# Research and Development of a Large-Scale Axial-Flux Generator for Hydrokinetic Power System
## 3 Results
### Feasibility Study
#### 3.5.2 Manufacturability and Mechanical Solution  
The generator has an outer diameter of nearly 14 m, and its manufacturing needs to use technologies typical of large structures. Several points could be raised at this phase of the product development process:  
* **Magnet production and mounting:** NdFeB magnets are manufactured by arranging the crystal orientation of NdFeB magnet powder in a particular direction using an external magnetic field, applying mechanical pressure to the mold, and sintering it through powder metallurgy. In line with this process, the electromagnet used to apply the magnetic field is incorporated into the press. Due to the limitations of the electromagnet's performance, the maximum size of a magnet manufactured in one press is approx. 100 mm\({}^{2}\). Existing studies on similar structures [27] use production technology for the magnet by assembling magnet blocks measuring 100 mm \(\times\) 100 mm \(\times\) 50 mm. These blocks are bonded to an iron plate, which is then bolted to the rotor disk, as NdFeB magnets are fragile, and threaded holes cannot be made in them (refer to Figure 9). Magnetic blocks are bedded and bonded to a back plate before magnetizing. Then, the unit is completed by one magnetizing process using the superconductive magnetizing equipment. Another problem is related to the mounting of magnets in  
\begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Preliminary Defined Design** & **Improved Final Design** \\ \hline Ratio of number of magnets to number of windings rows & Five rows of magnets against four rows of windings, which leads to sharing a magnet between two coils & Four rows of magnets against two rows of windings, which leads to a pair of magnets for each winding \\ Radial size of the winding against the radial size of the magnet & Almost equal radial sizes of the magnet and the winding, which causes the magnetic field to influence tangential winding branches and decrease efficiency & The windings are larger than the magnets, and their tangential branches are not influenced by the magnetic field \\ Air gap size & 140 mm air gap size decreases design efficiency & The air gap decreased to 40 mm \\ Coil thickness & 200 mm thickness of the coil in the axial direction has a high negative effect on the efficiency & The coil thickness is decreased down to 60 mm, leading to a decreased overall axial dimension (flat design) \\ Coil cross section & Its big cross section (40,000 mm\({}^{2}\)) is based on a high winding thickness, which has a high negative effect on the efficiency & A sufficient cross section (21,000 mm\({}^{2}\)) is the result of decreased thickness but an increased width of 350 mm \\ Magnet width & winding width causes a magnet to overlap neighbor winding radial branch & A 400 mm magnet width fits the 350 mm \\ \hline \end{tabular}
\end{table}
Table 1: Improvements and their influence to reach better efficiency.  
the structure. It is expected to be difficult to control the unit due to the strong magnetic attractive force toward the rotor, especially when the unit is near the rotor.
* **Windings' assembly:** The improved generator design has two rows of coils. Each row consists of 2\(\times\) 36 coils placed over a 10 mm-thick aluminum plate--each side of the plate has 36 coils. Other two-sided aluminum plates are mounted to improve the cooling of the module. This is shown in general in Figure 10. Another design specific of the winding is that it is flat--a 30 mm thickness allows for using a single coil row in its thickness, which facilitates winding manufacturing.