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High power density is required for power converter in more electric aircraft due to the strict demands of volume and weight, which makes silicon carbide (SiC) extremely attractive for this application. In this paper, a prototype of 50-kW SiC two-level three-phase voltage source inverter is demonstrated with a gravimetric power density of 26 kW/kg (without inclusion of filter). A gate assisted circuit is introduced to reduce the switching loss. In addition, the ringings of voltage and current due to parasitic parameters during the switching transition can also be mitigated. A mathematical model with consideration of various parasitic parameters is developed, which illustrates the parasitic effects in high-speed switching SiC power module. The converter is operated at a switching frequency up to 100 kHz and a narrow dead band of 250 ns. The measured efficiency is 97.91%.

Silicon carbide (SiC) metal–oxide–semiconductor field-effect transistor (MOSFET) is regarded as an attractive replacement for Si insulated gate bipolar transistor (IGBT) in high-power density applications due to its high switching speed and low switching loss. However, to fully utilise these benefits, the gate driver of the SiC MOSFET needs to be optimised to meet its special driving requirements. Fundamentally, both gate drivers for Si IGBT and SiC MOSFET share similar design methodology since these two power devices have the same MOS-gated structure. However, one of the major challenges in the gate driver design for SiC MOSFET is overcoming the electromagnetic interference, which is resulted from the much higher dv/dt and di/dt ratios during the switching transition. In this study, high-speed gate drivers for Si IGBT and SiC MOSFET power modules of similar ratings of 1.2 kV/120 A have been designed. The performances of gate driver have been experimentally evaluated by a double pulse test. The design considerations of gate driver to enable the replacement of Si IGBT by SiC MOSFET have been conclusively investigated and presented in this study.

The increasing demand for high power density requires the power converter to operate in high switching frequency. Silicon carbide (SiC) power module is regarded as one of the most promising candidates for high-frequency applications due to the superior switching speed and low switching loss. With the increase of switching frequency, the switching loss will be the limiting factor of efficiency. Hence, it should be minimized during each switching transition. The optimization of switching loss is normally achieved by the repetitive double pulse test experiments. It is time-consuming to find an optimum gate resistance to achieve the tradeoff between switching loss and electromagnetic interface. In this paper, an accurate subcircuit model for SiC power module is proposed to assist optimization of switching loss in converter design. By considering the device physics and structure, an accurate Miller capacitance model is obtained. Moreover, a parameter extraction procedure is presented, which is based on the datasheet. Good agreements are achieved between the PSpice simulation and experiment.

The high switching speed in a silicon carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET) will aggravate the parasitic effects (d i /d t and d v /d t ) arising from the interaction with parasitic elements. In this project, a high-speed gate driver has been developed and optimised for the commercially available SiC MOSFET power module. The impact of various parasitic parameters on parasitic effects is initially evaluated. Then, an improved gate-assisted circuit is proposed with a local low-impedance path for both discharging and C d v/ d t currents. It allows maximised turn-off speed (d v /d t up to 36 V/ns) and minimised turn-off loss (reduction up to 70%). It also produces a reduction in electromagnetic interference. The gate voltage spike due to C d v/ d t current is reduced below the threshold voltage at various testing conditions.

This paper presents the modeling and motion-sensorless direct torque and flux control of a novel dual-airgap axial-flux permanent-magnet machine optimized for use in flywheel energy storage system (FESS) applications. Independent closed-loop torque and stator flux regulation are performed in the stator flux ( x-y) reference frame via two PI controllers. This facilitates fast torque dynamics, which is critical as far as energy charging/discharging in the FESS is concerned. As FESS applications demand high-speed operation, a new field-weakening algorithm is proposed in this paper. Flux weakening is achieved autonomously once the y-axis voltage exceeds the available inverter voltage. An inherently speed sensorless stator flux observer immune to stator resistance variations and dc-offset effects is also proposed for accurate flux and speed estimation. The proposed observer eliminates the rotary encoder, which in turn reduces the overall weight and cost of the system while improving its reliability. The effectiveness of the proposed control scheme has been verified by simulations and experiments on a machine prototype.

