Electrical Troubleshooting Quiz — Feb. 5, 2019
Line Seven uses temperature sensors and a PLC to monitor the temperatures of its motors. The PLC initiates a line shutdown if any motor is running hotter than its set point temperature.
The thermal overloads in the supply protect against excess current (based on heat from overload) in the supply conductors. This scheme protects the motors based on the actual motor temperature.
Lately, those shutdowns have occurred too often and it’s not the same motor each time.
Another electrician left a recording digital multimeter (DMM) on the feeder for three days. Despite a shutdown, the DMM didn’t catch any voltage sags.
The process engineer wants you to solve for the cause of the nuisance shutdowns. How might you solve this?
DMMs are indispensable tools for troubleshooting, but you don’t bring a knife to a gunfight. You need a power analyzer so you can look for waveform distortion, excess harmonics, and low power factor. You may have all three present, and any one of them could be causing this problem.
This protection scheme is problematic. See if the process engineer will change the control logic so that maintenance gets a high temperature alarm at a slightly lower setpoint than the shutdown one (and maybe that can be removed or raised, at least temporarily). This will allow for troubleshooting during the conditions leading to the temperature fault.
What about motor ventilation? Is airflow to the motors blocked? Would fans and/or ducting help? Is waste process heat being properly removed?
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.