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
B_{m},H_{m}
on a recoil line inside the demagnetization curve. The point results from the application during stall of an effective mean field H
a
due 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
H_{m}/H_{a}
. The limiting values of this ratio (H
m
zero or H
a
zero) imply that the corresponding optimum magnet designs for minimum magnet volume should be based either on maximum recoil energy or on
(BH)_{max}
. In practice, the best extreme working point during stall should lie between the points for these two criteria, dependent on the actual
H_{m}/H_{a}
. In some existing motors
H_{m}/H_{a}
has been found to be between 0.5 and 1.5. For such values of
H_{m}/H_{a}
, the variation of B
m
and of
B_{m},H_{m}
with working point is illustrated for high coercivity ferrite and for two grades of cast alnico alloy. A note on design methods is appended