The trend towards SiC-based high power density converter requires to drive SiC MOSFET with high-speed switching. However, it tends to aggravate dv/dt effect due to the impact of parasitic parameters, resulting in shoot-through and high device stress in the half bridge configuration. In this work, a novel gate assisted circuit is proposed to eliminate shoot-through issue without compromise on switching speed. The mathematical model for the gate assisted circuit is developed. Then the switching test experiment is set up to validate its performance with commercial SiC half bridge module, which shows a high dv/dt up to 20 and 31 V/ns at turn-on and turn-off respectively and a 34% reduction of switching loss without any shoot-through issue.

In power transistor models, it is very important that device capacitances are modelled accurately so that switching losses, EMI filter requirements and gate timing requirements of the converter can be accurately determined. In this paper, a modelling technique utilizing modified sigmoid functions to describe the device capacitances is applied on a GaN HEMT and a silicon MOSFET to develop their corresponding SPICE models. Comparison of switching energies of the transistors under slow-switching conditions suggest that the technique is suitable for modelling both types of transistors. The developed model for the GaN HEMT is also shown to simulate faster than the manufacturer's model under fast switching conditions.

The effects of two-phase-flow modeling on nuclear reactor single-channel stability analysis are investigated with four two-phase-flow models, namely, the homogeneous-equilibrium model, the homogeneous-nonequilibrium model, the nonhomogeneous-equilibrium model, and the nonhomogeneous-nonequilibrium model. The models are applied to hot-channel analyses of a proposed typical supercritical-water-cooled-reactor (SCWR) design. The neutral stability boundaries derived by using the four models are compared and plotted on the traditional subcooling number versus phase change number plane. To ensure proper development of the models, they are benchmarked to the experimental data. It is found that the homogeneous models predict more conservative stability boundaries than the nonhomogeneous models and that the differences of the stability boundaries predicted by all four two-phase-flow models are reduced under higher-pressure conditions.

This paper presents the loss analysis of a dual-air-gap axial flux permanent magnet machine for a flywheel energy storage system. Losses in iron-core and permanent magnets caused by both space harmonics and time harmonics are respectively analysed using a three-dimensional finite element method. The effect of skew of magnets on machine losses is also taken into account. The analysis is validated by comparisons of calculated losses and measured values on a prototype. A solution to reduce eddy-current losses in the magnets for this machine is also presented. The eddy-current losses in the permanent magnets can be significantly lessened by both circumferential and radial segmentations.

This paper presents the design and analysis of a novel axial flux permanent-magnet (AFPM) machine for a flywheel energy storage system (FESS). Its design and control facilitate significant reduction in axial bearing stress and losses. Due to the unconventional flux distribution in this machine, a 3-D finite element method was employed for its design and analysis, including its electromagnetic torque and axial force performances. The effects of the rotor PM skew angle on the cogging torque and the axial force have been studied. It is found that an optimum skew angle is effective in reducing the overall cogging torque with negligible effect on the static axial force. The latter is crucial as it can be utilized to minimize the axial bearing stress in FESS application. The concept, design, and analysis methodology have been validated by experimental results from an experimental AFPM machine prototype.

In this study, the vibration distribution along the width direction in the output section of a piezoelectric transformer operating at the thickness shear vibration mode is investigated experimentally and theoretically. It is experimentally found that the vibration of output section has a spatial gradient and attenuates as the distance from the input section increases. A theoretical model is developed to estimate the vibration attenuation in the output section, which provides the guidelines of optimizing the transformer and explains some important phenomena such as the nonuniform temperature distribution in piezoelectric transformers. The larger the load resistance, the larger the vibration gradient is. At the resonant frequency, the vibration gradient decreases with increasing the width of the input or output section, and it changes little with the length and thickness of the transformer when the load resistance matches with the piezoelectric transformer. The vibration gradient increases with increasing the length or decreasing the thickness when the load resistance is constant, in which the load does not match with the transformer.

With the technology roadmap of LED moving towards high efficacy low power LEDs, the current AC distribution system suffers from system inefficiency. An energy efficient ELVDC power distribution for LED lighting system is proposed. The different design considerations affecting the inherent higher voltage drop at ELVDC level are discussed in this paper. Simulations are presented in the context of a lighting system running through the stairwell of a high rise built environment and are extended to a LED lighting system of an office context environment. It has been presented that the use of an ELVDC power distributed LED lighting system is highly efficient when integrated together with DC energy sources.

