||By Mike Johnson, Vice President & General Manager, SolaHD, Rosemont, IL
One of the most frequent power quality disturbances is not the complete loss of power, but the sag, or short loss of line voltage. This often underestimated and overlooked event accounts for a significant portion of lost revenue caused by equipment damage and production downtime. In the blink of an eye, a voltage sag can bring production to a halt.
Fortunately there are ways to prevent sags from disrupting operations. A study conducted by the Electrical Power Research Institute ( http://www.epri.com ) using data monitored over a three-year period, concluded that on average nine sags occur for every power interruption.
To the human eye, sags can outwardly cause the lights to dim. To automation equipment, sags can mean shutdown of equipment, loss of data, unexplained resets. Over time, sags can stress components resulting in premature wear and failure. For processes that rely on high speed, any interruption can lead to significant production shortages. For processes that take hours to create one part, or a single batch of parts, process interruptions have a significant impact on company profits. Shutdowns result in scrapped work in process, production shortages, lower service levels to customers and less income for the company.
There are ways to protect equipment and productivity against the losses created by a sag. The most easily recognized way is to use power conditioning devices to regulate and protect power. Several power conditioning technologies are available including constant voltage transformers, three phase power conditioners and uninterruptible power systems. The alternative to adding these mitigating devices to the production equipment would be to purchase and design equipment designed to tolerate sags. This proactive approach takes more planning but results in lower overall system costs.
Many manufacturing and process industries focus on ensuring sag protection to maintain maximum competitiveness, productivity and quality. The Semiconductor Equipment and Materials Institute (SEMI) has gone so far as to establish a minimum standard with regards to sag immunity performance for semiconductor tools and equipment. The SEMI F47 standard introduced a well thought out voltage-to-time curve that most equipment will be exposed to during normal operation. In addition, a specific method of test and reporting has been developed.
The SEMI F47 standard specifies that equipment must be able to tolerate 50 percent line voltage for 200ms, 70 percent line voltage for 500ms, and up to one second for 80 percent line voltage. Although you could do your own survey of your facility and develop your own standard, using the SEMI F47 standard saves considerable time and effort (http://www.semi.org).
Several third-party independent laboratories can test for conformance to the SEMI F47 sag immunity standard. This means that regardless of what industry is being served, there is a system in place that you can use to ensure the products you purchase can handle the sags they will be exposed to during normal operation. The first step would be to identify which components are critical to machine operation and would be adversely affected by a voltage sag. Most motors, lighting and indicators can tolerate a short duration sag with negligible detriment to production — perhaps a fan slows briefly, or a light flickers, but production goes on. The most critical and normally sag sensitive component is the AC-DC power supply used to power all DC control and logic circuits.
Any type of computing or automation equipment must use an AC-DC power supply to supply the right power level for chips and components. A majority of power supplies currently on the market average 10 to 20ms of hold-up time at full load. These devices will not meet the sag immunity performance needed to work during common sag events without special considerations taken by the system designer.
One consideration is to use a universal input power supply (85-264 VAC) and power from the higher line voltage (208/240). This, of course, only meets the needed level of performance when powered from the higher line voltage. Operating from 208 or 240VAC is not always practical or possible. In North America, the most popular input line voltage is 120VAC.
Another option would be to de-rate the power supply to a lower output current in the hopes that it will perform better when exposed to input sags. This relies on the designer to test and verify they do not add loads that bring the power supply above the level at which it would meet the performance standard. This method is risky as any rework after the initial design or modifications by the customer could make the system susceptible to input sags. The preferred method is to use a power supply which meets the standard at full power and all voltage ranges. This gives the designer maximum flexibility with minimum design effort. Look for manufacturers who offer third party tested, F47 compliant power supplies or pay for verification of your current supplies to ensure they will offer you trouble-free operation in all environments.
Contact: EGS Electrical Group, 9377 W. Higgins Road, Rosemont, IL 60018 847-268-6613 fax: 847-268-6016 Web: http://www.emerson.com or http://www.SolaHD.com
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