Nigel Schofield

Battery balancing methods: A comprehensive review

The trend toward more electric vehicles has demanded the need for high voltage, high efficiency and long life battery systems. A complete battery system consists of the following parts: protection, management and balancing. Of the three parts, balancing is the most important concerning the life of the battery system because without the balancing system, the individual cell voltages will drift apart over time. The capacity of the total pack will also decrease more quickly during operation which will result in the fail of the total battery system. This condition is especially severe when the battery has a long string of cells (high voltage battery systems) and frequent regenerative braking (charging) is done via the battery pack. This paper presents the theory behind the proposed balancing methods for battery systems within the past twenty years. Comparison between the methods is carried out and different balancing methods are grouped by their nature of balancing.

The Impact of Transport Electrification on Electrical Networks

In order to satisfy the growing expectation for energy-efficient ecofriendly transportation, a number of vehicle concepts have emerged, including the hybrid electric vehicle (HEV) and battery electric vehicle. Vehicle dynamics necessitate careful sizing of onboard energy storage systems. As the penetration of electric vehicles, in both the public and private sector increases, the requirement to facilitate and utilize them becomes paramount. A particular class of vehicle, the plug-in HEV also poses a greater challenge to existing terrestrial-based electrical supply systems. This paper establishes a series of well-defined electric vehicle loads that are subsequently used to analyze their electrical energy usage and storage in the context of more electrified road transportation. These requirements are then applied to a European Union residential load profile to evaluate the impact of increasing electrification of private road vehicles on local loads and the potential for vehicle and residential load integration in the U.K.

Novel Switched Reluctance Machine Configuration With Higher Number of Rotor Poles Than Stator Poles: Concept to Implementation

There is a great demand for efficient, quiet, reliable, and cost-effective motor drives for propulsion systems in hybrid and plug-in hybrid electric vehicles. Owing to a rigid structure and the absence of magnetic source on the rotor, a switched reluctance machine (SRM) is inherently robust and cost effective. In spite of these advantages, several challenges in the control of this machine remain an issue, including high levels of torque ripple, acoustic noise, and a relatively low torque density. This paper presents a new family of SRMs which have higher number of rotor poles than stator poles. Using a newly defined pole design formula, several novel combinations of the stator-rotor poles have been proposed. From the simulation and experimental analysis of a prototype 6/10 configuration, it has been observed that this machine produces higher torque per unit volume and comparable torque ripple when compared to a conventional 6/4 SRM with similar number of phases and constraints in volume. The results presented in this paper make this family of machines a strong contender for survivable high-performance applications for automotive propulsion systems. The simulation and experimental results for the prototype 6/10 configuration have been presented and compared to a conventional 6/4 design for verification.

Plug-in hybrid electric vehicle developments in the US: Trends, barriers, and economic feasibility

There is a growing interest in Plug-in hybrid electric vehicle (PHEV) concepts for private, public, and utility services across the USA. This has encouraged the establishment of a number of small companies providing expertise and components for evaluation and demonstration system vehicles, and interest by auto manufacturers in future mass-produced PHEVs. In this paper, we present the principles of Plug-in hybrid electric operation, discuss the practical implementation issues associated with the various technology platforms, and propose power-train options for various classes of vehicle. We also discuss current US hybrid and PHEV trends, summarize major national and state projects, the charging impact on the power grid, vehicle-to-grid technology (V2G) and other related technologies.

Design of a Switched Reluctance Machine for Extended Speed Operation

This paper presents results from a design study on the feasibility of employing high-efficiency switched reluctance (SR) machines in minimal hybrid-electric vehicles. The application requirements are presented and highlight the constraining influences of the vehicle drive-line topology on the machine design. The benefit of continuous phase current excitation is reported for the first time, demonstrating that constant power at an extended-speed operation can be realized with a higher number of phase winding turns per pole than would otherwise be achieved with conventional discontinuous current control. Thus, the torque/Ampere capability, when operating at or below base speed, is not as significantly compromised, an important consideration for the power inverter rating and, hence, drive system cost. The design procedure and simulated results are validated by measurements from a prototype machine. The results demonstrate the potential of SR technology for high-performance low-cost automotive applications, which often combine arduous environmental and volumetric constraints. In addition, the results highlight the benefits of continuous current control for extended-speed operation.

