Influence of dummy slots on noise and vibration performance in permanent magnet synchronous machines

Introducing dummy slots is often applied to reduce cogging torque in permanent magnet synchronous machines. Although this solution always helps for smooth operation, its influence on noise and vibration (NV) performance needs more focused analysis. This paper employs two surface-mounted machines to investigate the effect of dummy slots, one of which is a 10-pole/12-slot machine (M1) with few magnetic-field units and another is a 12-pole/18-slot machine (M2) with multi magnetic- field units. It is shown that dummy slots not only influence the harmonic components of cogging torque, but also those of local force distributing on the stator bore, both of which contribute to NV. In nature, it is by the way of exerting influence on airgap flux density do dummy slots affect these excitation sources. As the contribution degrees of cogging torque and local force to NV differ widely for machines with few and multi magnetic-field units, the influence of dummy slots on NV performance is quite different for these two machines. The results indicate a large reduction of 15.7 dB in noise level for M2 while only a little variation of 0.9 dB for M1.