Electrical Troubleshooting Quiz — April 7, 2020
On the Line 7 drying oven, one of the motors was replaced for the first time in 15 years. That was 18 months ago, and three replacements have been made since then.
The motor has a variable speed drive, and the drive has been thoroughly checked and ruled out as a possible cause. Voltage imbalance, low voltage, high voltage, voltage distortion, and excess harmonic distortion have all been ruled out as well. A portable power analyzer was hung on the branch circuit after the second failure, and it recorded no transient events during that time.
One theory was that a bonding jumper was missing so undesired current flowed through the motor casing; that, too, was ruled out. Can you solve this?
Are you sure it's the right replacement motor? Perhaps the wrong motor data got entered in the computerized maintenance management system (CMMS). Go back to the original drawings and specs to find which motor should be installed. It's on a drying oven, and that's a good clue. The original motor was probably temperature-rated, and maybe that rating did not get entered in along with the other motor data. Thus, after a few months, the motor insulation fails.
What is the failure mode? If it's bearing failure, the problem is most likely incorrect lubrication. If it's insulation failure and the replacements match the original spec, determine if something has changed with the drying oven to require a higher temperature rating.
Another place to look is at the replacements. Where are these coming from? Have they been properly stored? Does anyone take baseline test data on them, or even test them at all before installation?
About the Author
Mark Lamendola
Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.
Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.
Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.