The effects of coupled inductors in parallel interleaved buck converters

Interleaved switching and coupled inductors are proven methods for reducing DC-DC converter output ripple. This paper furthers discussions of these techniques to arrangements of many buck converters connected in parallel. The different possible arrangements of the DC-DC converters are discussed and criteria for fair comparisons between them are chosen. The effects of interleaved switching on ripple values are presented and subsequent effects of coupling the inductors is then investigated. A generalised solution for current ripple in n coupled inductor converters is presented. Simulations are used to verify the solution and characterise the converter and output ripple for all configurations.

Ironless wheel motor for a direct drive vehicle application

An ironless motor for use as direct wheel drive is presented. The motor is intended for use in a lightweight (600kg), low drag, series hybrid commuter vehicle under development at The University of Queensland. The vehicle will utilise these ironless motors in each of its rear wheels, with each motor producing a peak torque output of 500Nm and a maximum rotational speed of 1500rpm. The axial flux motor consists of twin Ironless litz wire stators with a central magnetic ring and simplified Halbach magnet arrays on either side. A small amount of iron is used to support the outer Halbach arrays and to improve the peak magnetic flux density. Ducted air cooling is used to remove heat from the motor and will allow for a continuous torque rating of 250Nm. Ironless machines have previously been shown to be effective in high speed, high frequency applications (+1000Hz). They are generally regarded as non-optimal for low speed applications as iron cores allow for better magnet utilisation and do not significantly increase the weight of a machine. However, ironless machines can also be seen to be effective in applications where the average torque requirement is much lower than the peak torque requirement such as in some vehicle drive applications. The low spinning losses in ironless machines are shown to result in very high energy throughput efficiency in a wide range of vehicle driving cycles.

A method of extracting switching loss from a high efficiency MOSFET based half bridge converter

The measurement of losses in high efficiency / high power converters is difficult. Measuring the losses directly from the difference between the input and output power results in large errors. Calorimetric methods are usually used to bypass this issue but introduce different problems, such as, long measurement times, limited power loss measurement range and/or large set up cost. In this paper the total losses of a converter are measured directly and switching losses are exacted. The measurements can be taken with only three multimeters and a current probe and a standard bench power supply. After acquiring two or three power loss versus output current sweeps, a series of curve fitting processes are applied and the switching losses extracted.

The UltraCommuter : a viable and desirable solar-powered commuter vehicle

The University of Queensland UltraCommuter project is the demonstration of an ultra-light weight, low drag, energy efficient and low polluting, electric commuter vehicle equipped with a 2.5m2 on-board solar array. A key goal of the project is to make the vehicle predominantly self-sufficient from solar power for normal driving purposes , so that it does not require charging or refuelling from off-board sources. This paper examines the technical feasibility of the solar-powered commuter vehicle concept, as it applies the UltraCommuter project. A parametric description of a solar-powered commuter vehicle is presented. Real solar insolation data is then used to predict the solar driving range for the UltraCommuter and this is compared to typical urban usage patterns for commuter vehicles in Queensland. A comparative analysis of annual greenhouse gas emissions from the vehicle is also presented. The results show that the UltraCommuter’s on-board solar array can provide substantial supplementation of the energy required for normal driving, powering 90% of annual travel needs for an average QLD passenger vehicle. The vehicle also has excellent potential to reduce annual greenhouse gas emissions from the private transport sector, achieving a 98% reduction in CO2 emissions when compared to the average QLD passenger vehicle. Lastly, the vehicle battery pack provides for tolerance to consecutive days of poor weather without resorting to grid charging, giving uninterrupted functionality to the user. These results hold great promise for the technical feasibility of the solar-powered commuter vehicle concept.

Simulated lifecycle costs of ultracapacitors in battery electric vehicles

The pulse power characteristics of ultracapacitors appear well suited to electric vehicle applications, where they may supply the peak power more efficiently than the battery, and can prevent excessive over sizing of the battery pack due to peak power demands. Operation of ultracapacitors in battery electric vehicles (BEVs) is examined for possible improvements in system efficiency, vehicle driving range, battery pack lifetime, and potential reductions in system lifecycle cost. The lifecycle operation of these ultracapacitors is simulated using a custom-built, dynamic simulation code constructed in Matlab. Despite apparent gains in system efficiency and driving range, the lifecycle cost benefits as simulated appear to be marginal, and are heavily influenced by the incremental cost of power components. However, additional factors are identified which, in reality, will drive ultracapacitors towards viability in electric vehicle applications.

Multi-converter topology evaluation for connection of low voltage DC sources

A design for a cascaded multilevel DC-DC converter is proposed. The applications of a multilevel converter and the design issues involved in changing from a single converter to multiple converters are discussed. Implementation of the multilevel system using multiple Cuk converters is suggested and explanations of design decisions are given. The merits of the proposed design are discussed.

