When reviewing shaft grounding options for addressing electrically induced bearing damage, there are several key factors to consider when making a selection. For example, can the grounding system decrease shaft voltages below what is needed to prevent dielectric breakover through the grease and then on to the bearing? Will the device maintain that voltage for the long-term and avoid what we call “voltage creep” over time? Is the grounding device design robust enough to withstand industrial environments?
All these questions are important, but there is probably one question that should stand out amongst the others when making your decision as it likely has the most immediate and consequential impact on system performance. Can the grounding device withstand a dirty environment? What classifies as a dirty environment? For the sake of this conversation, a dirty environment would be just about any place where dust (or worse) is allowed to accumulate on the running surface of a shaft grounding device.
Where current flows is dependent on a path-of-least resistance. In a variable frequency drive situation with no shaft grounding device present, the path-of-least resistance from shaft-to-frame is through the dielectric (bearing grease) and then to the bearing. Shaft current discharging through the bearing leads to electrical discharge machining (EDM) and electrically induced bearing damage. Per our field studies of a large group of VFD motors running 24/7, bearing failure incidence rate is 25% at 18 months and 65% at 30 months with the incidence rate very likely increasing beyond 30 months.
To prevent this problem, a shaft grounding device is needed to divert the current away from the bearing and back to the motor frame. In short, the shaft grounding device becomes the path of least resistance in the system and is considered the most effective and practical method of eliminating electrically induced bearing damage. As a side note, devices such as common mode chokes, line filters, etc. may decrease voltage spikes of the power to the motor, but they do not decrease shaft voltages down the level needed to prevent dielectric breakover. At best, these products only minimally slow down EDM.
Most shaft grounding devices significantly decrease shaft voltage out of the box and consequently will slow down EDM to a degree. However, operating environment will quickly negate out-of-box performance as the path-of-least resistance becomes rapidly compromised with the introduction of dirt, dust, grease, corrosion, etc. leading to voltage creep. Some grounding system brands have realized this problem and will retreat their devices internally to the motor. Unfortunately, the inside of a motor can become quite dirty as well. Anybody who has rewound a motor knows just how dirty the inside of the motor can become. Retreating to the inside of the motor may buy a bit more out-of-box performance, however, voltage creep will eventually set-in as the motor’s internal environment becomes dirty.
Through smart product design and a deep understanding of what it takes to eliminate electrically induced bearing damage for the long-term, Shaft Grounding Systems, Inc. developed their patented non-drive end shaft grounding system. These grounding systems have been operating in the field for more than 3 decades and are the only known long-term fix for electrically induced bearing damage. What makes SGSTM products perform so well? It’s the ability to establish and maintain a path-of-least resistance away from the bearing for the long-term. How does an SGSTM non-drive end shaft grounding system accomplish this unique performance?
- SGSTM products do not run directly on the shaft. Running on a steel shaft will create a film that quickly eliminates the grounding system’s path-of-least resistance and returns that path back to the bearing. Any carbon-based brush system that runs directly on the shaft will quickly lead to system performance failure. In short, stay away from these cheap grounding systems.
- SGSTM non-drive end shaft grounding systems are fully sealed from the environment. The running surface is impervious to harsh industrial environments. Patented designs and proprietary materials ensure a clean running surface at all times and for the entire wear life of the brush. Actual long-term field studies have proven SGSTM non-drive end shaft grounding systems will run for 10 years at 1800rpm prior to needing an inexpensive brush change that can be done with the motor running and in less than 5 minutes. The out-of-box shaft voltage performance is maintained for the entire wear life of the brush and is a direct result of not being impacted by the operational environment. Of note, this system is so well designed that not only is the running surface protected from the environment, but the grounding system’s wearables do not escape the grounding system. Contamination testing shows the motor outputs more environmental contamination than an SGSTM CR Series shaft grounding system.
Located inside the body of the
SGSTM CR Series is the fully sealed running surface that is not impacted by the environment, whatsoever, and will maintain a path-of-least resistance away from the bearing for the entire wear life of the brush. Wear life is expected to be 10 years at 1800rpm. Brush changes to be done in less than 5 minutes with motor running.
