Some aspects of permanent magnet DC motors

The linear relationships of speed, current, and torque in a motor with permanent magnet stator imply no change in armature flux over the test range. On this basis the torque per unit volume of magnet material at a specified fraction of the theoretical no-load speed can be expressed in terms of machine dimensions, conductor resistivity, and the square of mean air gap flux density. is shown to be proportional to the no-load energy density in the stabilized magnet . Stabilization is normally obtained from the armature reaction at stall but its effective value is only a small fraction of the total armature magnetomotive force. The relationships derived are used first, to examine the product values theoretically obtainable from three different magnet materials, and second, to obtain some experimental values of which are in the range 0.01-0.1.