| Abstract |
In a permanent magnet motor a cogging torque manifests itself by the tendency of the rotor to align in
a number of stable positions when unexcited. Under dynamic conditions the resulting pulsating
torque, of zero net value, may cause undesirable speed pulsation, and also may induce vibrations and
acoustic noise. It is thus of great interest to get a deeper understanding of the cogging torque
phenomenon and to find possible ways to reduce it.
By means of advanced two dimensional field calculations a new design rule has been demonstrated
for a rotor design with slot mounted or embedded magnets. If the quantity Në, the relative magnet
width compared to the slot pitch, is chosen as an integer, the cogging torque will be significantly
reduced by a factor of ten or more to less than 0.5% of the rated torque. This can also be given a
physical explanation.
The calculated results have been verified by experiments on two different motors. The agreement
between measurements and calculations of the cogging torque as a function of the rotor angle is good
both for an nonoptimized rotor design as well as for an optimal design with integer relative magnet
width.
The results are generally valid for motors with embedded permanent magnets. For motors with airchannels
around the magnets, the value of the relative magnet width for minimum cogging torque
somewhat deviates from an integer.
One conclusion from the study is that the use of Maxwell stresses increases the calculation accuracy
significantly compared to calculating the cogging torque from the change of magnetic energy and that
the calculation of the torque should be made on the rotor surface or as near the surface as possible. |
