Demagnetisation phenomena in high-performance d.c. servomotors

The paper describes the experimental results obtained from mounting Hall plates in the airgap of a high-performance permanent-magnet d.c. machine. It is found that the flux-density distribution over the pole face during transient operating conditions is nonuniform. These results suggest that the conventional single-recoil-line theory is deficient, and that each point on the surface of the permanent-magnet material operates along a separate recoil line. The spread of these recoil lines in the second quadrant is particularly significant if the armature of the machine is subjected to reversals, and the permanent-magnet poles are only short-circuit stabilised.