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?
