Maximising the saliency ratio of the synchronous reluctance motor

Recent interest in the synchronous reluctance motor has increased in the context of possible applications in field oriented AC drives. The absence of rotor slip losses, and the apparent simplicity of the control, suggest the possibility of performance and cost advantages over the induction motor. With field oriented control, and continuous shaft-position feedback, the synchronous reluctance motor does not need a starting cage and can be designed for maximum saliency ratio (Ld/Lq ratio). This ratio is by far the most important parameter for achieving high power factor, torque/ampere, and constant-power speed range. This paper analyses the various known forms of the synchronous reluctance motor, to determine the maximum achievable saliency ratio and identify the parameters on which it depends. The main originality is the analysis of the effect of the number of flux guides/barriers. It is shown that a minimum number is required for the performance to surpass that of the induction motor. The analysis also reveals the optimum ratio of flux guide/flux barrier thicknesses and the effects of the tooth/slot geometry and stator saturation. Test results are included from three motors ranging from 50 W at 2000 rpm to 7.5 kW at 1500 rpm, covering both axially laminated and transversely laminated types.