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.