With the recent focus on DC power distribution in the built environment due to ease of integration of DC energy sources such as solar photovoltaic and battery energy storage, the Extra Low Voltage DC (ELVDC) power distribution system suitable for high rise building with direct integrated DC energy sources and LED lighting loads is discussed in this paper. Design considerations and system level modeling are described. The architectural concept has high system level efficiency, and does not need complex re-design when retro-fitting into existing buildings. Simulations and experimental results are presented in this paper which supported our proposed design.

With the recent shift towards human centric lighting and the built environment, the Extra Low Voltage DC (ELVDC) power distribution system suitable for high-rise building with LED lighting loads is discussed in this paper. A driverless ELVDC LED lighting system is proposed and its relevant centralized dimming scheme is discussed in this paper. Mathematical modeling of individual LED lighting is presented and compared for the different ELVDC LED lighting driverless topologies. System level efficiency and cost benefit analysis against the conventional AC distributed and its counterpart ELVDC distributed LED lighting system are presented. A driverless ELVDC LED lighting system is cost effective for a high-rise built environment.

This paper discusses an energy-efficient LED lighting distribution system in high-rise buildings based on 48Vdc bipolar ELVDC distribution nano-grid fed from a centralized AC/DC converter. The paper first introduces the LED lighting system performances and its associated challenges. It then introduces the preference for ELVDC distribution system in the built environment. The paper then presents the mathematical model of the two different ELVDC distribution systems: the unipolar and the bipolar systems and their associated electrical component models. An approximated analysis was done using a constant power load model on the two distribution systems with voltage drop and power loss as the main indices. Simulations were carried out to determine the voltage level and system efficiencies at each node point. Experimental results of the two ELVDC LED lighting systems are verified and are compared with theoretical results. The 48Vdc bipolar ELVDC distribution system has reduced voltage drop and better system efficiency compared to the 24Vdc unipolar system.

This paper presents a new technique of shifting the phase of a low-frequency AC voltage. The technique is derived from the multioutput piezoelectric transformer. The piezoelectric phase shifter is made of a piezoelectric ceramic plate operating at the thickness shear vibration mode. The ceramic plate consists of input, output and control parts. The control part is connected to a tunable inductor or capacitor, which can tune the phase shift between the output and input voltages smoothly. The phase shift can be increased by decreasing the inductance or capacitance at the control part. Tuning a DC bias voltage applied to the input part may also change the phase shift. A maximum phase shift of 128° and minimum phase shift of −10° are achieved by the prototype of the phase shifter. The phase shifter has a wide phase shift range, good ability to manage high power and relatively high energy transmission efficiency.

In our previous study, the multioutput piezoelectric transformer operating at the thickness-shear vibration mode was proposed and experimentally investigated. By designing a new construction of support and lead wire connection, a power density of 52.7 W/cm3 and a total output power of 169.8 W were achieved at a temperature rise less than 20 degrees C. In this work, a theoretical model was developed for the dual-output piezoelectric transformer operating at the thickness-shear vibration mode. The equivalent circuit parameters of the piezoelectric transformer were derived. Based on this, the impedance characteristics, equivalent inductance, capacitance ratio, voltage gain, and efficiency of the piezoelectric transformer were calculated. The theoretical results were verified by experimental data. Furthermore, the effect of the transformer size on the voltage gain, efficiency, output power and power density, and the effect of the load of one output on the voltage gain of another output were analyzed. Some useful guidelines were achieved by these analyses.

Active magnetic bearings, which provide non-contact suspension for high-speed rotors, have received increasing attention in recent years. Although the contactless nature of magnetic bearings brings up many advantages over conventional bearings, one of the challenging problems is to stabilize the rotor of the magnetic bearing systems that is very sensitive to outside disturbances and plant uncertainties. In this paper, a stabilizing synchronization design of rotor-magnetic bearing systems is proposed by incorporating cross-coupling technology into the optimal control architecture, which can be decomposed into two problems: a robust optimal control problem to improve the synchronization performance of the rotor in the radial directions and a stability problem. The control scheme is based on minimization of a new quadratic performance index in which the synchronization errors are embedded. Stability of the control scheme is also investigated through the linear quadratic Gaussian (LQG) optimal control technique. Simulations on a compact and efficient flywheel energy storage system with integrated magnetic bearings demonstrate that the proposed approach is very effective to recover the unstable system when the outside disturbances are present.

