Mohammad Kimiabeigi

High-Performance Low-Cost Electric Motor for Electric Vehicles Using Ferrite Magnets

Permanent-magnet motors with rare-earth magnets are among the best candidates for high-performance applications such as automotive applications. However, due to their cost and risks relating to the security of supply, alternative solutions such as ferrite magnets have recently become popular. In this paper, the two major design challenges of using ferrite magnets for a high-torque-density and high-speed application, i.e., their low remanent flux density and low coercivity, are addressed. It is shown that a spoke-type design utilizing a distributed winding may overcome the torque density challenge due to a simultaneous flux concentration and a reluctance torque possibility. Furthermore, the demagnetization challenge can be overcome through the careful optimization of the rotor structure, with the inclusion of nonmagnetic voids on the top and bottom of the magnets. To meet the challenges of a high-speed operation, an extensive rotor structural analysis has been undertaken, during which electromagnetics and manufacturing tolerances are taken into account. Electromagnetic studies are validated through the testing of a prototype, which is custom built for static torque and demagnetization evaluation. The disclosed motor design surpasses the state-of-the-art performance and cost, merging the theories into a multidisciplinary product.

Electromagnetic Considerations for a Six-Phase Switched Reluctance Motor Driven by a Three-Phase Inverter

The switched reluctance machine (SRM) offers advantages over other topologies, but low torque density, high torque ripple, and use of a nonstandard power converter are limitations. This paper develops a drive configuration, which facilitates the operation of a six-phase SRM using a standard three-phase inverter in order to address these limitations. The focus of the paper is an investigation of electromagnetic design aspects of two candidate SRM topologies in this six-phase context for a pure electric or hybrid electric vehicle-type application. Advances are made in the understanding of the electromagnetic design of suitable SRMs, and the conventional SRM is demonstrated as the preferred topology through parametric and finite-element analysis (FEA) design studies with reference to a given specification. Laboratory test results for a prototype machine are presented in verification of the machine design and demonstration of this drive concept as a high-torque-density candidate suitable for electric vehicle applications.

On Selection of Rotor Support Material for a Ferrite Magnet Spoke-Type Traction Motor

Interior permanent magnet motors with ferrite magnets and distributed windings can be a cost effective alternative to rare-earth magnet based motors for demanding applications such as automotive traction. Among different rotor topologies, the spoke type may be preferred, due to its advantages for high flux concentration and resistance to demagnetization, when carefully designed. When high speed operation is required, to increase the power density of the motor, the spoke type rotor must be comprised of two sections: a) the ferromagnetic rotor pole to provide the path for the magnetic flux, and b) the non-magnetic rotor support to provide the structural integrity. In this paper, the multiphysics and cost implications of the rotor support material, as part of a high performance ferrite magnet traction motor, are reviewed, and an optimal selection with respect to those criteria is proposed.

Three-Dimensional Modelling of Demagnetization and Utilization of Poorer Magnet Materials for EV/HEV Applications

High performance electric motor designs with ferrite magnets have recently gained interest due to the high and volatile price of rare earth magnets. However, due to the relatively poor coercivity of ferrite magnets, these designs are highly susceptible to demagnetization, as a result of which accurate modelling and better understanding of this phenomenon is particularly important. In this paper, the impact of the motor stack length and level of magnetic saturation on the demagnetization risk are studied based on 3-dimensional finite element simulations and a proposed lumped circuit model. It is shown that reducing the stack length can significantly enhance the demagnetization resistance with the effect being more pronounced for designs with a higher level of magnetic saturation. To benchmark the practicality of the concept, a previously presented high-performance ferrite-based design is modified by using a 30% weaker grade of ferrite magnet whilst shortening the stack length. It is shown that the demagnetization withstand capability of the design was significantly enhanced and exceeded the short circuit requirement with a good safety margin. The theoretical findings have been supported by prototype testing.

Comparative Assessment of Single Piece and Fir-Tree-Based Spoke Type Rotor Designs for Low-Cost Electric Vehicle Application

Permanent magnet motors with ferrite magnets may be applicable to demanding electric vehicle applications, provided that designs with high power density and efficiency, as well as high resistance to demagnetization are developed. One rotor topology that allows meeting those challenges is a design with a so-called spoke arrangement of the magnet poles. Although, a spoke type rotor topology is widely known, the influences of the rotor mechanical design on the electromagnetic performance have largely been overlooked, while a comparison of different feasible design solutions has not been provided to date. In this paper, a novel spoke type traction motor using ferrite magnets and based on a single piece rotor topology has been presented. The design is based on a coupled structural-electromagnetic optimization, where the key design features influencing the structural stress and the electromagnetic torque density have been explained. This design is, further, compared against a previously disclosed fir-tree spoke type rotor solution, which has been designed for the same set of geometrical and output requirements. Through the comparisons, and for the first time, it is shown that the two alternative spoke type designs show significant differences in performance, including torque density, efficiency, and demagnetization.

