Brushless dc (BLDC) machines with a surfacemounted
permanent magnet (SMPM) rotor meet the high-torque
and high-efficiency requirements for automotive applications.
However, their constant-power operation region is limited due to
the low phase inductance. As an alternative to the electrical fieldweakening
methods, the speed range of radial-flux BLDC machines
can be extended by mechanically reducing the axially
overlapping length of the stator and the rotor. In this paper, the
no-load performance of an SMPM-rotor BLDC machine with
an axially displaceable permanent-magnet rotor is analyzed. The
effectiveness of this mechanical field-weakening method is limited
through the flux components due to the stator/rotor misalignment
and the additional losses. The cause of the flux components due
to the stator/rotor misalignment and the dependence of backelectromotive-
force waveforms on the axial rotor position are
investigated by using 3-D finite-element method (FEM) analysis,
where the effects of the end-winding geometry and design are
taken into account. Moreover, the additional loss mechanisms due
to the stator/rotor misalignment are identified, and the no-load additional
losses are determined by using the experimental and 3-D
FEM analysis results. Finally, the numerical results are verified by
using test-bench measurements.