Shaft currents

Bearings must be insulated wherever necessary, to prevent them from shaft currents (Section 10.4.5). 

Cognisable shaft currents may exist in large LT and HT motors of 2000 kW and above, using circular laminations and all motors with segmented laminations due to the magnetic field cau.sed by asymmetries. 

The problem of shaft currents may also be due to dielectric leakages that may take cognisance in HT... 

Windmills (induction generators) as an unconventional energy source, vertical hollow shaft motors and submersible pump sets, selection of belts for transmission of load, the phenomenon of shaft currents are discussed. 

The du/dt of motor voltage can be reduced by an output filter. Shaft current (ground current I3) and circular current are thereby reduced. Good common-mode rejection is important here. 

Turbines and compressors can occasionally act as electric generators due to high shaft currents, resulting in equipment damage and production losses. Engineers unaware of the real problem might simply replace wornout parts, only to find that after a period of time the problem recurs. This article explains how shaft currents are generated, how to detect them, and how to control them. 

Shaft currents can be located by finding equipment damage, which occurs where resistance is encountered. Lx)ok for pits. and grooves and temperature hot spots on the surface of the material, all of which can be caused by sparks. 

A simple method for detecting shaft currents is to touch... 

To minimize the generation of shaft currents, the following maximum levels of residual magnetism are recommended ... 

First, one must know the locations of the shaft currents. Brushes are then positioned so that the current can travel the path of least resistance to leave the unit without any sparking. They can be placed either axially or radially, as shown in Fig. 2. Axial brushes are effective when shaft currents are generated at the extreme end of a turbomachinery set, such as at the end of a compressor-turbine train. Radial brushes can be used in a turbine where the shaft is projected between the casing and bearing housing, and, if possible, should be mounted on the shaft inside the bearing housing. 

Install sensing devices to continually monitor shaft currents and internal magnetic fields. 

Specific examples from the authoring team s experience at a deep vertical shaft currently being sunk in the Rustenburg area in. South Africa will be discussed. 

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