On winding design of a high performance ferrite motor for traction application

Design of low cost traction motors with ferrite magnets needs to meet challenges such as minimizing the risk of demagnetization, and maximizing the torque and power density, via a suitable choice of rotor and stator winding topology and parameters. With regards to the stator, distributed and concentrated windings may have both advantages and disadvantages when considering manufacturing cost, slot fill factor, the contribution factor of reluctance torque and parasitic effects. Furthermore, the trend toward high speed operation of the traction motors may increase the AC loss effects in the windings, contributing to motor deficiencies and risk of thermal failure. In this paper, the performance of a high speed ferrite motor with a distributed and concentrated wound stator, and with regards to torque and power performance as well AC loss effects is assessed. The dynamic performance of a full scale prototype design based on a distributed aluminum wound stator is presented.

Electromagnetic considerations for a six-phase switched reluctance motor driven by a three-phase inverter

The Switched Reluctance Machine (SRM) offers advantages over other topologies, but low torque density, high torque ripple, and use of a non-standard power converter are limitations. This paper develops a drive configuration which facilitates the operation of a six-phase SRM using a standard three-phase inverter in order to address these limitations. The focus of the paper is an investigation of electromagnetic design aspects of two candidate SRM topologies in this six-phase context for a pure electric or hybrid electric vehicle type application. Advances are made in the understanding of the electromagnetic design of suitable SRMs, and the conventional SRM is demonstrated as the preferred topology through parametric and FEA design studies with reference to a given specification. Initial laboratory test results for a prototype machine are presented in demonstration of this drive concept and verification of the machine design.

Production and Application of HPMS Recycled Bonded Permanent Magnets for a Traction Motor Application

Due to the volatility of the cost and sustainability concerns associated with the rare-earth permanent magnets, alternative product designs using less or no rare-earth contents have, recently, gained popularity. Another method to address this need is to apply a magnet recycling process, such as the novel hydrogen processing of magnetic scrap (HPMS) which can be applied to the end-of-life products such as hard drive disks. Despite the growing research on the background science of different recycling techniques, a practical make, use and evaluation of recycled magnets in a real-life application, is rarely attended. To address this gap, in this paper and for the first time, the viability of the HPMS recycled magnets for use in a permanent magnet traction motor is investigated. On this basis, a detailed description and testing of the recycling process and the magnet production for a customized traction motor design is provided. Furthermore, the behavior of the motor using the final magnet product is analyzed using simulations and prototype testing. Based on the results, the proposed recycled magnets satisfy the overall requirements, while demonstrating similar or better electromagnetic performance compared to the alternative low-cost ferrite magnets.

A fourier approach for computationally efficient modelling of the operating envelope in PMSMs

This paper proposes a novel approach to modelling the torque/speed envelope of PMSMs which considers harmonic components in the voltage waveform. The proposed model is computationally efficient and can produce precise results rapidly, the accuracy of the model and improvement over previous dq based techniques is validated against experimental measurements of a 21-slot 14-pole IPM motor for a hybrid electric vehicle. The technique is then applied to a model of a ferrite based PMSM for a 48 V application; it is shown how use of the dq method here can give a significant over estimation of the available peak power when compared to the Fourier approach.

Shaft cooling and the influence on the electromagnetic performance of traction motors

This paper addresses the heat transfer coefficient associated with a shaft-cooling of traction motors. In such shaft-cooling systems, the 50–50 ethylene glycol-water is made to flow through the shaft hole in order to cool the machine. The heat transfer coefficient is estimated using a computational fluid dynamics(CFD) method, where the effect of the rotational velocity as well as the liquid flow rate have been accounted for. The results from two different turbulence models were compared. As a result of the simulations, it is concluded that the rotational speed can significantly increase the convective heat transfer in the shaft hole above the stationary condition. Finally, the benefits of implementing a shaft-cooling to an existing ferrite magnet traction motor, in terms of the continuous torque capability, is described.