The conveyor on a PLC-controlled packaging machine occasionally stops for no reason, and operators are unable to restart it. This problem has occurred on four occasions. Each time, the tech has gone through the PLC logic, found nothing wrong, and simply restarted it.

Is the process of just checking the control logic really solving this problem each time? Nobody can come up with a better explanation, yet nobody can say why this seems to work. How can you determine what's really going wrong?

A major clue is presenting itself. The act of going through the PLC logic is having an effect. It's doing so by wasting time. Ordinarily, we don't consider it beneficial to waste time, but in this case, it is — sort of.

During the time the operators waste on failed restart attempts and the time the tech wastes going through the logic, something is changing. What changes over time for idled equipment? Temperature. Thus, the motor cools off enough to permit a restart. You can use a thermal camera to confirm this is correct.

To determine why that motor overheats, begin by checking for the following “top causes” of motor overheating:

  • Inadequate airflow to the motor. Clean the motor vents, and replace any filters.
  • Voltage imbalance. Phase-to-phase maximum is 2%.
  • Poor power quality. This includes low power factor, high harmonics, and distorted waveforms.
  • Improper bonding. See Article 250, Part V.