# A New Multi-Axial Flux Pm Motor-Generator System for Flywheel Energy Storage
## 4 Discussion  
In this study, a new flywheel design with a discharge winding that could give high current in a short time for BMF, and two AFPM machine structures are suggested. The suggested design consists of a low-speed stage AFPM motor, a high-speed stage AFPM motor, and an additional discharge winding on the low-speed stage motor winding. Thus, the flywheel reaches the reference speed in a short time with the high discharge current provided by the suggested system and has the advantage of low energy consumption at the reference speed. For high discharge current output from the FESS, a separate winding was used instead of the existing winding of the 2p = 12 pole AFPM motor-generator. This discharge winding is called C. In addition, data were obtained by different connections with motor windings called A and B in 2p = 12 pole AFPM motor-generator and compared with the experimental data of the C winding. In the study, first of all, magnetic analyses were carried out with 3D Maxwell for 2p = 2 and 2p = 12 pole AFPM machines. Since the increase in the Lw parameter causes a decrease in the number of turns, the \(\sigma\) value is taken as 0.4 for the Lw parameter. At minimal values of \(\sigma\), saturations occur in the core feet. So, the parameters were obtained with the ANSYS Maxwell program by classical optimization (iteration), considering the magnetic saturation points of the core. * As a result of the magnetic analysis for 2p = 2 pole, L\({}_{\rm w}\), L\({}_{\rm h}\), M\({}_{\rm d}\), and M\({}_{\rm h}\) values were found to be 2.5 mm, 12 mm, 5\({}^{\circ}\), and 7 mm, respectively;
* As a result of the magnetic analysis for 2p = 12 pole, L\({}_{\rm w}\), L\({}_{\rm h}\), and M\({}_{\rm p}\) values were found to be 5.5 mm, 12 mm, and 30\({}^{\circ}\), respectively. For the no-load state of the AFPM machine that was suggested for the flywheel energy storage system, the induced voltage values per rpm were obtained for the 2p = 2, 2p = 12, A, B, and C windings. When the AFPM machine operates as a motor, V/rpm values are an important parameter for drive design. * For the 2p = 2 pole AFPM machine, the induced emf value per rpm was obtained as 0.0305;
* For the 2p = 12 pole AFPM machine, the induced emf value per rpm was obtained as 0.197 for A and B windings and 0.024 for C winding. Since C winding is 1/8 of A an B windings, the V/rpm value is also measured proportionally. The study suggested a new Flywheel energy storage system for CML. In this system, three windings (A, B, and C), are designed for the 2p = 12 pole AFPM motor-generator structure. For CML systems, a high energy discharge current must be taken in a short time such as 0-2 s. In the literature, discharge processes are carried out by making a single or a 2-part (A, B winding) motor winding. Unlike the literature, the experimental results obtained from a separate generator winding (C winding) suggested in this study are given below. Experimental data were obtained by recording current and voltage values on the Fluke 190 series oscilloscope. * For the winding A, 332.42 Watts of power were obtained in 0.5 s at 460 rpm, with the highest at 2.61 A;
* For the A + B series winding, 474.03 Watts of power energy was obtained in 0.5 s at 460 rpm and 2.32 A;
* For the A / / B parallel winding, 463.25 Watts of power energy was obtained in 0.5 s at 460 rpm and 3.81 A;
* For the C winding (designed for BMF), 255.78 Watts of power energy was obtained in 0.5 s at 460 rpm and 24.02 A;* For the C winding, 1499.89 Watts of power energy was obtained in 0.5 s at 1188 rpm and 38.4 A. Although more current is obtained by connecting the winding in parallel on the AFPM motor-generator system, there is more voltage drop due to internal resistance and leakage reactance, which reduces the energy obtained. In the C winding, on the other hand, the current load increases with the design that increases the number of parallel arms of the winding. Thus, the amount of energy at higher speeds also increases.