolve this problem. An option to install special power monitoring and supply to eliminate power fluctuations was not affordable.

In response to this need, the NIST-EPRI collaboration conducted extensive testing of the machining center. The machine performance was measured while subjecting it to varying levels of voltage sag across electrical phases for varying durations. At 60 % of line voltage (a 40 % drop in line voltage on two phases, A and B) and for as little as 5 cycles in duration, the machine controller shuts down in an error mode and the spindle consistently drops up to 18 mm due to gravity. Voltage sags of this level are common in raw, utility-supplied power. The testing also established that the machine electrical systems, with the exception of the controller power supply, stay functional until the 50 % threshold is exceeded.

The testing was able to replicate the machine behavior such that the problem could be quantified. By wiring a constant voltage transformer (CVT) in series with the controller power supply, the problems of machine malfunctions and damaged in-process parts were overcome. This solution allows the controller to sense an under-voltage condition and shut down the equipment in a controlled fashion. The CVT has been installed permanently on the machine in question, and NIST staff will be monitoring the resulting installation for long-term performance under actual conditions. Further collaboration between NIST and EPRI will add to the existing knowledge through evaluation of additional machine tools and power quality effects.