This digest introduces a novel BL-WRSM having a unique two-layer stator winding configuration that surpasses the performance of the BL-WRSMs presented in [1] - [5]. Moreover, the new two-layer stator winding is such that it utilizes all the slot space available, and it can produce fundamental MMF components for torque generation and sub-harmonic MMF components for field excitation. As it can be seen from Fig. 1, two sets of series-connected stator windings, ABC and XYZ, are wound in a two-layer arrangement. Although the bottom layer’s slots are entirely covered by the ABC winding, the top layer consists of both ABC and XYZ winding, where every other slot has XYZ winding conductors in the whole periphery of the top layer, which is shown in Fig.2. By this arrangement, a single current source inverter can drive the machine with a maximum number of slots utilization. It is important to state that the rotor of the machine is also comprised of two separate windings itself and they are: the field winding and the harmonic winding. While the field winding is wound in an 8-pole manner, the rotor harmonic winding is a 2-pole winding that is employed for inducing the sub-harmonic MMF generated by the XYZ winding of the stator. Both the rotor windings are connected via a diode bridge rectifier circuit mounted on the periphery of the rotor. To extend the understanding, the fundamental MMF component is produced by winding ABC, and it has an 8-pole arrangement with the frequency of ωm, where ωm is the frequency in the mechanical space domain. Whereas in the case of winding XYZ with its 2-pole configuration, it delivers the sub-harmonic MMF component with the frequency of (ωm)/4. Thus, the two components rotate at a different speed. In the rotor, on the other hand, the harmonic winding is placed on the rotor such that it aligns magnetically with the stator's XYZ winding, and both are wound in a 2-pole manner. Adjacent to the harmonic winding, which is connected through the diode rectifier, field winding having an 8-pole arrangement aligns magnetically with winding ABC having 8 poles. Given all that, the novel BL-WRSM achieves its brushless operation by inducing the sub-harmonic MMF to the rotor harmonic winding and by magnetically locking the stator fundamental MMF with the rotor field winding. The main manuscript of this topic will cover an extensive literature review, structure of the machine, winding configuration, and working principle of the proposed BL-WRSM. Along with these, a 2-D finite element analysis (2-D FEA) has been performed to test the validity of the machine, which is also partially reported in this digest. An 8-pole, 48-slot BL-WRSM with a 3-phase, 2-layer distributed stator winding is designed to validate the proposed topology. Based on the stator winding structure, both ABC and XYZ windings are placed throughout the 360-mechanical degree of the stator, and the rated power of the machine is considered 1281 W having a constant speed of 900 rpm. To examine the effectiveness of the proposed machine, its performance is compared with the conventional WRSM used in [3] and the single inverter BL-WRSM in [4]. For the proposed machine, however, all design parameters are kept exactly similar to the BL-WRSM presented in [4], which includes, outer diameters, air-gap length, stack length, the current density in the stator, and rotor winding to name but a few. While all the machines are supplied with an RMS current of 4.5 A, the rotor field current is found in the proposed BL-WRSM (21.61 A) is remarkably higher than that of the BL-WRSM (9.8 A) in [4]. The reason for the higher current is 2-pole sub-harmonic component and the proposed 2-layer stator winding arrangement. As a result, the 2-D FEA shows that the average torque of the new machine is found 12.03 Nm that is approximately 65% more than the average torque production of the machine in [4]. Despite the performance enhancement in torque generation, the torque ripple is found 34%. Moreover, the flux density and flux lines are somewhat evenly distributed throughout the stator rotor frame for the novel BL-WRSM, unlike in the case of BL-WRSM in [4], where the distribution was found uneven because of the under-utilized stator slot space. It also shows the proposed BL-WRSM works under the flux density of 2 T. The focal point of this work is to develop a brushless wound rotor synchronous machine. To study the performance and feasibility the new machine has substantiated with the 2-D finite element method. Although operation wise it resembles the BL-WRSMs, performance-wise it surpasses many crucial aspects, and it demonstrates some certain advantages over the newly developed BL-WRSMs in [2] to [5]. Hence, the proposed machine could be an alternative solution considering all the attractive features provided, and they are listed as follows, 1. Single inverter brushless operation. 2. Utilizes the maximum allowable slot capacity. 3. No need for implying complex current control to mitigate unbalanced current circulation in the stator. 4. No need for imposing a complex unbalanced-cooling system. 5. Greater torque generation. With all these advantages, it can certainly be a competitive alternative not only to the BL-WRSMs but also with the state of the art PMSMs those are used in contemporary electrified transportation application. Moreover, the proposed topology could potentially adopt the design presented in [6] to [9], and used in converter topologies presented in [10], [11]. These designs will extensively be covered in the main manuscript.