An observer-based robust adaptive nonlinear position and speed tracking controller is developed for a permanent magnet synchronous motor with initial rotor angle uncertainty. The unknown initial rotor position is treated as a constant motor parameter in the development of the controller. An incremental encoder, which provides relative position variation of the rotor, is used along with stator current signals to achieve stable control. However, the controller does not require the knowledge of motor parameters and it only assumes friction, external disturbances, and model uncertainties are bounded. By using state observers, the measurement of acceleration and load torque, which is required usually in the nonlinear controller design with high tracking performance, is avoided. The stability of the control system and tracking convergence are guaranteed using Lyapunov theory. Finally, the stability and efficacy of the proposed drive system are verified by experimental results.

We investigated the complementary effects of TiO/sub 2/ and MoO/sub 3/ additives on the magnetic properties, the core loss, and the dielectric properties of MnZn ferrites. We analyzed the composition factors influencing the permeability of MnZn ferrites. A high initial permeability base ZnO-MnO-Fe/sub 2/O/sub 3/ composition ratio was selected. Electrical and dielectric analyses indicated that the introduction of TiO/sub 2/ can improve the high-frequency loss properties of MnZn ferrite cores, but such introduction will also have a negative effect on the magnetic properties of the sintered samples. Introduction of MoO/sub 3/ can alleviate the negative influence of TiO/sub 2/ and also reduce the internal polarization intensity. As a result, the dielectric constant and corresponding dielectric loss may be reduced. We give experimental results in the paper.

In this study, a high power-density dual-output piezoelectric transformer is proposed and experimentally investigated. This transformer consists of a lead zirconate titanate (PZT) ceramic plate with a size of 30 mrn x 5 mm x 1 mm. It has a high mechanical quality factor Q m and high electromechanical coupling coefficient k eff . The PZT ceramic plate is poled along the width direction. The electrodes of input and output parts are on the top and bottom surfaces of the ceramic plate and separated by narrow gaps. Lead wires are pressed on the electrodes by a proper elastic force. At a temperature rise lower than 20 °C, the piezoelectric transformer has a maximum output power density of 52.7 W/cm 3 with an efficiency of 88%. The maximum efficiency of this piezoelectric transformer is 95%. Voltage gains larger than one and smaller than one are obtained in one transformer. By using a parallel combination of the two outputs of the proposed transformer, a higher output power density of 57.3 W/cm 3 can be achieved at the temperature rise lower than 20 °C.

In this paper, a mathematical model of permanent-magnet synchronous motor (PMSM) with initial rotor position uncertainty is derived and its control methodology is proposed. Based on Lyapunov stability theory, an observer-based robust adaptive position and speed-tracking control system for the PMSM is developed given that all parameters including load inertia and motor parameters are unknown, acceleration is not measurable and friction and external disturbances are bounded. An incremental encoder which provides relative position of the rotor is used along with stator current signals to achieve stable control. The simulation and experimental results have proven the stability and efficacy of the proposed control law.

In a permanent-magnet (PM) brushless DC motor, the waveform of back electromotive force (EMF) is related to the rotor position; hence, the back EMF can be used for position sensorless control. However, in practical implementation, the terminal voltage or phase voltage is used instead, as the back EMF is difficult to be sensed directly. Thus, detection error of the rotor position can occur. This paper documents the calculations and analyses on the detection error and the motor commutation angle, and presents an error compensation circuit to ensure proper commutation. Finite-element field simulation and experimental results are also given to verify the calculations as well as the compensation circuit.

Single-phase line-start permanent magnet (LSPM) synchronous motors have always been far less amenable to detailed computer-aided performance analysis compared with three-phase LSPM synchronous motors. The main reason is the lack of an accurate mathematical model of the motor characteristics arising from the unbalanced stator field and the rotor saliency. However, there is great potential for these types of motors to replace conventional single-phase induction motors in many domestic applications on account of their higher efficiencies when properly designed. In this paper, a new model that is applicable to both synchronous and asynchronous operation of the motor is proposed in which the parameters can be readily obtained by the two-dimensional (2-D) static finite-element method (FEM). It includes both the forward and backward rotating magnetic fields. It can be used to analyze the torque versus slip characteristics and predict the steady-state performances of the motor quickly with reasonable accuracy. The model has also been extended to simulate the transient start-up process and other dynamic performances. The models are suitable for the initial design and optimization of the motor geometry because of its low run-time overheads. Experimental results have verified the practicability of the models.

