Like most industrial technologies, Variable Frequency Drive (VFD) units have grown increasingly affordable over the past few years and have become common in many facilities to better control processes. While there are many advantages to implementing VFDs on existing equipment, there are some pitfalls as well. The rectangle wave (pulse code modulation) approximations created from the drive can cause notable distortion of the electrical signal into the motor. Without proper grounding, this can lead to high-frequency discharge, which can cause significant damage.
Here’s an example from an ATS client’s facility:
During bi-monthly data collection, a chilled water pump running on a VFD started showing signs of pump bearing degradation in the vibration data. Confirmed raceway defect harmonics began to appear at elevated levels at the outboard pump bearing location. These changes appeared out of nowhere.
Typically, in motors that draw energy from the line, bearing degradation happens more gradually and in stages. There had been no changes in process or plant maintenance for the asset in question, so the sudden change in vibration data on a machine with a VFD indicated that an electrical discharge from the motor and through the pump shaft had caused the bearing issues.
The facility had encountered grounding and electrical discharge issues in another area where VFD units had been recently installed. As a result, a Faraday ring counter was used to monitor the motor and pump bearing locations for the presence of electrical discharge. Initial counts at the motor and pump were in the range of 100 discharges per 30 seconds. Over the next few vibration collection cycles, bearing defects remained present at the pump with some slight progressions, but instances of discharges were increasing more rapidly. During the last collection before the confirmed replacement of the pump bearings, the ring counter registered well over 4000 discharges per 30 seconds at the pump.
We recommended to the maintenance staff that the pump bearing be replaced, the grounding be inspected, and a grounding ring be installed. However, maintenance personnel resisted the idea of bearing damage due to electrical discharge. They had been monitoring the pump bearings with their ultrasonic detection system and had not found any notable increases.
It was not until one of the maintenance technicians noticed the pump bearings were getting hot to the touch that the pump bearings were replaced and subsequently sent out for root cause analysis. The report came back confirming that these bearings were severely damaged due to electrical discharge. Because of the damage found in the pump bearings, grounding rings were installed on the drive motor.
The post-repair data shows significantly lower impacting at the pump in the vibration data, and the high-frequency discharge counter registered zero incidents throughout this asset.
We’re always ready to help you determine the cause of a problem with your equipment and to offer informed recommendations about how to proceed. A case like this demonstrates the value of having ATS as your predictive maintenance partner.