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Demagnetizing motor shafts to prevent bearing failures

Publication date: 
October 2005
Cyndi Nyberg Esau, Former EASA Technical Support Specialist

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.

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