The axial permanent magnet magnetic bearingless flywheel machine (APM-BFM) mostly operates under the state of high-power fast charging and discharging. Due to the characteristics of high-power density per unit volume, the centralized windings have small heat dissipation area and high thermal loads, the temperature rise on APM-BFM is a very important issue. Thermal analysis is thus of particular importance particularly for modern machines since the temperature rise and distribution are significant during their running under higher electromagnetic loads commonly used nowadays. In this paper, the equivalent thermal network method is proposed to evaluate the temperature rise of the APM-BFM. The equivalent thermal network model is established based on the basic theorem of heat transfer. The thermal resistance and heat source in the machine are analyzed and calculated, and the thermal balance equation is established. The temperature rise of each node is solved, and finally the validity of the model is verified by finite element magnetic-thermal coupling simulation.

In this paper, a novel topology for the flywheel battery is proposed, based on the axial split phase (ASP) bearingless switched reluctance machine (BSRM) concept. The topology and operating principle of the ASP-BSRM are explained, the electromagnetic characteristics, torque, suspension force and coupling performance are analyzed by 3-D finite element analysis (FEA). In order to improve the torque and suspension force of the ASP-BSRM, the single variable method by FEA is adopt to screen out the sensitive structure parameters, and a new multi-objective optimization design mothed based on random forest regression (RFR) and differential evolution (DE) is proposed to optimize the key parameters. The 3-D FEA of the complete system is performed, and simulation results verify the proposed topology overcomes the dead zone of suspension force and realize the decoupling between torque and suspension forces. Besides, the optimized torque and suspension force are improved by 4.15% and 12.91%, respectively. An experimental prototype is built to provide an experimental basis for investigation of the actual operating characteristics. The experimental results of the system are evaluated under different operating conditions and compared with FEA results.

This paper proposes a topology of special axial split phase (ASP) bearingless switched reluctance (BSR) machine for flywheel applications named ASP-bearingless flywheel (ASP-BF) machine, which adopts hybrid-inner-stator permanent magnet (PM) structure and exhibits complete decoupling of torque and radial suspension force, four degrees-of-freedom (DOF) actively controlled suspension, high torque and radial force density. The topologies of conventional radial split phase (RSP)-BSR machines and the proposed ASP-BF machine are comparatively investigated. It is found that problems of conventional RSP-BSR machines, i.e., the radial suspension force generation dead zone and strong coupling of radial suspension force and torque, can be solved in ASP-BF machines and the suspension DOF can be doubled. Electromagnetic performances, including magnetic field distribution, inductance, torque, radial force are comprehensively investigated. Further, the concept of degree-of-coupling (DOC) is introduced, and the DOCs of the proposed machine and conventional machines are quantitatively analyzed and compared. It shows that the DOC of ASP-BF machine in two cases are 3.41% and 4.05%, less than a quarter of the conventional 8/10 pole machine (22% and 16%) and less than half of the 12/14 pole machine (8.57% and 11.3%). Finally, a 3 kW/100 Wh prototype is built and the experiments proved the validity.

In this Letter, three types of bearingless switched reluctance machines (BSRMs) with different hybrid stator structures are designed and analysed. First of all, topologies of three BSRMs, the conventional 8/10 pole, 12/14 pole BSRMs and a novel 12/12 pole BSRM with axial split phase inner stator permanent magnet structure, are given. Then, the electromagnetic performances, including the distribution of magnetic fields, suspension force and electromagnetic torque are comprehensively compared by means of 2D and 3D finite‐element method (FEM). The results based on theoretical analysis and FEM reveal that the proposed 12/12 pole BSRM would exhibit complete decoupling, shorter magnetic circuit, higher torque (power) density and suspension force than the conventional ones. Further, it is found that four degree‐of‐freedom suspension of the rotor can be realised by two‐phase suspension windings in the proposed 12/12 pole BSRM. Finally, a prototype machine is designed and manufactured to verify the validity of the topology design and electromagnetic performance.