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.B_{g}^{2}is shown to be proportional to the no-load energy density in the stabilized magnetB_{m}H_{m}. Stabilization is normally obtained from the armature reaction at stall but its effective value is only a small fractioneof the total armature magnetomotive force. The relationships derived are used first, to examine theB_{m}H_{m}product values theoretically obtainable from three different magnet materials, and second, to obtain some experimental values ofewhich are in the range 0.01-0.1.