In this study, a mechanical field weakening method that is applied to
enhance the high-speed performance of a PM brushless machine by reducing
the active axial machine length is analyzed in terms of its operational
characteristics and efficiency. For this purpose, an existing
multi-phase brushless DC (BLDC) machine with an axially displaceable
rotor is used. First, the parameters of this machine, such as back EMF,
torque constant and phase inductances are determined for various axial
rotor positions by using 2-D and 3-D FEM analyses. Then, the determined
machine parameters are used to build a dynamic simulation model of the
drive system. By using this developed system model, the operational
characteristics including the limits of the mechanical field weakening
method are analyzed. In the second place, the efficiency of the BLDC
machine is investigated, laying the main focus of attention on the
additional losses in the field weakening range. In order to determine
these losses, new numerical and measurement methods are developed.
Furthermore, the measurement results are used to validate the numerical
results. Finally, recommendations for PM brushless machines with an
axially displaceable stator/rotor including possible design measures to
reduce the additional losses and the application of mechanical field
weakening are presented.
