## 3 Semi-Stranded Coils: A New Concept  
To combine high fill factor and high AC performance, a novel semi-stranded coil is proposed. In this design, the lower half of the coil has a single strand, and the upper half has multi-strands. The main reason is that the upper turns (i.e., near the slot opening) are more exposed to leakage flux, and they possess over 70% of the total AC losses [27]. Additionally, unlike coils with randomly placed strands, a controlled strand positioning is enabled using AM. This means that each strand has a well-defined location inside the stator slot, which allows for an accurate finite element simulation. In order to demonstrate the concept of the semi-stranded coil, the electromagnetic performances of five case studies are investigated, as shown in Figure 2. As can be noticed, Coil 1 is the reference coil case which represents the conventional approach of rectangular windings. In this design, eight flat solid copper conductors are used per slot. Coil 2 is a semi-stranded coil with the upper three turns divided into two strands. Coil 3 is a semi-stranded coil with the upper four turns divided into four strands with fixed locations. Coil 4 is similar to Coil 3, except for using different strand positions. The concept of strand transposition is compared with the fixed strand location, as shown in Figure 3. The strands of the four upper turns are transposed so that each strand moves one location when going to the next turn. The design of the end turns in 3D is very compact and precise especially at the front end of the coil (Figure 3). The one of a kind design allows for a smooth and continuous transition between the series turns. Coil 5 is identical in the design of Coil 4; however, an aluminum material is used instead of copper. The main target is to compare the performance of these materials at high frequency under the same geometries. The electrical circuit of Coils 3-5 is shown in Figure 4. As can be seen, the first four turns have a single conductor and are carrying the same current. On the top four turns, each turn has four strands. The strands in the positive  
Figure 2: Current density distribution at high frequency (2 kHz) in semi-stranded coils compared to the solid single-strand coil. and negative slots are electrically connected. For a fair comparison between all cases, the same slot geometries are used for all the designs. Additionally, all the designs are simulated under the same current level. The current densities in the five designs are computed and compared at high frequency of 2 kHz, as shown in Figure 2. As can be seen, Coil 1 has the worst current density distribution. The main reason is that the turns are solid causing an excessive current density of 44.5A/mm2 at the upper turns. In Coils 2 and 3, the maximum current density is not reduced much due to using a fixed strand location. On the counterpart, Coils 4 and 5 have reduced the current density significantly by 43% and 63%, respectively. The aluminum coil (Coil 5) has a better current density distribution compared to the copper one (Coil 4). The main reason is that the aluminum has higher resistivity than copper, resulting in more limitation for the eddy current losses in the high frequency domain. The losses are also calculated in each turn for all the five case studies at 2 kHz, as shown in Figure 5a. As can be noticed, in Coils 2 and 3, the losses are reduced by stranding in only turns 7.8. On the other hand, with the transposed strands in Coils 4 and 5, all the stranded turns have remarkably lower AC losses. Furthermore, the total AC losses are compared for all coil cases at different frequency levels, as shown in Figure 5b. It is clear that using the semi-stranded coil with transposed strand location can provide a remarkable performance starting from low frequencies (200 Hz). It is also obvious that using aluminum instead of copper can provide even lower losses above 1 kHz. With this being said, a semi-stranded aluminum coil is to be built using AM with transposed strand locations.