Experimental Study on the External Shaft Axial Stray Flux in Squirrel-Cage Induction Motors

Nowadays, due to the strong worldwide trend toward Industry 4.0, in industrial facilities with a large number of three-phase induction motors, nonintrusive motor condition monitoring and fault diagnosis techniques/devices are being used to provide key data for the implementation of predictive maintenance strategies. Most of the retrofitting nonintrusive sensing devices that are being used to measure vibration and temperature at the motor frame incorporate wireless communication, allowing for the implementation of wide wireless sensor networks. It is well known that the existence of a homopolar/zero-sequence axial stray flux in rotating three-phase machines is associated with electromagnetic asymmetries, core saturation and/or supply voltage unbalances. This axial stray flux can be partially measured in the external drive-end shaft with a concentric search coil, thus it can potentially be used to monitor the motor operating condition or to diagnose some motor faults. In this paper, the results of a set of experimental measurements of the external shaft axial stray flux in low-voltage, three-phase, squirrel-cage induction motors under different operating conditions are presented and briefly discussed, contributing to a better understanding of this zero-sequence stray flux.