On-line machine fault detection is preferable since it is difficult to move and dismantle an electrical machine once it is installed and operated on-site. Hence we propose an on-line stator winding inter-turn fault detection approach based on electromagnetic field signature. Search coil technique will be used to capture the main flux that pass through stator tooth. The magnetic flux distribution is obtained by an electromagnetic field model. This fault detection scheme is suitable for non-stationary operating electrical machine system. A permanent magnet synchronous machine will be used as the candidate machine in this study. Finite element method simulations are used to validate this proposed on-line detection scheme.

This paper presents an experimentally verified single-pulse-operation switched reluctance motor converter topology based on the basic buck-fronted topology and the modified C-dump topology. Using only (n+1) power switches, this topology is able to achieve full single-pulse operation, thereby reducing switching losses and acoustic noise. It also achieves a greater demagnetizing voltage compared to the basic buck-fronted topology, without having to increase the DC-source voltage. The converter is more robust as its dump capacitor stops being charged if the chopper switch fails. The principle of operation, modes of operation, transient simulation results, and experimental results are presented.

This paper presents the computer-aided design (CAD) and performance analysis of a novel direct-driven wheel brushless DC motor drive for electric vehicles (EVs). The proposed motor is a permanent magnet square-wave motor, whose rotor with rare earth magnets forms the exterior of the motor, which can be fitted with a wheel tire to realize the direct drive for each wheel of an EV. The interior stator with its windings is rigidly mounted onto the suspension and frame structure of the vehicle. In order to achieve the direct drive without any mechanical transmission for EVs, the wheel motor has been designed as a low-speed high-torque motor. The design and optimization of the motor geometry was achieved with the aid of finite-element electromagnetic field analysis. Simulation studies on the transient performance of the motor drive were also carried out. This involved the creation of the motor transient model and formulation of a motor control strategy to ensure the wheel motor drive runs efficiently in the entire permitted speed and load range. The application of CAD techniques in the design of this very unconventional drive is described in this paper.

Singapore being a city state with 712.4km2, 5.183million people, S$59,813 Gross Domestic Product (GDP) per capital, population density of 7,126 per square kilometer [1] and limited natural resources, identified her GHG emission as carbon dioxide (CO2) mainly from combustion of fossil fuels and natural gas to generate energy meeting development and human needs. By 2006 highly efficient combined cycle technology was deployed to generate 78% primary energy by burning natural gas [2]. One of Singapore key strategies to further mitigate GHG emissions is to increase the energy efficiency of various sectors and/or introducing renewable energy sources. Singapore Industry, Buildings and Households sector consumes 54%, 16% and 9%, respectively, of generated secondary energy [2]. Three areas of pilot test beddings are presented to enhance Singapore energy efficiency. First, Energy Market Authority (EMA) launched a pilot project Intelligent Energy System (IES) in 2010 to test and evaluate new applications and technologies around a smart grid, thereby enhancing Singapore's power system efficiency and resilience, reducing wastage, saving peak loads and deferring capital investments to meet consumer demand in the future. Second, EMA will show case in 2012 how clean and renewable energy can be deployed at the system level in an environmentally, socially and economically sustainable manner for an off-grid community at Pulau Ubin, an island located at North East of Singapore. Third, Agency for Science, Technology and Research (A*STAR), and four industry partners developed a test facility Experimental Power Grid Centre (EPGC) [4], housing a 1MW power grid at Jurong Island in South West Singapore.

The world today is continuously striving towards carbon neutral clean energy technology. Hence, renewable energy sources like wind power system is increasingly receiving the attention of mankind. Energy production is now no more the sole criterion to be considered when installing new megawatt (MW) range of turbines. Rather some important design parameters like structural rigidity, cost effectiveness, life cycle impact, and, above all, reduced mass come into the scenario from new installation point of view. Accordingly, these issues are followed up in this article from wind turbine design perspective. The study, at the outset, aims to establish blade and tower material selection indices on the basis of inherent structural constraints and potential design objectives. Next, it highlights entire blade and tower material selection aspects for small and large scale horizontal axis wind turbines, both for onshore and offshore application. Finally, it distinguishes advanced blade and tower materials in agreement with multiple constraint, compound objective based design optimization procedure. Findings from the study can be deployed to harness massive scale wind energy from structurally more promising, economically more competitive and environmentally more clean and green turbines.

