This thesis presents a study of electric machine topology aimed at maximizing the specific power of a powertrain motor for a 120 kW electric aircraft propeller drive. The initial design is a radial flux permanent magnet (RFPM) machine with an inner rotor, surface-mounted permanent magnets (PMs), a winding of single-layer 12-tooth-14-pole (12T14P) layout, and flux barriers in alternate stator teeth. The study of RFPM machine topology includes the following four: i) higher number of poles, ii) outer rotor, iii) Halbach-array PMs, and iv) double-layer 12-tooth-10-pole (12T10P) layout. The research scope of axial flux permanent magnet (AFPM) machines is divided into two categories: i) single-sided and ii) yokeless and segmented armature (YASA) structures. In single-sided machines, a comparison between double-layer 12T10P and single-layer 12T14P layouts is conducted, and the latter is in combination with flux barriers. Then, two multi-level excitation systems are studied, i.e., double-segment structure and double-layer winding of unequal radial length. It is shown that more efficient iron utilization is possible by using a combination of high and low excitation levels. The design of the YASA machine adopts a segment-oriented design method, which converts the design of AFPM machine into the manipulation of individual segments. Optimized YASA machines with continuously increased specific torque are generated in a deterministic way by this method. The last topology study is that the surface-mounted PM rotor of the YASA machine is replaced by a Halbach-magnetized PM rotor. This thesis provides an in-depth understanding of the impact of these different topological structures on machine performance.