Selection of copper loss minimizing D-axis inductance in IPMSM design

High motor saliency is required to increase the power density, and to reduce the use of permanent magnets (PM). It is a common practice to reduce the d-axis inductance (L
d
) as much as possible to obtain a high saliency ratio. But, we should look at the loss in the motor operation stage. Specifically in the field weakening region, a big negative d-axis current needs to be supplied to satisfy the voltage limit even when the torque is equal to zero. It causes copper loss which is interpreted as a cost paid for field weakening. That loss is enlarged if we reduce L
d
, since the field weakening power is determined by L
d
i
d
, i.e., the d-axis current needs to be supplied more as much as L
d
is reduced. Considering the two perspectives of L
d
, there is an optimal choice. It is investigated here to find an optimal L
d
that minimizes the copper loss when the torque, speed, and the maximum (or DC link) voltage are specified. Computed results are presented that support the design strategy.