Direct axis, Quadrature axis

Two-axis theory can be used as a way of analyzing a rotating machine. In this theory, elements such as magnetic flux, current, and inductance are divided into two electrical orthogonal axial components and discussed. As a standard way of thinking, the direction of the field flux of the separately excited DC motor in fig. (a) can be considered as the direct axis (d axis), and the direction of the magnetic flux in the armature (the typical component in the armature core is considered to be horizontal), as the quadrature axis (q axis). Fig. (b) shows both axes in the case of a 4-pole brushless motor using an embedded type permanent magnet. Note that here the d-axis is taken for the flux due to the permanent-magnet and q-axis is for the flux due to the stator windings.