Design optimisation of ironless motors based on magnet selection

This paper considers the design of a radial flux permanent magnet ironless core brushless DC motor for use in an electric wheel drive with an integrated epicyclic gear reduction. The motor has been designed for a continuous output torque of 30 Nm and peak rating of 60 Nm with a maximum operating speed of 7000 RPM. In the design of brushless DC motors with a toothed iron stator the peak air-gap magnetic flux density is typically chosen to be close to that of the remanence value of the magnets used. This paper demonstrates that for an ironless motor the optimal peak air-gap flux density is closer to the maximum energy product of the magnets used. The use of a radial flux topology allows for high frequency operation and can be shown to give high specific power output while maintaining a relatively low magnet mass. Two-dimensional finite element analysis is used to predict the airgap flux density. The motor design is based around commonly available NdFeB bar magnet size

A method of switch selection for an electric vehicle drive

The key to reducing cost of electric vehicles is integration. All too often systems such as the motor, motor controller, batteries and vehicle chassis/body are considered as separate problems. The truth is that a lot of trade-offs can be made between these systems, causing an overall improvement in many areas including total cost. Motor controller and battery cost have a relatively simple relationship; the less energy lost in the motor controller the less energy that has to be carried in the batteries, hence the lower the battery cost. A motor controller’s cost is primarily influenced by the cost of the switches. This paper will therefore present a method of assessing the optimal switch selection on the premise that the optimal switch is the one that produces the lowest system cost, where system cost is the cost of batteries + switches.

A DC circuit breaker for an electric vehicle battery pack

Electric vehicle battery packs require DC circuit breakers for safety. These must break thousands of Amps DC at hundreds of Volts. The Sunshark solar racing car has a 140V 17Ahr battery box which needs such a breaker. A static design using 200V MOSFETs to interrupt the fault current is presented. The design specification, decisions and proposed solution circuit are given. The current sensing technique,MOSFET overvoltage protection, and DC bus capacitor precharging scheme are specific focuses. Simulation results are presented and discussed.

An isolated MOSFET gate driver

Traditional methods of isolated MOSFET/IGBT gate drive are presented, and their pros and cons assessed. The best options are chosen to meet our objective— a small, high speed, low cost, low power isolated gate drive module. Two small ferrite bead transformers are used for isolation, one transmits power at 2.5MHz, the other sends narrow set reset pulses. On the secondary these pulses drive a transistor totem pole to ensure high current drive, and the value is held by CMOS buffers with positive feedback. An alternative design for driving logic level devices uses only an HC buffer on the secondary. Double sided SMDconstruction (primary one side, secondary on the other) yields an upright module 40x18x5mm. Propagation delaywas 20ns, and rise/fall time 15ns with a 1nF load. The design places no limits on frequency of operation or duty cycle. Power supply requirementswere 5V@20mA for operation below 100kHz, dominated by magnetising current.

Implementing natural PWM digitally

For dynamic closed loop control of a multilevel converter with a low switching frequency, natural sampled PWM is the best form of modulation. However previous natural sampled PWM implementations have generally been analog. A digital implementation has advantages, particularly for a multilevel converter. Re-sampled uniform PWM, a modulation technique which approaches the performance of natural PWM, is implemented both in software and hardware. Results demonstrating the improvement over uniformPWM techniques are presented.

Robust control algorithm for grid-interfaced three-phase inverters with high-bandwidth LCL filter

The paper introduces the design of robust current and voltage control algorithms for a grid-connected three-phase inverter which is interfaced to the grid through a high-bandwidth three-phase LCL filter. The algorithms are based on the state feedback control which have been designed in a systematic approach and improved by using oversampling to deal with the issues arising due to the high-bandwidth filter. An adaptive loop delay compensation method has also been adopted to minimize the adverse effects of loop delay in digital controller and to increase the robustness of the control algorithm in the presence of parameter variations. Simulation results are presented to validate the effectiveness of the proposed algorithm.

Students' experience of problem-based learning in virtual space

This paper reports outcomes of a study focussed on discovering qualitatively different ways students' experience problem-based learning in virtual space. A well accepted and documented qualitative research method was adopted for this study. Five qualitatively different conceptions are described, each revealing characteristics of increasingly complex student experiences. Establishing characteristics of these more complex experiences assists teachers in facilitating students engagement and encouraging deeper learning.

Work integrated learning: Exposure to professional practice : expectations and challenges

Prior to graduation engineering students are expected to provide evidence of relevant experience in the workplace. This experience is expected to provide opportunities for exposure to the profession and to help students develop confidence, skills and capabilities as emerging professionals. This investigation considers the expectations and challenges in implementing WIL programs in different contexts. While this will inform the next iteration of engineering course development at QUT the issues and interventions described provide useful insights into options available and engineering curriculum design more broadly. This comparative analysis across three phases highlights expectations and challenges including stakeholder responsibilities, expectations, and assessment. The study draws on the findings of a 2005 investigation into the purpose and provision of WIL and findings of a 2012 Faculty review of the current WIL model. The enhancement of WIL through a series of developmental phases highlights strengths and weaknesses of various models. It is anticipated that this investigation will inform course development decisions on a whole-of-course approach to WIL that improves student engagement and learning experience. The importance of WIL is not disputed. However with industry expectations, increasing student numbers and cohort diversity the ways in which students and industry currently engage in WIL are not sustainable and more creative, flexible and engaging approaches are needed.