###### Abstract  
Recent advances in electrical machines and energy storage technologies make electric vehicles (EVs) feasible replacements to conventional internal combustion engines. One of the main challenges of high speed electrical machines is providing maximum output power with minimum energy losses, weight, and volume. At high frequency operation, the conductors of AC electrical machines can suffer from skin and proximity effects. This results in high AC losses in the machine windings and can eventually lead to machine failure. In this paper, a novel design for a semi-stranded coil is proposed to limit these undesirable effects. Enabled by additive manufacturing (AM) technology, this sophisticated design is 3D printed using ultralight aluminum alloy. Finally, the AC performance of this coil is measured and compared with conventional single-strand copper coil at different frequency levels. It is found that the proposed design can effectively limit the eddy current losses in the high frequency domain. electric machines; 3D printed windings; additive manufacturing eddy currents +
Footnote †: journal: Applied Sciences  
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Footnote †: journal: Applied Sciences  
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Footnote 0:  
utilizing such wires, it is expected that the overhang will be long in spite of their superior bendability, especially if there is an overlap between windings. For a further improvement of the performance of stranded wires at high frequencies, the strands transposition can be considered one of the helpful solutions to reduce the eddy current losses in the conductors. With insulated and twisted sub-strands, litz windings can effectively limit the high-frequency eddy current losses and circulating currents. However, using fully stranded coils is not the optimal solution for the above-mentioned issue because of two main reasons. The first reason is that the fill factor of the stranded coils is very low compared to other types of magnet wires (e.g., rectangular hairpins). As a result, the DC losses are much higher. The second reason is that stranded wires are usually placed randomly inside the stator slot, and it is extremely hard to control the strand location. This can eventually result in higher AC losses than expected. On the counterpart, using a single strand coil with flat conductors can guarantee a high fill factor and low DC resistance. However, the skin effect and AC losses will be too high. Flat hairpin conductors can fit in a rectangle closed or semi-closed slot. However, a post-process of welding is necessary to join the hairpins. This time consuming process can cause a delay in the supplier chain specially. Alternatively, a radially-inserted flat conductor coil can be used with a single continuous conductor. Yet, an open slot design is required. The disadvantages of flat solid conductor include the high cost and the less mechanical flexibility. They cannot also fit in different slot shapes. Besides, there are excessive AC losses at high frequencies. This study is aiming to find effective solutions for the aforementioned issues by combining between the reduced AC losses of stranded windings and the high fill factor of single-strand windings. This is achieved by adopting a new design concept of \"semi-stranded coil\" with half-solid half-stranded conductors. Additionally, the strands' locations are controlled and transposed to minimize the AC losses in this coil. Combining between the design novelty, controlled strand positioning, and ultralight weight structure, is only possible using a flexible and unique technology such as additive manufacturing (AM). To the best of the authors' knowledge, there is no published work concerning 3D printed stranded windings with such structure, especially for high speed electrical machines. The paper is divided as follows. Section 2 is outlining the potentials and recent developments in the 3D printing of electrical machines and the windings in particular. In Section 3, a new concept of semi-stranded windings is introduced and different possibilities are simulated using finite element modeling (FEM).