Pat Douglas Kirby Risk
Mechanical Solutions & Service
Shaft currents have always been a concern for large motors due to magnetic asymmetries within the motor. Manufacturers strive to keep these to a minimum.
With the widespread use of Variable Frequency Drives (VFDs), shaft current issues have become a concern in all sizes of motors. If these currents are discharged through the bearings, electrical discharge machining (EDM) occurs. Proper installation of VFDs can play a large part in mitigating issues with shaft currents.
Many end users are not aware of shaft currents or their destructive paths. All too often they think that the motor bearings keep failing because the motor repair was not completed properly.
Service centers need to be on the lookout for these issues when repairing a customer’s equipment. Many repairs arrive at the service center with no history and no hint of what the problem with the motor might be. The technicians have to do an “autopsy” of the motor to be sure the causal problem is repaired and not just the symptom.
Start with the bearings
One of the many places to find clues is inside the bearings. They should be dissected and analyzed on every repair. Often this is done by cutting, but that can cover up potential clues. A better way is to remove the cage by machining off the rivet heads. Many things can be uncovered by adopting this process; one of which is the subject at hand, shaft currents. Rolling element and raceway frosting (Figure 1) may be early signs of shaft currents. Fluting or a washboard pattern (Figure 2) in the raceway is a good indication of current flowing through the bearing while the shaft is turning. Damage will be more pronounced in the load zone. The grease inside the bearing is usually black due to electrical arcing (Figure 3).
Finding a bearing that looks like this would be a good reason to get the customer into the service center to show him what you found and explain the possible causes and solutions. If the customer is having similar issues on other equipment a service trip on site to test for shaft voltages could lead to additional opportunities.
Inadequate ground could be source
An inadequate ground on the system could be a contributor. When installing a VFD, it is crucial to explicitly follow the installation instructions. Using the specified type and size of wire, routing according to the applicable electrical code, and terminating grounds at the specified location all must be done. For example, just because a ground is terminated close to the specified location does not mean it will work properly. Grounding the motor and VFD “per code” is not sufficient. A dedicated low impedance ground with braided cable, from the motor frame to the case of the VFD, is part of a viable solution to shaft currents.
Preventing future problems
There are different opinions as to what should be done to the motor to prevent future problems. Any one remedy will most likely not work in all applications. There are a few products on the market for grounding the shaft which may be needed on one or both ends. Another piece to solving the puzzle might be to insulate one or both bearings from ground. This can be accomplished with an insulated bearing journal, shaft bearing fit or an insulated sleeve in the end bracket. Bearings are available with an insulated outer race or ceramic balls to help prevent current from traveling through the bearing. Insulating one end, without installing a shaft grounding device on the other end, will not solve the problem.
Shaft currents can cause significant problems resulting in costly down-time and repair charges. These costs can be dramatically reduced by identifying this failure mode and working with your customer to identify and solve the root cause.
EASA Technical Manual
More information on this topic can be found in EASA's Technical Manual- Section 2: AC Machines
- Section 8: Bearings
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