An update on causes of, solutions for shaft currents
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. When a conductor is passed through a magnetic field, voltage is induced into the conductor. It is not the voltage that damages a bearing, but rather the current. (Fuses fail because the current is too high, not the voltage.) We don't have a practical way to measure the current through the shaft, so we measure the magnitude of the voltage instead. We used to blame magnetic dissymmetry for shaft currents, and rightfully so. Magnetic dissymmetry just means gaps in the iron, such as segmented laminations used to build stator cores over approximately 35" (900 mm) diameter, uneven airgap, circulating currents in the parallel circuits of a 3-phase winding, or variations between bolt-in DC poles. Since the electromagnetic field in the stator rotates around the stator bore, those dissymmetries are one source for induced voltage in the frame. Through-bolts in rotors were another cause - but more about them later.