Influence of Air-Gap Length on the Performance of a Three-phase Induction Motor with a Capacitive Auxiliary Stator Winding

In order to avoid lower efficiency, poor power factor and lower torque density, the airgap length of a three-phase squirrel cage induction motor (SCIM) should not be designed very large. On other hand, the distortion of airgap flux density distribution can be minimized by enlarging the airgap length. This has informed machine designers to choose very carefully the airgap length while designing a three-phase SCIM. This paper deals with the influence that the airgap length has on the performance of a three-phase SCIM having a capacitive auxiliary three-phase stator winding. The three-phase auxiliary winding is only magnetically coupled to the stator main winding. A conventional 5.5 kW, 50-Hz, and 4-pole three-phase SCIM is modified to accommodate both main and auxiliary windings in the same stator slots, while maintaining the rated power to 5.5 kW. The results obtained from practical measurements evidenced that for a large airgap it is possible to obtain high efficiency and good power factor with the presence of a capacitive auxiliary stator winding. The high efficiency is mainly due the decrease in iron and stator copper losses. The iron losses decrease with increase in airgap length for the SCIM with or without a capacitive auxiliary winding, while the stator copper loss is obtained by reactively exciting the three-phase auxiliary stator winding.