Permanent magnet utilization criteria in a DC motor

In a dc motor with permanent magnet stator, the mean no-load working point of the magnet material can, after stabilization by stall current, be represented by a point on a recoil line inside the demagnetization curve. The point results from the application during stall of an effective mean field Hadue to armature reaction in addition to the self-demagnetizing field due to circuit reluctance. It is shown that the motor specification and sizes lead to a specific value for . The limiting values of this ratio (Hmzero or Hazero) imply that the corresponding optimum magnet designs for minimum magnet volume should be based either on maximum recoil energy or on . In practice, the best extreme working point during stall should lie between the points for these two criteria, dependent on the actual . In some existing motors has been found to be between 0.5 and 1.5. For such values of , the variation of Bmand of with working point is illustrated for high coercivity ferrite and for two grades of cast alnico alloy. A note on design methods is appended