This webinar explains what shaft currents are, what causes them, and differentiate between the two common causes. It also discusses and compares methods to mitigate shaft currents and explains why the different causes of shaft currents require different solutions.

There are a few areas of concern involving the misapplication variable frequency drives (VFDs) on a standard induction motor. This article looks at some of those.

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. 

There are a number of ways that the shaft of an electric motor can become magnetized in service. The most likely culprit is electric current through the motor and shaft, either from internal dissymmetry, welding or from a variable frequency drive. It can also be caused by electrical faults in the system, or even a lightning strike. If enough current does pass through the shaft, then it can remain magnetized, even after it is taken off line. The problem is that when a shaft is magnetized, it can further lead to bearing failures, unless something is done to eliminate the residual magnetism. The first reason for bearing failures is that the residual magnetism can cause shaft currents, which can quickly lead to bearing failures. But in addition, a magnetized shaft will attract bits of metal to the bearings. This reduces bearing life because it damages the bearing surfaces.

While shaft currents are not a new problem (papers on the subject date back prior to 1930), what is "new" is our understanding of how to solve the problem. Shaft currents have been described as shaft voltages, circulating currents, bearing currents and circulating voltages. This article will refer to the phenomenon as "shaft currents" because it is the current that causes the damage.