The Cold Facts — A Case of Overheating Motors
Motor failures can be a mystery, especially when their causes originate outside a facility’s walls. That’s the conundrum that faced a small motor shop recently, when a spate of failures sparked an investigation of the seemingly random breakdowns. The machinists called for assistance from the motor-rewind shop that had been repairing the failed models—a not-unusual resource for those facing motor meltdowns.
The winding-shop supervisor began his investigation by searching for patterns that could tie the failures together. Age wasn’t an issue, as problems seemed to be affecting both old and new units. However, all the affected motors had been carrying full loads, and their windings showed they’d been overheating.
The machine shop hadn’t undergone any changes since the motors started failing, but an adjacent building had taken on a significant new tenant—and insurance company—and the two operations were served by the same utility transformer.
The rewind-shop supervisor’s prime suspect became possible nonlinear loads within the insurance building, and he proceeded in the best deductive fashion to prove his theory, with the help of a handheld power-quality analyzer.
He started by connecting the unit phase-to-phase at the machine shop’s main service panel, and discovered a flat-topped voltage waveform with a total harmonic distortion (THD) of 7.8%. Another key discovery: the 5th harmonic was the dominant bar on the harmonics display. The supervisor also used the analyzer to compare the current drawn by fully loaded motors to the full-load-amp values on the units’ nameplates, and found the motors were drawing more than the rated values by a small amount.
This collection of observations helped the supervisor nail down the cause of his customer’s problems. The 5th harmonic on a three-phase induction motor produces a magnetic field that tries to run the motor backwards. As a result, the motor draws more fundamental current to offset this effect, which results in overheating. In this case, the boosted 5th harmonic was caused by the nonlinear loads generated by computers and office machines in the adjacent insurance building.
The utility, when notified about the problem, agreed to install a separate transformer to serve the machine shop alone. Again, the handheld analyzer came in handy, measuring distortion on the new transformer was measured at an acceptable 3.3%.