Question: Occasionally, a drive motor speeds up and slows down with no change of control input. But the production process depends on exact speed, and this problem is causing excess scrap rates. What steps should you take to troubleshoot this?

Answer: One common response to such a situation is to look in the motor control logic. However, that’s probably the last place you should look. Begin by putting a power logger on the motor supply. The speed variations could be due to voltage sags and swells or other power anomalies.

While the power logger is collecting data, perform these other steps:

  • Inspect the bonding connections. Look for corrosion and broken connectors. Make sure there’s no difference of potential between the motor case, the supply transformer, and the building steel.
  • Review the power cable testing data and trends. A small change in slope near the end of the trend may indicate the start of deterioration that has accelerated since the testing. Periodic current leakage can happen when degraded insulation leaks into normal condensation around a cable. After the condensation dries, the leakage stops — thus the cycling effect observed here. The good news is this problem will solve itself. The bad news is this problem will solve itself, because it’s a prelude to total failure.
  • Inspect and test the signal wiring connections. Loose connections could be causing swings in signal input to the controller.
  • Suspect inductive coupling on the signal wires. Ideally, signal wires should be separated from other wiring by as wide a space as is practical. Sometimes, what’s practical isn’t much, and signal wires are too close to other wiring. Use an appropriate instrument to monitor/record voltage on the signal wires. If you discover a “ghost signal,” move the wires (physical distance is your only defense against magnetic lines of flux).
  • With the motor under load, take thermographic images. Look for excess heat at the bearings, the load coupling, and the load. This is a quick way to spot gross mechanical defects, although lack of detection doesn’t rule them out.