Optimal Design of Special High Torque Density Electric Machines based on Electromagnetic FEA

Electric machines with high torque density are essential for many low-speed direct-drive systems, such as wind turbines, electric vehicles, and industrial automation. Permanent magnet (PM) machines that incorporate a magnetic gearing effect are particularly useful for these applications due to their potential for achieving extremely high torque density. However, when the number of rotor polarities is increased, there is a corresponding need to increase the number of stator slots and coils proportionally. This can result in manufacturing challenges. A new topology of an axial-flux vernier-type machine of MAGNUS type has been presented to address the mentioned limitation. These machines can attain high electrical frequency using only a few stator coils and teeth, which can simplify construction and manufacturing under certain conditions. Additionally, the inclusion of auxiliary small teeth within the stator main teeth can generate a noteworthy increase in output torque, making it a unique characteristic of this motor. By analyzing the operating principle of the proposed VTFM PM machine, possible pole-slot combinations have been derived. The process of designing an electric machine is complicated and involves several variables and factors that must be balanced by the designer, such as efficiency, cost, and performance requirements. To achieve a successful design, it is crucial to employ multi-objective optimization. Using a 3D FEA model can consider the impact of magnetic saturation, leakage flux, and end effects, which are not accounted for in 2D. Optimization using a 3D parametric model can offer a more precise analysis. Validating the machine\u27s performance requires prototyping a model and testing it under different operating conditions, such as speed and load, which is a crucial step. This approach provides valuable insights into the machine\u27s behavior, allowing the identification of any areas for improvement or weaknesses. A large-scale multi-objective optimization study has been conducted for an axial-flux vernier-type PM machine with a 3-dimensional (3D) finite element analysis (FEA) to minimize the material cost and maximize the electromagnetic efficiency. A detailed study for torque contribution has indicated that auxiliary teeth on each stator main teeth amplify net torque production. A prototype of optimal design has been built and tested

Traction motors for electric vehicles: Maximization of mechanical efficiency – A review

With the accelerating electrification revolution, new challenges and opportunities are yet emerging, despite range anxiety is still one of the biggest obstacles. Battery has been in the spotlight for resolving this problem, but other critical vehicle components such as traction motors are the key to efficient propulsion. Traction motor design involves a multidisciplinary approach, with still significant room for improvement in terms of efficiency. Therefore, this paper provides a comprehensive review of scientific literature looking at various aspects of traction motors to maximize mechanical efficiency for the application to high-performance Battery Electric Vehicles. At first, and overview on the mechanical design of electric motors is presented, focusing on topology selection, efficiency, transmission systems, and vehicle layouts; Special attention is then paid to the thermal management, as it is one of the main aspects that affects the global efficiency of such machines; thirdly, the paper presents a discussion on possible future trends to tackle ongoing challenges and to further enhance the performance of traction motors

Impact of slot/pole combination on inter-turn short-circuit current in fault-tolerant permanent magnet machines

This paper investigates the influence of the slot/pole (S/P) combination on inter-turn short-circuit (SC) current in fault-tolerant permanent magnet (FT-PM) machines. A 2-D sub-domain field computational model with multi-objective genetic algorithm is used for the design and performance prediction of the considered FT-PM machines. The electromagnetic losses of machines, including iron, magnet, and winding losses are systematically computed using analytical tools. During the postprocessing stage, a 1-D analysis is employed for turn-turn fault analysis. The method calculates self-and mutual inductances of both the faulty and healthy turns under an SC fault condition with respect to the fault locations, and thus SC fault current, considering its location. Eight FT-PM machines with different S/P combinations are analyzed. Both the performance of the machine during normal operation and induced currents during a turn-turn SC fault are investigated. To evaluate the thermal impact of each S/P combination under an inter-turn fault condition, a thermal analysis is performed using finite element computation. It is shown that low-rotor-pole-number machines have a better fault tolerance capability, while high-rotor-pole-number machines are lighter and provide higher efficiency. Results show that the influence of the S/P selection on inter-turn fault SC current needs to be considered during the design process to balance the efficiency and power density against fault-tolerant criteria of the application at hand

