## 5 Hybrid Coils for AC Loss Mitigation  
At high frequency operation, the flat copper coil has an unbalanced distribution of losses through the turns. Over 70% of the total coil losses are found in the upper three strands. With this being said, hybrid coils are proposed to have a fair distribution of losses and limit them in the turns near the slot opening. This section introduces alternative materials and new design solutions to mitigate AC losses at a high-frequency operation. Different combinations of materials and magnet wires are used to form different hybrid-strand designs, as explained in Figure 21. Five magnet wire options are used to evaluate the losses at each strand, which are solid copper, flat litz, solid commercial aluminum (Alu), a 3D-printed aluminum powder made from Al-Si-Mg-based alloy (Alu3D), and finally, a Roebel bar with four transposed strands made from Alu3D. Typically, Alu3D powder is used as a feedstock for the additively manufactured part due to its higher printability compared to pure aluminum. The electrical conductivity of the Alu3D after thermal treatment is lower than that of commercial aluminum, with a value of ~20 M S/m. Yet, the mass density of both materials is almost the same. Using the aforementioned five wires, the core is simulated using homogenous turns, and the losses are calculated in each turn. In Figure 22, the losses in each turn are compared in percentage using these materials. As can be seen, using a certain material for all turn  
Figure 21: Proposed hybrid coils and materials for AC loss mitigation. locations is not the optimal design. For instance, in turn 1, copper has the lowest losses among all materials. However, in turn 8, it has the highest loss percentage. On the counterpart, the Roebel bar has the highest losses when used in turn 1. However, it comes in second place when it is used for turn 8, having nearly the same losses as the litz wire, which has the lowest losses. Considering the aforementioned, each type of wire has an optimal location. To give another example, the losses in the turn at the bottom of the slot are higher for aluminum wire compared to copper wire, whereas near the slot opening, the situation is reversed. Having said that, three hybrid coils are proposed as follows. Figure 23: Hybrid coil (Hy-1): flat solid (3 Cu + 5 Alu). Figure 22: Losses in each turn for different magnet wires. In the first two coils (Hy-1 and Hy-2), copper is used for the first three turns because it has the lowest losses compared to all the other materials. However, this was not feasible for the printed coil because multi-material printing is not as advanced as single-material printing. In the third coil (Hy-3), the stranded section of the Roebel bar is transposed using the same pattern described earlier in Figures 12 and 13. In order to achieve a higher fill factor inside the slot, the strand transposition is performed at the coil head. As a result, the current density in the semi-stranded coil is damped remarkably in the 3D semi-stranded coil compared to the conventional coil. Finally, the coil is 3D printed using Al-Si-Mg-based material produced via Laser Powder Bed Fusion (L-PBF), as shown in Figure 26. The electrical insulation is added in a post-process by dipping the coil in a varnish bath. Figure 24: Hybrid coil (Hy-2): solid and stranded, 3 Cu + 3 Alu+ 2 Litz. Figure 25: Hybrid coil (Hy-3): 3D-printed solid and Roebel (4, 4 Alu3D). The losses in each turn are calculated for the three proposed hybrid coils, as shown in Figure 27. Compared to the reference copper coil, all coils have remarkably lower losses, especially in the top three strands. This is also verified by measuring the total losses at the same frequency level, as listed in Table 4. In Hy-1, the total coil losses are reduced by 16%, besides a 43% weight reduction. In Hy-2, the same design is used. However, the upper two turns are replaced with flat litz copper wire. The weight saving in this case is only 36% compared to the copper coil. However, this coil has the lowest losses with 41% lower losses.