Axial flux (with disc type rotors) and radial flux (with cylindrical rotors) are two ways in which air gap orientation can be defined in PMSG as shown in Figure 5. If the flux direction is perpendicular to the axis of the air gap, the machine is called radial; whereas, if it is parallel with the air gap axis, it is axial. The conventional types of PMSG used in wind systems are radial machines (RFPM), and because of its popularity, cost of production is lower. Axial flux (AFPM) topologies have been the earliest electrical machines but were shelved for RFPM due to the following reasons (Gieras et al., 2008):
Strong axial (normal) magnetic attraction force between stator and rotor;
Fabrication difficulties in making slots;
High cost of manufacturing;
Difficulty in assemblage and air gap construction.
In recent times, breakthroughs in PM materials have revived the interest in axial flux machines, which possess high torque density and compactness, and are good candidates for low-speed direct-drive wind systems since a large number of poles can be accommodated. However, a major drawback of AFPM is its very complicated manufacturing process.