| 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 resultant 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. In an earlier paper a new design rule, Design rule 1 for normal motors, was 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 (n), 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. Here a complementary Design rule 2 is described for skewed motors, where the rotor is split into two equal parts that are displaced half a slot pitch to each other. If the quantity Në, the relative magnet width compared to the slot pitch, is chosen to (n+1/2), n being an integer, the cogging torque will also be significantly reduced to less than 0.5% of the rated torque. We show that this rule can be understood as a consequence of rule 1 for skewed rotors. Design rule 2 is to be used in cases where a motor according to Design rule 1 is not suitable because of a too low rated torque. The two rules 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 un-optimised skewed rotor design as well as for an optimal normal design with an integer relative magnet width. |