Generator Operation of a Switched Reluctance Starter/Generator at Extended Speeds

Switched reluctance (SR) drive systems are a candidate technology for electric vehicle applications, particularly where a high degree of component integration is required within a thermally demanding environment-typical of engine-mounted power-assist solutions. This paper discusses the design, prototype realization, and test validation of an SR machine for a mild-hybrid power-assist starter/alternator application that has performance requirements of both high starting torque and a wide speed range at full power. While a number of papers on SR machine design for automotive starter/alternator applications have been published, a comprehensive case study discussing the design and application issues has not been published. This paper focuses on the impact of the performance requirements and volumetric and environmental constraints on the machine design. It is shown that a higher number of stator and rotor poles is more appropriate for the utilization of the available active volume. While various operation and fault scenarios have been presented for SR drive systems, this paper also discusses test observations that suggest the potential for the transient self-excitation of the SR generator via residual magnetization of the machine rotor-a fault scenario that has only been briefly reported on.

Hybrid energy sources for electric and fuel cell vehicle propulsion

Given the energy (and hence range) and performance limitations of electro-chemical batteries, hybrid systems combining energy and power dense storage technologies have been proposed for electric vehicle propulsion. The paper will discuss the application of electro-chemical batteries, supercapacitors and fuel cells in single and hybrid source configurations for electric vehicle drive-train applications. Simulation models of energy sources are presented and used to investigate the design optimisation of electric vehicle on-board energy source in terms of energy efficiency and storage mass/volume. Results from a case study considering a typical small urban electric vehicle are presented, illustrating the benefits of hybrid energy sources in terms of system mass and vehicle range. The models and approach can be applied to other vehicles and driving regimes.

Design Concepts for High-Voltage Variable-Capacitance DC Generators

The transmission requirements of the next generation of offshore wind farms, such as the Round Two U.K. offshore development proposals, may rely on high-voltage direct current (HVDC) technology for at least a part of their power collection and transmission requirements. HVDC technology is particularly suited for the transmission of high powers through large lengths of submarine cables; however, its application is limited by the high cost of offshore ac-dc converter stations. This paper therefore investigates the feasibility of the direct generation of HVDC power using a novel generator topology, as an alternative solution to the generation-transmission requirements of large offshore wind farms. A variable-capacitance generator uses electrostatic fields to generate an HVDC output with a minimum of power conditioning and is based on a previous work that suggests system power densities comparable with the conventional generator-transformer-rectifier systems to be achievable.

Generator operation of a switched reluctance starter/generator at extended speeds

Switched reluctance (SR) drive systems are a candidate technology for electric vehicle applications, particularly where a high degree of component integration is required within a thermally demanding environment-typical of engine mounted power assist solutions. The paper discusses the design, prototype realisation and test validation of a switched reluctance (SR) machine for a mild-hybrid power assist, starter/alternator application that has performance requirements of both high starting torque and a wide speed range at full power. Whilst a number of papers have published on SR machine design for automotive starter/alternator applications, a comprehensive case study discussing the design and application issues has not been published. The paper focuses on the impact of performance requirements and volumetric and environmental constraints on the machine design. It is shown that a higher number of stator and rotor poles are more appropriate for utilisation of the available active volume. Whilst various operation and fault scenarios have been presented for SR drive-systems, the paper also discusses test observations that suggest the potential for transient self-excitation of the SR generator via residual magnetisation of the machine rotor, a fault scenario that has only been briefly reported upon.

Double-Rotor Switched Reluctance Machine (DRSRM)

With the era of modern vehicle electrification, electric machines with high traction torque-speed output and compact volume are highly desired. This paper presents a family of switched reluctance machine configurations that are composed of double rotors and one stator integrated in one machine housing. The machines are potentially more compact and lower cost, while providing two independent mechanical outputs suitable for hybrid electric vehicle transmissions. The detailed design process of a double-rotor switched reluctance machine (DRSRM) is presented, comprising of analytical calculations, finite-element field analysis, and full-drive system simulation. Additionally, some optimizations are applied to maximize the machine performance and minimize the machine weight and volume. A scaled prototype machine is designed and built according to chosen vehicle drive cycles to evaluate and validate the DRSRM prototype.