Investigation of Doubly Salient Stator Slot Permanent Magnet Machines

Variable flux reluctance machines (VFRMs), which have both field and armature windings on the stator, are novel types of magnetless machines with a simple and robust mechanical structure and a low manufacturing cost. However, their electromagnetic performance, especially their overloading capability, is limited by high magnetic saturation due to field excitation. Therefore, circumferentially magnetized permanent magnets (PMs) are placed in the stator slot openings in order to (a) alleviate the magnetic saturation and (b) increase the torque capability based on VFRM, which leads to novel machine topologies, i.e. hybrid excited stator slot PM machines (HESSPMs) and stator slot PM machines (SSPMs). The effects of PMs in the stator slot openings are comparatively investigated for VFRMs, HESSPMs and SSPMs together with the discussion of the unique fault tolerant feature in stator slot PM machines. Furthermore, the overlapping winding (OW) layouts with coil pitches of 3 stator slot pitches are proposed in the three machine topologies in order to enhance the torque density. The electromagnetic performance of each machine topology, with OW and non-overlapping winding (NOW) and various feasible stator slot/rotor pole number combinations, is comparatively studied by finite element method. It shows that the proposed OW layout can improve the average torque of VFRM, HESSPM and SSPM with the optimal stator/rotor pole number combination. The proposed OW layout will be more competitive for the machines with a longer axial length and reduced end-effect. Prototype machines for these three machine topologies with both NOW and OW are built and tested to validate the finite element predicted results

Mathematical Models for the Design of Electrical Machines

This book is a comprehensive set of articles reflecting the latest advances and developments in mathematical modeling and the design of electrical machines for different applications. The main models discussed are based on the: i) Maxwell–Fourier method (i.e., the formal resolution of Maxwell’s equations by using the separation of variables method and the Fourier’s series in 2-D or 3-D with a quasi-Cartesian or polar coordinate system); ii) electrical, thermal and magnetic equivalent circuit; iii) hybrid model. In these different papers, the numerical method and the experimental tests have been used as comparisons or validations

In-wheel Motors: Express Comparative Method for PMBL Motors

One of the challenges facing the electric vehicle industry today is the selection and design of a suitable in-wheel motor. Permanent Magnet Brushless (PMBL) motor is a good choice for the in-wheel motor because of its lossless excitation, improved efficiency, reduced weight and low maintenance. The PMBL motors can be further classified as Axial-Flux Twin-Rotor (AFTR) and Radial-Flux Twin-Rotor (RFTR) machines. The objective of this dissertation is to develop a fast method for the selection of appropriate in-wheel motor depending on wheel size. To achieve this, torque equations are developed for a conventional single-rotor cylindrical, twin-rotor axial-flux and twin-rotor radial-flux PMBL motors with slot-less stators based on magnetic circuit theory and the torque ratio for any two motors is expressed as a function of motor diameter and axial length. The theoretical results are verified, on the basis of magnetic field theory, by building the 3-dimensional Finite Element Method (FEM) models of the three types of motors and analyzing them in magnetostatic solver to obtain the average torque of each motor. Later, validation of software is carried out by a prototype single-rotor cylindrical slotted motor which was built for direct driven electric wheelchair application. Further, the block diagram of this in-wheel motor including the supply circuit is built in Simulink to observe the motor dynamics in practical scenario. The results from finite element analysis obtained for all the three PMBL motors indicate a good agreement with the analytical approach. For twin-rotor PMBL motors of diameter 334mm, length 82.5mm with a magnetic loading of 0.7T and current loading of 41.5A-turns/mm, the error between the express comparison method and simulation results, in computation of torque ratio, is about 1.5%. With respect to the single-rotor cylindrical motor with slotless stator, the express method for AFTR PMBL motor yielded an error of 4.9% and that of an RFTR PMBL motor resulted in an error of -7.6%. Moreover, experimental validation of the wheelchair motor gave almost the same torque and similar dynamic performance as the FEM and Simulink models respectively

Investigation of a tubular linear transverse flux machine for use with a free-piston engine

PhD ThesisThis thesis explores different topologies of permanent magnet linear electric machines for use with a renewable engine – Free-Piston Engine. By comparing their performances, it aims to prove that a new linear modulate pole (transverse flux) machine can be the best overall topology. Five candidate topologies including longitudinal flux and transverse flux types are presented and simulated by finite element analysis method in MagNet software. The designer optimises each topology with the same effort and strategy by using an advanced algorithm in OptiNet software. The transverse flux and flux switching topology are detailed compared with different optimisation strategies to investigate machine performances. The designer performs an advanced design study on the modulated pole machine to simplify the manufacturing process, reduce building cost and improve output performance. The process includes the tooth combination design, the single tooth winding design and the lamination design. The designer expands the insight of the final designed modulated pole machine. The force production and power factor derivation are performed theoretically and numerically. The numerical solution is compared to the simulation result for validation. Detailed mechanical design for the final designed modulate pole machine is performed consisting of: the shaft strength investigation, the stator assembly strategy, the winding assembly and drive linear actuator selection. The designer tests the final build modulate pole machine in both static and dynamic conditions. Static tests include cogging force test and DC test. Dynamic tests include EMF test and short circuit current test. All results are compared with simulations for machine quality check. The thesis demonstrates that the tubular transverse flux machine is manufacturable and fulfil the requirements of a Free-Piston Engine