## 6 Conclusions  
This paper investigates the AC copper losses for different winding topologies in inverter-fed electrical machines. Also, different winding topologies are simulated at the strand level using the FEM under PWM currents. This includes flat solid conductors and stranded windings. Moreover, the effect of strand transposition is simulated and verified experimentally using 3D-printed parts with controlled strand locations. The results are finally verified using different coil prototypes such as flat solid, parallel strands, and flat litz. Also, the impact of conductor placement inside the slot is investigated by comparing the same magnet wire under different conductors per layer. Even more, a more practical approach for core loss measurement is introduced for iron loss separation. A comparison is also made between all the implemented coils at different frequency levels, highlighting the best case. Finally, new hybrid coil topologies are suggested that incorporate various magnet wires within the same slot. The results demonstrate that employing a mixed wire arrangement can efficiently minimize the negative impact of eddy current losses, leading to a reduction in losses by 16-41% and weight reduction of 36-70%.  
The proposed hybrid coil topologies offer several primary benefits, including:  
* Reduced AC losses: The hybrid coils significantly reduce AC losses, resulting in improved overall efficiency in electrical machines.
* Weight savings: These designs lead to substantial weight reductions, which can be advantageous for applications where weight is a critical factor.
* Balanced loss distribution: The coils achieve a more balanced distribution of losses, preventing the concentration of losses near the slot opening. However, it is essential to acknowledge that there may be potential drawbacks, including:
* Complexity: Implementing hybrid coils with multiple materials and designs may introduce manufacturing complexity, which could impact production costs.
* Material compatibility: Ensuring the compatibility and performance of different materials within a single coil design may require careful engineering and testing.  
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline  & **Coil** & **Ref.** & **Hy-1** & **Hy-2** & **Hy-3** \\ \hline Total losses & [Watt] & 53.92 & 45.07 & 31.89 * & 35.57 \\ @ 1 kHz & [p.u.] & 1 & 0.84 & 0.59 * & 0.66 \\ \hline Weight & [gm] & 172.8 & 98.6 & 111.6 & 52.3 * \\  & [p.u.] & 1 & 0.57 & 0.64 & 0.30 * \\ \hline \hline \end{tabular} (*) Lowest value.  
\end{table}
Table 4: Measured losses and weights for the hybrid coils.  
Overall, the benefits of reduced losses and weight savings outweigh the potential drawbacks, making hybrid coil topologies a promising solution for enhancing the efficiency of electrical machines.  
Conceptualization, A.S.; Investigation, A.S. and P.S.; Resources, P.S.; Writing --original draft, A.S.; Writing--review & editing, K.B.T., M.N.I. and P.S.; Project administration, P.S.; Funding acquisition, P.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The author would like to acknowledge Punch Powertrain NV for supporting this research work. Punch Powertrain is a global partner of choice for innovative, affordable, and sustainable powertrain solutions.  
The authors declare no conflict of interest.