Selection of PM Flux Linkage for Maximum Low-Speed Torque Rating in a PM-Assisted Synchronous Reluctance Machine

In recent years, there is an increasing interest in adopting synchronous reluctance machines for several applications. This is due to two main factors. The first is the increasing and highly variable cost of rare-earth permanent-magnet (PM), such as NdFeB and SmCo. The second is the requirement for more efficient machines with respect to induction machines in general applications. However, a drawback of the synchronous reluctance machine is the low power factor. Such a drawback is compensated by assisting the machine by means of a proper volume of PM material, generally ferrite, which is introduced in the rotor flux barriers. The PM flux saturates the rotor iron bridges, increases the power factor of the machine (which corresponds to a decrease of the volt-ampere ratings of the inverter), and adds a PM flux torque component. This paper investigates how to select the amount of ferrite magnet in a PM-assisted reluctance machine, adopting a model that considers the machine operation at low speed with both a current and a flux linkage limit. It is shown that, by means of the aforementioned analysis, the machine performance at low speed can be improved by means of a proper choice of the PM flux linkage.