A motor drives a pellet feeder system to a reactor tank, which is supposed to output a batch every 20 minutes. But sometimes an hour goes by with no output. The motor, which is under PLC control, seems to “konk out.” The plant manager, in his current position only six weeks, is understandably nervous. He said, “Either fix the PLC logic or replace the motor. It has to be one of the two.”
What’s your next step?
With all due respect to the plant manager, it’s probably neither. Your next step should be the first step in troubleshooting a PLC-controlled system: check the final control element(s).
If that drain valve is sticking, for example, its limit switches won’t provide the signal that the drain cycle has occurred. So the PLC won’t tell the motor to provide more material for the next batch; this one’s still in the vessel.
If the valve is OK, check the limit switches. Maybe work this system backwards from the final control element(s) to the PLC, and keep going until you’ve addressed everything on the output side. Then look at the input side.
