Shaft Grounding for Motors Greater than 100 Horsepower

DP&A Sales and Shaft Grounding Systems, Inc. recently sat down together to discuss several recent customer interactions regarding shaft grounding of larger framed motors. A large supplier of microfilament style shaft grounding systems has heavily promoted the following concept:

For motors 100hp or greater an insulated bearing on the non-drive end and a grounding system on the drive end of the motor is necessary to eliminate capacitive coupling and potential circulating currents in the motor.

Regarding circulating currents, the following are what DP&A Sales and Shaft Grounding Systems, Inc. know to be best practices and is based on 30 years of field-based experience and knowledge:

Circulating currents may be present in 25-35% of motors with the following motor specifications:

1. Between 100-200hp with rpm less than or equal to 1200
2. Above 200hp regardless of rpm

Our recommendation for when to be most concerned about circulating currents is more nuanced and best reflects what actually is occurring in the field.

Determining when to be concerned about circulating currents can be a complex decision-making process that should account for the above mentioned risk relative to the cost of insulated bearings, the grounding systems, and the downtime associated with electrically induced bearing damage. Because we’ve periodically seen insulated bearings fail due to electrical bearing damage, our general recommendation is to provide shaft grounding on both ends of the motor to eliminate both capacitive coupling and circulating currents.

That said, we understand that the market trend to address circulating currents is to use an insulated bearing on the non-drive end and a grounding system on the drive end. Assuming the insulated bearing performs as expected and there is no connected non-drive end equipment such as encoders and tachs, this set-up will prevent bearing damage due to capacitive coupling and circulating currents. SGS^TM is best known for providing non-drive end shaft grounding systems, most notably the CR Series. Most OEM motor manufactures, such as ABB/Baldor, provide an SGS^TM non-drive end shaft grounding system. We learned through field feedback that SGS^TM will be side-lined, despite being specified by the end user, due to the misconception that SGS^TM can only provide non-drive end shaft grounding systems. If a specification calls for an insulated non-drive end bearing, then any discussion of providing an SGS^TM system stops, particularly at the OEM motor manufacturer level.

Shaft Grounding Systems, Inc. has provided for 30 years a class leading, heavy duty, industrial grade split ring shaft grounding system for the drive end of the motor known as the SR Series. This field installable shaft grounding system is particularly suited for 24/7 operation in some of the harshest industrial environments such as mining, pulp & paper, marine and nuclear power. One end user installed the SR Series in an environment where the grounding device is bathed in salt water. The SR Series has even been fully submersed in oil and will continue to function as a long-term fix for electrically induced bearing damage. In short, the SGS^TM SR Series drive end system will handle just about any environment and for the long-term.

As an end user, you have the power to influence your preferred OEM motor manufacturer, applications engineer or mechanical contractor to provide the best fix for electrically induced bearing failure. DP&A Sales and Shaft Grounding Systems, Inc. are ready to work with your chosen partners and will ensure they receive what has been proven to be the only long-term fix for electrically induced bearing failure. See below for field installation examples of the SGS^TM SR Series shaft grounding system.

The SGS^TM SR Series split ring shaft grounding system installed onto the drive end of a large AC motor. This highly robust system is field installable with handheld tools and without the need for uncoupling the motor. These systems can also be easily installed at an OEM motor manufacturer. It’s exceptionally heavy duty with very high wear tolerances and is key for ensuring a long-term fix for electrical bearing damage. Hold a microfilament ring in one hand and an SGS^TM split ring in the other and it’s very apparent which grounding system will perform for the long-term. We continue to service originally installed rings from over 20 years ago with very infrequent brush changes. The ring has such a high life expectancy, we’ve actually not seen a single ring wear down enough to warrant replacement.

The SGS^TM SR Series Compact split ring shaft grounding system installed onto the drive end of a large close coupled AC motor. This system is also field installable with handheld tools and without the need for uncoupling the motor. It can easily be installed at the motor factory as well. Close coupled applications require the brush bracketing assembly to accommodate the tighter tolerances related to the close coupling. This model possesses a dual brush design that ensures long-term performance. SGS^TM also provides split rings that can accommodate narrow axial shaft space.

The SGS^TM SR Series split ring shaft grounding system installed onto the drive end of a 1000hp motor in a power plant. This customer was concerned about circulating currents and therefore also installed an SGS^TM CL Series shaft grounding system onto the outboard end of this motor. Although not typically necessary, this end user opted to install a grounding strap as well. Mounting the brush bracketing is simple and uses the motor’s existing bearing retainer bolts for bracket installation.