With year-end outages approaching, many motors will be repaired and returned to service during the holiday season, when crews are stretched thin and attention is pulled in a lot of directions. Nobody sets out to make a mistake, but the date and time stamp on a test record may give insight into the root cause of a bad measurement. A recent case study from a motor repair shop shows why. A motor repaired between late December and the New Year was given a final Quality Control test before shipping to the end user. Inductive imbalance was under 1%, but resistive imbalance was 22%. The cause was a single coil connection that had been rolled over and taped but never soldered. The high frequency AC inductance measurement passes right through a connection that is barely touching, while the DC phase resistance measurement exposes it. Had the motor been shipped and started, it would have run…until that connection began to arc. Instead, the shop’s final testing caught it. They heated the coils to locate the bad joint and corrected it before the motor ever left the shop, a great example of a repair shop using the right test technology to deliver a reliable motor.
Don’t let a good inductance reading clear a repaired motor on its own. Make phase resistance and resistive imbalance part of your acceptance criteria, compare them to the pre-repair baseline, and take note of when the work was done.
To hear Todd and Noah discuss this and other case studies from the field, visit the PdMA YouTube Channel at A Forensic Review of Going Green: https://www.youtube.com/watch?v=TpBSGV-7SGs&t=1334s
