Large ac generators are made of solid magnetic steel rotors of cylindrical shape, whereas small- and medium-size generators are made with laminated salient poles (protruding poles).
The salient-pole construction provides more physical space for the rotor winding and also produces additional torque (called reluctance torque) that increases the machine power output at a small power angle δ.
The flux in the main magnetic axis (called direct axis or d-axis) has a low-reluctance path (less air gap) than the flux in the quadrature axis (called q-axis).
For this reason, the protruding salient poles have a natural tendency to align with the stator field axis under ferromagnetic attraction even in the absence of rotor current. This requires additional torque from the prime mover, twice in one electrical cycle, to drive the rotor away from said natural alignment.
The total torque input and hence the power output of the generator has a sin(2δ) component superimposed on the sinδ component.
The power output analysis of the salient-pole generator requires employing the two-axis theory using Park transformation.