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Guard Against Voltage Sags

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 using data monitored over a three-year period, concluded that on average, nine sags occur for every power interruption. For more information on the EPRI study, visit

To the human eye, sags can cause the lights to dim. To automation equipment, sags can mean shutdown of equipment, loss of data and 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, reduced service levels to customers and less income for the company.

One method employed to protect against short-term power sags is to use power-conditioning devices to regulate and protect power. The alternative to adding these mitigating devices to the production equipment is to purchase and develop equipment designed to tolerate sags. This proactive approach takes more planning, but results in lower overall system costs.

Sag standard

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 regard to sag immunity performance for semiconductor tools and equipment. SEMI F47 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. (See graphic.)

The first evaluation step is to identify which components are critical to machine operation and would be adversely affected by voltage sag. Most motors, lighting and indicators can tolerate short-duration sag with negligible detriment to production.

The most critical and normally sag-sensitive component is the AC-DC power supply used to power all DC control and logic circuits. A majority of power supplies currently on the market average 10 to 20 ms 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 option 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. Another 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.

The preferred method is to use a power supply that 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 that they will offer you trouble free operation

Mike Johnson,, is a product manager at Sola/Hevi-Duty, Rosemont, Ill.

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