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Achilles Heel of VFDs – The Problem and Solution

This post focuses on a fantastic product – Variable Frequency Drives (VFDs) and how to protect it. VFDs must be in your arsenal to improve production and lower energy costs. Practically all process engineers, project managers, and operations personnel know the value of adding VFDs. The significant-excellent investment adds process flexibility, can save 30% or more on energy, and, on average, have a 60% IRR as a stand-alone investment! But there is a little-known issue with VFDs.

Achilles Heel of VFDs

So, just what is this issue, this Achilles Heel? VFD-induced shaft currents or Electrical Discharge Machining (EDM) can transmit a voltage discharge through the shaft. These voltage discharges are like little shocks to the system that will pit the metal due to what is known as Electrical Discharge Machining (EDM.) Over time, these discharges eat away at the bearings, housing, and the motor itself. These pictures say it far better than words:

edm pitting bearing race wall from Pulmac

electrical discharge from Pulmac

Unfortunately, many specifying Variable Frequency Drives are unaware of this issue. They may have spec’d out dozens, even hundreds, yet unaware of the destructive environment on every VFD! Unless the process engineer or whoever spec’d the VFD is aware of this, they continue specifying VFDs as usual. VFDs can be found on agitators, extruders, gearboxes, mixers, paper machine rolls, mixers, pumps, screw conveyors, sleeve-bearing motors, steam turbines, etc.

Maintenance Personnel

Where does this issue come to light? Typically, from the Maintenance Group in a mill. Operations called the Maintenance Group, complaining that their VFD had just stopped working. And this is the second time it has occurred within the last year! Shop personnel replace the broken parts, then disassemble the damaged parts trying to analyze what happened and why. It is a head-scratcher. As it turns out, most of the time, AEGIS® is called into a mill by the maintenance department after a VFD has failed multiple times and asked Why are my bearings failing?

What is the Solution?

AEGIS® pioneered and introduced a shaft-ground protection ring technology in 2009. This protection ring completely encircles the motor shaft, provides continuous grounding regardless of RPM, and requires no maintenance once installed. For motors over 100 HP, AEGIS® further recommends adding a silver coating on the shaft to help ensure that all stray voltages are picked and discharged through their grounding ring.

the solution from Pulmac

In an ideal world, these bearing protection ring kits would be specified as part of a VFD project, rebuilt or new. On motor rebuilds, installing an AEGIS® Impro/Seal® with an integrated AEGIS® shaft grounding ring is also considered a best practice. These shaft-grounding kits that AEGIS® markets are modest in cost yet effective in protecting your VFD investment. This ensures your VFD investment will continue to provide the benefits expected for years to come.