that is, a stationary d-axis (often chosen
to be aligned with the a-axis of the stator with θda = 0), leads to the
rotor and the stator dq winding voltages and currents oscillating at the
synchronous frequency in a balanced sinusoidal steady state. The choice
of ωd = ωm results in dq winding voltages and currents in the stator and
the rotor varying at the slip frequency; this choice is made for analyzing
synchronous machines, as we will discuss in Chapter 9.