Salient-pole torque

This is the torque due to the salient-pole structure and armature current, even when there is no field flux, in a DC motor. Figure (a) shows the magnetic flux in blue due to field current and windings. This current and flux are referred to as direct-axis current and direct-axis flux, respectively. Figure (b) shows the armature current and flux due to it, these being called the quadrature-axis current and quadrature-axis flux, respectively. More succinctly they are referred to as the d-axis or q-axis. By the interaction of both axes’ currents or flux, a torque T is generated. This is given by
T=p(L_d－L_q)i_di_q

where
p = number of pole pairs,
L_d = d-axis inductance
L_q = q-axis inductance
i_d = d-axis current, and
i_q = q-axis current.
Figure (c) shows that a new torque is produced even when no field current is supplied but the brushes are oblique, because the q-qxis current produces a d-axis flux due to the salient-pole structure. This is the salient-pole torque. In a brush type DC motor this is not employed in normal applications because of strong sparks between the brushes and commutator. In a brushless DC motor using an expensive permanent magnet, the salient-pole structure is employed to reduce magnet volume.