Causes of Bearing Failures in Motors and Generators
The purpose of shaft monitoring is to indicate the presence of high levels of voltage or current on the motor or generator shaft in order to detect poor grounding shaft performance prior to bearing failures.
Poor performance of a shaft grounding brush is one of the most frequent reasons for the build-up of shaft voltage and current. During the normal operation of electrical machines, AC and DC voltages can be induced in the shaft, or created by the rotating elements of the turbine, connected to the same shaft.
Other key causes include:
Importance of Shaft Voltage and Current Monitoring
If shaft voltages are high enough, shaft currents can cause arching between the shaft and the bearing. The arching elevates the temperature enough to cause the metal to vaporize and result in pitting of the bearings.
Shaft voltage monitoring will also provide additional benefits that may include:
Methods For Shaft Voltage and Current Monitoring
On-line shaft monitoring using the Guard II requires the permanent installation or modification of 1 or 2 voltage brushes. Shaft current and voltage signals are continuously measured by the monitor to trend and alert when grounding maintenance is required.
Installation of the GuardII can be performed without a unit shutdown, but the installation of the voltage brushes and connection to the shaft grounding brush might require a shutdown depending on the type of the brush installed.
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