Facilities Support Services

VFDs are not infallible , sometimes they need to be repaired or replaced. The VFD is often the first indicator of a process change or application problem.

Many VFDs communicate using an LCD or LED display, or through an open interlock or fault indication. In most applications, the VFD interacts with operator controls, process control signals, and PLCs. A problem with the interaction between the VFD and these external controls may appear to be a drive issue, when actually the problem is with the process. Discussing process and drive symptoms with the plant operations often can help determine the problem area.

If the external controls are working correctly, use the VFD to identify problems systematically. If the display status indicator does not operate, verify incoming ac power. If the status indicator still does not display after verifying or restoring ac power, then verify control power, and restore it if necessary.

If the VFD has been operating successfully, but suddenly fails to start, or if the drive starts but does not run properly, check to see if the diagnostics status display indicates a fault. The instruction manual for the VFD should have a description of faults and troubleshooting steps. Use diagnostics or a keypad control to monitor variables such as incoming voltage, dc bus, carrier frequency, output frequency, voltage, current, and I/O and control status. These parameters are displayed on most common VFDs. I/O status uses bits to monitor required start conditions to ensure they are enabled and to determine what may be inhibiting start. Control status indicates the source of the speed reference and can be used to verify incoming speed or direction signals.