During the brutally hot summer of 2012, nobody was particularly surprised when it turned out that overheating was responsible for the high rates of motor failure. But now, in October, it's much cooler, yet the failure rates are still high, and the motor shop's "autopsy" reports still show overheating as the cause of motor failure. The plant manager says this can't be right. He and several others have asserted that if motor failures were due to overheating then they should decrease with cooler weather.
Now the plant manager has tasked you with finding the failure cause. What should you do?
It's unlikely a motor shop would misdiagnose overheating once, much less misdiagnose this consistently over time. Trust this diagnosis until you can rule it out. Problems other than high ambient temperatures can cause motor overheating. In this case, these two conditions might not have any causal relationship. You need to investigate why the motors might be overheating.
As a first step, have a qualified thermographer with experience in motor thermography conduct a thermographic survey on the running motors. Also, ask the motor shop to provide more specifics on the overheating. Solving for overheated bearings is different from solving for overheated windings. What exactly do they mean by "overheating?"
Then, look for these likely causes:
· Voltage imbalance. Ensure less than 2% between phases.
· Vibration. Use a portable tester if there's no monitor.
· Harmonics. Use a power analyzer.
· Grounding. Ensure the motors are bonded (Art. 250, Part V), not grounded.