# A New Multi-Axial Flux Pm Motor-Generator System for Flywheel Energy Storage
###### Abstract  
This study presents a flywheel energy storage system utilizing a new multi-axial flux permanent magnet (MAFPM) motor-generator for coil launchers. The traditional winding structure of the flywheel is effective for energy recovery over several minutes. However, because the projectile is launched from coil launchers in less than one second, the traditional winding structure experiences insulation deterioration and winding damage due to the high current. This study proposes a winding structure made of an \(8\times 0.5\) mm conductor with eight turns to meet the energy requirements of coil launchers. Furthermore, the motor winding was divided into two sections, which were compared using both series and parallel connection methods as described in the literature. The proposed system produces energy that is 29.96%, 85.63%, and 81.11% lower than the A winding (where A and B are identical), the A + B winding (series connected), and A /B winding (parallel connected) at the same speed. However, as the speed increases by 258.26%, the energy output rises by 215.88%. The flywheel motor-generator's series-parallel winding structure reaches its current carrying capacity at 1188 rpm. By utilizing a separate winding instead of the traditional motor-generator winding, a current of 38.4 A is achieved, ensuring that the winding's current carrying capacity remains within the design parameters. Experimental data have proven that the proposed multi-wire winding structure is an innovative solution for coil launchers, surpassing various combinations of motor-generator windings found in the literature. Furthermore, the placement of the proposed winding in a single slot in the design ensures a compact structure. e +
Footnote †: journal: Applied Sciences  
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Footnote †: journal: Applied Sciences  
magnet poles and the slot edges in no-load conditions. There are many methods in the literature on the rotor and stator side to eliminate the cogging torque. Among them, the methods applied to the rotor side are more economical and more accessible. The most common techniques that have been used on the rotor side can be listed as skew methods (classic, parallel, trapezoidal, and rounded skew), rotor shifting [16], slot use with different openings, magnet grouping [17], magnet placement angle changing [18], and different shaped magnet usage. Flywheel energy storage systems are divided into low and high-speed systems. While low-speed energy storage systems are up to 6000 rpm, high-speed energy storage systems reach 60,000 rpm. Flywheel energy storage systems consist of parts such as a flywheel that stores the energy, a motor that accelerates the flywheel, a generator where the energy is recovered, a mechanical and magnetic bearing, and a vacuum body [19]. As the speed of the flywheel energy storage system increases, the amount of energy stored increases. However, the use of high-speed or magnetic bearings has disadvantages, such as increasing the hardness of the flywheel. So, this study suggests a low-speed flywheel energy storage system for the coil launcher. FESS and supercapacitors are more advantageous than chemical batteries due to their high discharge current and high cycle life [20]. On the other hand, FESS comes to the fore with a higher energy density than supercapacitors [19]. FESS is recommended for a wide range of uses. Some of those are power fluctuations in wind power plants, energy storage in space shuttles, and aircraft launcher systems as well as the recovery of regenerative braking energy in electric vehicles, rail, and coil launchers [21]. When examining the literature, supercapacitors provide the high energy required to ensure the movement of the projectile in the coil launcher within less than 2 s [5,6]. However, the energy discharge time for a flywheel is 2 min, compared to just 1 min for supercapacitors [22].