James Goss

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.

Design Considerations of a Brushless Open-Slot Radial-Flux PM Hub Motor

Initial sizing of a brushless radial-flux permanent-magnet (PM) machine is typically based on assumptions regarding the electric and magnetic loadings together with the motors active volume. Additional design assumptions include the winding current density and magnetic flux density within the core pack. These initial design decisions are crucial as they largely define the resultant electromagnetic and thermal behaviors of the motor. The general rules regarding the choice of the initial design parameters are well known. However, the effects of the initial sizing on the final motor performance are not widely reported. This paper presents an analysis of the design considerations for a brushless open-slot radial-flux PM hub motor. A number of alternative motor designs are compared to demonstrate the effect of the initial design decisions on the final motor performance. Both electromagnetic and thermal aspects of the motor design are considered. The design variants are characterized by the same electric and magnetic loadings together with active volume while the winding rated current density and no-load/open-circuit magnetic flux density within the core pack are varied. The employed sizing methodology combines the classical approach with a nonlinear magnetostatic finite-element solver and an optimization routine. A prototype hub motor has been manufactured to validate the theoretical findings from the design process. The experimental data show good agreement with the theoretical findings.

The design of AC permanent magnet motors for electric vehicles: A design methodology

This paper presents a complete methodology for the design of AC permanent magnet motors for electric vehicle traction. Electromagnetic, thermal and mechanical performance aspects are considered and modern CAD tools are utilised throughout the methodology. A 36 slot 10 pole interior permanent magnet design example is used throughout the analysis.

A comparison of an interior permanent magnet and copper rotor induction motor in a hybrid electric vehicle application

This paper compares a permanent magnet motor to an induction motor in a hybrid electric vehicle application. The comparison considers replacing an existing 50kW Interior Permanent Magnet (IPM) motor with a copper-rotor induction motor (CR-IM). The 2004 Toyota Prius Hybrid system is used as the baseline for the analysis. The size, weight and cost of the two topologies are compared and the difference in lifetime energy use over various drive cycles is analysed. Additional considerations including the impact on battery sizing and the inverter VA rating are also considered and discussed.

A comparison between maximum torque/ampere and maximum efficiency control strategies in IPM synchronous machines

This paper presents a comparison between maximum torque/ampere and maximum efficiency control strategies for interior permanent magnet synchronous machines (IPMSMs) in an electrical vehicle propulsion application. A mixed theoretical and experimental approach is adopted to demonstrate how an improvement in performance may be achieved when maximum efficiency control is utilised. The study is completed on three machines, a 36 slot 10 pole fractional slot per pole distributed winding, a 30 slot 10 pole integer slots per pole distributed winding design and a 12 slot 10 pole concentrated winding design. The findings are experimentally validated on the 36 slot 10 pole IPM motor. It is shown that the largest improved in efficiency is achieved with a concentrated winding design however for all three designs a significant efficiency gain is achieved in the most common operating regions of the standard international and US urban drive cycles, WLTP and UDDS.

Design considerations of a brushless open-slot radial-flux PM hub motor

Initial sizing of a brushless radial-flux permanent magnet (PM) machine is typically based on assumptions regarding the electric- and magnetic- loading together with the motors active volume. Additional design assumptions include the winding current density and magnetic flux density within the core pack. These initial design decisions are crucial as they largely define the resultant electromagnetic and thermal behaviour of the motor. The general rules regarding the choice of the initial design parameters are well known. However, the effects of the initial sizing on the final motor performance are not widely reported. This paper presents an analysis of design considerations for a brushless open-slot radial-flux PM hub-motor. A number of alternative motor designs are compared to demonstrate the effect of the initial design decisions on final motor performance. Both the electromagnetic and thermal aspects of the motor design are considered. The design variants are characterised by the same electric- and magnetic- loading together with active volume while the winding rated current density and no-load/open-circuit magnetic flux density within the core pack are varied. The employed sizing methodology combines the classical approach with a non-linear magneto-static finite element solver and an optimisation routine. A prototype hub-motor has been manufactured to validate the theoretical findings from the design process. The experimental data shows good agreement with the theoretical findings.

Modern heat extraction systems for electrical machines - A review

This paper presents a review of modern cooling system employed for the thermal management of electrical machines. Various solutions for heat extractions are described: high thermal conductivity insulation materials, spray cooling, high thermal conductivity fluids, combined liquid and air forced convection, loss mitigation techniques.

Brushless AC interior-permanent magnet motor design: Comparison of slot/pole combinations and distributed vs. concentrated windings

This paper compares the attributes of 36 slot, 33 slot and 12 slot brushless interior permanent magnet motor designs, each with an identical 10 pole interior magnet rotor. The aim of the paper is to quantify the trade-offs between alternative distributed and concentrated winding configurations taking into account aspects such as thermal performance, field weakening behaviour, acoustic noise, and efficiency. It is found that the concentrated 12 slot design gives the highest theoretical performance however significant rotor losses are found during testing and a large amount of acoustic noise and vibration is generated. The 33 slot design is found to have marginally better performance than the 36 slot but it also generates some unbalanced magnetic pull on the rotor which may lead to mechanical issues at higher speeds.

Experiment informed methodology for thermal design of PM machines

The common approach used in the thermal design of electrical machines is calibrating thermal models based on the designers previous experience, or hardware tests on a prototype machine. This allows for various manufacture and assembly nuances to be accounted for in the design process, assuring accurate and computationally efficient predictions of the machine thermal behaviour. The post-manufacture calibration of thermal models from tests on a complete machine has limited use in development of machine topologies, where no previous experience or machine hardware exist. In this context, an experiment informed design technique that makes use of reduced order machine sub-assemblies presents an attractive alternative. In particular, the hardware manufacture cost and time is significantly reduced compared to the prototyping of the complete machine assembly. This allows for numerous hardware samples to be constructed and tested, to inform the machine design process. The use of the machine sub-assembly testing is focused, but not limited to identifying and quantifying various power loss and heat transfer phenomena. This paper reviews the applicability of the sub-assembly testing in a broader context of the machine design. The aim of the research focuses on formulating a basis for sub-assembly based, experiment informed methodology for the thermal design of electrical machines.

Experimental calibration in thermal analysis of PM electrical machines

Thermal design of electric machines frequently involves tests on a fully constructed prototype to calibrate various build factors associated with the manufacture, assembly and materials used in the hardware construction. The prototype machine is usually instrumented with multiple temperature sensors providing a detailed insight into the temperature distribution. The resolution of the experimentally gathered data is usually limited by the number of temperature sensors, and therefore the quality of model calibration is highly affected by the input data. This paper investigates the issue of thermal model calibration in the context of available machine hardware and measured data resolution. Also, the research evaluates the most suitable thermocouple location with reference to the model complexity, from reduced-order lumped-parameters circuit to high-fidelity finite element method (FEM). The investigation is focused on the stator-winding assembly, which is frequently associated with the main source of power loss within a PM machine body. A prototype of a PM generator has been selected to illustrate the effects associated with the model calibration. Tests on a representative stator-winding sub-assembly (motorette) have been used in the analysis. The results suggest that the measured data from alternative sensor locations for a given machine region has a significant impact on the quality of the model calibration and consequently temperature predictions.