Your company recently acquired another company's manufacturing facility. The previous management had cut costs by replacing experienced maintenance techs with recent tech school grads. Your boss sent you to solve an equipment problem that has dropped production to 40%. The problem is poor speed control on the main line, with symptoms ranging randomly from speed surges to nearly stopped.

The line has three 50-hp drive motors controlled by VFDs. Over the past two years, each VFD has been replaced at least three times. The repair records contain some voltage measurements and a long list of "drive adjustments."

How can you accomplish the mission your boss gave you?

We can rule out "defective drive" as a possible cause. Obviously, the "parts replacement instead of troubleshooting" approach didn't work. A drive is rarely defective, so just replacing one is unlikely to provide anything but added expense.

What about the many drive adjustments? If you set up a drive properly, you still may need to do some tuning for actual conditions. That's a methodical process, and it doesn't require a large number of adjustments.

Assume everything is wrong, because there was no rationale for the changes made thus far. You can solve this only if you "reboot" to a proper starting point. For example:

  • Ensure the motors and drives are properly matched to each other and to the application.
  • Ensure the installation conforms to Art. 250, Part V.
  • Measure for 2% maximum voltage imbalance between phases; repair as needed.
  • Perform insulation resistance tests on the motors.
  • Set up the drives per the manual.
  • Inspect signal wire routing for inductive effects.