**References:**

1. F. Yao, D. Sun, L. Sun, and T. A. Lipo, ‘‘Dual third-harmonic-current excitation principle of a brushless synchronous machine based on double three-phase armature windings,’’ in Proc. 22nd Int. Conf. Electr. Mach. Syst. (ICEMS), Harbin, China, Aug. 2019, pp. 1–4. 2. Q. Ali, T. A. Lipo and B. Kwon, "Design and Analysis of a Novel Brushless Wound Rotor Synchronous Machine," in IEEE Transactions on Magnetics, vol. 51, no. 11, pp. 1-4, Nov. 2015, Art no. 8109804. 3. Ali, Q., Atiq, S., Lipo, T. A., & Kwon, B. (2016). PM Assisted, Brushless Wound Rotor Synchronous Machine. Journal of Magnetics, 21(3), 399-404. 4. Hussain, A., & Kwon, B. (2017). A new brushless wound rotor synchronous machine using a special stator winding arrangement. Electrical Engineering, 100(3), 1797-1804. 5. Hussain, A., Atiq, S., & Kwon, B. (2018). Optimal Design and Experimental Verification of Wound Rotor Synchronous Machine Using Subharmonic Excitation for Brushless Operation. Energies, 11(3), 554. 6. M. Ayub, S. Atiq, Q. Ali, A. Hussain and B. Kwon, "Dual-Mode Wound Rotor Synchronous Machine for Variable Speed Applications," in IEEE Access, vol. 8, pp. 115812-115822, 2020 7. G. Jawad, Q. Ali, T. A. Lipo and B. Kwon, "Novel Brushless Wound Rotor Synchronous Machine With Zero-Sequence Third-Harmonic Field Excitation," in IEEE Transactions on Magnetics, vol. 52, no. 7, pp. 1-4, July 2016, Art no. 8106104. 8. M. Ayub, A. Hussain, G. Jawad and B. Kwon, "Brushless Operation of a Wound-Field Synchronous Machine Using a Novel Winding Scheme," in IEEE Transactions on Magnetics, vol. 55, no. 6, pp. 1-4, June 2019, Art no. 8201104. 9. A. Hussain, S. Atiq and B. Kwon, "Consequent-Pole Hybrid Brushless Wound-Rotor Synchronous Machine," in IEEE Transactions on Magnetics, vol. 54, no. 11, pp. 1-5, Nov. 2018, Art no. 8206205. 10. Rafin, S. M., Lipo, T. A., & Kwon, B. (2014). A novel topology for a voltage source inverter with reduced transistor count and utilizing naturally commutated thyristors with simple commutation. 2014 International Symposium on Power Electronics, Electrical Drives, Automation and Motion. doi:10.1109/speedam.2014.6872057 11. Rafin, S. M., Lipo, T. A., & Kwon, B. (2015). Performance analysis of the three transistor voltage source inverter using different PWM techniques. 2015 9th International Conference on Power Electronics and ECCE Asia (ICPE-ECCE Asia).

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Fig.2

 A sub-harmonically excited novel brushless wound rotor synchronous machine using a new two-layer winding topology.

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