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Pinpoint Voltage Sags from Hard-Starting Motors
When customers complain of flicker or unexplained equipment resets, the culprit is often a neighboring load whose inrush current drags feeder voltage below tolerance. This case study follows a Midwestern utility that deployed a PMI ViP three-phase recorder at a 1 MVA grain-elevator transformer and an Eagle monitor at a nearby business after multiple early-morning flicker calls. Time-correlated waveforms revealed two 150 HP motors hard-starting without the mandated soft-start setting, pulling current peaks near 1,300 A and producing 38 V dips at the adjacent service entrance. An hour-by-hour timeline, supplemented by strip-charts and RMS plots, links each dip to motor-control selections at the customer’s MCC panel. Engineers confirmed the finding during a field inspection, then recommended permanent soft-start configuration or VFD retrofits to eliminate future events. The paper demonstrates how dual-location monitoring turns finger-pointing into data-driven root-cause analysis and provides a template for documenting voltage-sag disputes.
Key topics include:
- Initial Conditions
- Review of Evidence
Why utilities should care:
Three-phase motor starts are routine, but when soft-start circuits are bypassed they can impose deep, feeder-wide voltage sags that spark customer claims and erode reliability indices. This study supplies unmistakable PQ fingerprints of a hard-start event, illustrates the value of synchronized monitoring at both the suspected source and point of complaint, and quantifies the operational and financial impact of unchecked voltage dips. Reading it arms distribution engineers with the diagnostic workflow and mitigation options needed to resolve sag complaints quickly, justify equipment upgrades, and protect SAIFI/SAIDI scores—before the next high-horsepower motor comes online.
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