Why Shaft Current Occurs in Line-Frequency Operated Motors

The two fundamental prerequisites for current generation are voltage and a closed circuit. Shaft current can only form when shaft voltage exists alongside a closed conductive path. Then why does shaft voltage emerge in motors running at industrial power frequency? Two root mechanisms generate shaft voltage in rotating machinery: alternating magnetic flux and static charge accumulation.

Shaft voltage induced by alternating magnetic flux is continuous and periodic. Under normal operating conditions, the motor rotor rotates within a symmetrical sinusoidal alternating magnetic field. The alternating electromotive force (EMF) induced as the rotor cuts magnetic flux generates balanced alternating currents, so no asymmetrical voltage exists between the two ends of the rotor shaft in ideal scenarios. However, uneven magnetic reluctance distributed circumferentially along the stator core creates unbalanced alternating EMF, which in turn produces axially directed shaft voltage. In contrast, shaft voltage stemming from static charge is intermittent and non-periodic. During motor operation, friction between process fluid loads and rotating components builds static charge on the rotor assembly; continuous charge accumulation subsequently develops shaft voltage.

For medium and large AC motors, once a conductive closed loop forms across the rotor shaft voltage, shaft current will arise, featuring the typical characteristic of low voltage with high current. The shaft and bearing bushes are separated by lubricating oil, with motor bearings supported on an oil film. Given the relatively low magnitude of shaft voltage, the insulating oil film generally remains intact without breakdown.

During high-speed rotor rotation, substandard lubricant quality or insufficient oil supply ruptures and breaks down the oil film, bringing the shaft and bearing bush into direct metallic contact. A closed circuit instantly forms across the shaft voltage, triggering low-voltage breakdown. The resultant shaft current reaches several hundred or even thousands of amperes instantaneously, sufficient to burn out the shaft journal and bearing bushes.

Static charge generated by operational friction accumulates progressively on the shaft, continuously elevating the shaft potential as it charges. Discharge takes place immediately if the rotating shaft makes contact with any stationary external component. If the shaft remains isolated from non-rotating parts, static charge keeps building up until an excessively high voltage develops. Once this voltage exceeds the dielectric strength of the bearing oil film, rapid discharge occurs and produces shaft current.

This current circulates through a closed loop composed of the rotating shaft, bearing inner ring, bearing outer ring and bearing housing. The most evident failure sign is tiny, deep circular pitting corrosion on the journal surface and bearing inner race caused by electric arcing discharge. Such pitting destabilizes oil film formation and integrity, and ruins the precise fit between the shaft and bearing, rendering the bearing inoperable. In severe cases, intense electric arcs form at the contact surfaces of the shaft journal and bearing bushes due to heavy shaft current, severely damaging these components. This leads to excessive motor vibration and abnormal noise, ultimately rendering the motor unable to run normally.