Field Weakening Methods for Axially Displaceable Brushless DC Machines

Brushless DC (BLDC) machines are known to have a high torque/power density but it is still a challenge to design a BLDC with a wide extended speed range while keeping its torque/power density high. A mechanical field weakening method, in which the active axial length of a BLDC machine is reduced on run by axially displacing its rotor with respect to its stator, was proposed to achieve a wider extended speed range with a limited DC-link voltage. However, its effectiveness and optimal application are not considered in detail. The aim of this study is to analyze the extended speed range of an axially displaceable rotor brushless DC (ADR-BLDC) machine. First of all, factors that limit the speed range of an ADR-BLDC machine are discussed. Afterwards, rotational speed limits of the analyzed ADR-BLDC machine are compared for different field weakening approaches. Based on these results a combined field weakening approach that combines both the electrical and mechanical field weakening methods is proposed.