The selected magnetic wedge has permeability, which is
much higher than air. However, this permeability is much
lower than the lamination steel used in the stator. This choice
of the magnetic wedge minimizes the leakage (short-circuit)
flux into the magnetic wedges, however, is still sufficient to
improve the machine inductance. The selected material has
low permeability compared to lamination steel and it does not
saturate. Thus providing a path for leakage flux (to improve
inductance) at all torque levels. The improvement of the
machine inductance both improves the high-speed constant
power range and the high-speed efficiency. Another benefit of
having magnetic wedges is to improve the machine back emf
thus improving torque without increasing the magnet content,
or increasing cost. This improvement in machine back emf is
achieved by the improvement of permeability of the magnetic
circuit, due to the addition of the magnetic wedges, to the
magnet flux. The introduction of the magnetic wedges also
reduces the slotting effect, which is major source of no-load
spin loss. As a consequence, the spin loss is also reduced
significantly.
The effect of the short pitching of the magnets and addition
of magnetic wedges in the stator slot is quite obvious in the
back emf waveform of the developed machine. Typically,
slotting introduces harmonics in the back emf. The measured
back emf of the machine, as shown in Figure 4, is nearly
sinusoidal, showing very low harmonic content. As a
consequence, the developed machine has very low cogging
torque and torque ripple. The other conventional way of
reducing the cogging torque is to skew the magnet. However,
this approach also reduces the machine torque. The above
implemented method of the developed machine does not
compromise machine torque.