This paper proposes a variable speed control scheme for a tidal in-stream power generation system. The generator used in the study is a Squirrel Cage Induction Generator (SCIG) deliberately selected due to its robustness to boisterous conditions and ease of maintenance which makes it suitable for undersea application. An in indirect vector control mode, where the d-axis stator current controls the excitation level and the q-axis stator current controls the generator torque, is utilized. In addition to the conventional dq controller the use of a second controller which dictates the machine electrical torque is proposed. Simulation results are shown to verify the validity of the proposed scheme.

In a typically commercial or residential building, the indoor lighting system can be considered as one of the largest single consuming units amongst many electrical loads. Saving energy in such a lighting system is utmost important whilst satisfying lighting user's preference as well. This paper presents a controllable LED lighting system embedded with ambient intelligence gathered from a distributed smart wireless sensor network (WSN) to optimize and control the lighting system to be more efficient and user-oriented. In this proposed WSN-based control system, there is an inherent problem with the operational lifetime of its wireless sensor nodes limited by the finite energy capacity of their onboard batteries. To overcome this WSN's problem, an energy-aware communication protocol is introduced to reduce the power consumption of the WSN in order to prolong its operating time. The proposed smart WSN-based LED lighting system has been test-bedded and retrofitted into an existing workplace to conserve up to 10 % of waste light energy without compromising the workplace lighting condition. As for the operational lifetime of the proposed energy-aware WSN-based control system, the experimental results show that the wireless sensor nodes are able to operate for a longer period of time, from 87 days to 102 days, with about 20 % increase in their operational lifetimes.

Inefficiency of the common lightings like incandescent, fluorescent and halogen lamps presents an ample opportunity to retrofit high efficacy, long life-time and low maintenance LED solid-state lighting, which has significantly reduced in cost on an annual basis. LED lights and many electronic loads used within in-built environment are DC in nature. As such, in this paper, an energy-efficient low voltage DC (LVDC) grid powered LED lighting system with automated energy saving control is simulated, designed and implemented. An energy saving mechanism is introduced into the LED lighting system to tune its illuminance level accordingly to the users' preferences and to further achieve energy conservation. Experimental results show that the proposed LED lighting system with an energy saving mechanism incorporated is able to achieve similar lighting performance as the conventional lighting condition, while at the same time, able to attain about 44.2% energy saving as compared to the original AC fluorescent system. The LED lighting system test-bed has also demonstrated that the users have the flexibility to choose their own lighting preferences for their desks.

A compact flywheel energy storage system assisted by axial-flux partially-self-bearing permanent magnet motor has been proposed by the authors. The proposed machine combines axial magnetic bearing and motoring functionality into a single magnetic actuator, which not only spins the rotor-flywheel but also generates a levitation force to overcome the gravity. The mathematical model of the levitation force and rotational torque are developed. The control system of the position and speed is designed based on PID method. The experimental setup is built and the test results prove the feasibility of the proposed flywheel system concept, the correctness of the mathematical analysis and the validity of the designed control system.

This paper presents the loss analysis of a novel axial flux permanent magnet machine for flywheel energy storage system. Losses in iron-core and permanent magnets caused by both space harmonics and time harmonics are respectively analyzed using three-dimensional finite element method. Effect of skew of magnets on machine losses is also taken into study. The analysis is validated by comparisons of calculated losses and measured values on a prototype. A solution to reduce eddy-current losses on the magnets for this machine is also presented: the eddy-current losses in the permanent magnets can be significantly lessened by both circumferential and radial segmentations.

A compact flywheel energy storage system assisted by axial-flux partially-self-bearing permanent magnet motor has been proposed by the authors. The proposed machine combines axial magnetic bearing and motoring functionality into a single magnetic actuator, which not only spins the rotor-flywheel but also generates a levitation force to overcome the gravity. The mathematical model of the levitation force and stiffness of the active magnetic bearing are developed. The relationships between the stiffness and the key motor parameters are studied. High stiffness can be achieved for the special motor design. The derived force model is validated by finite element analysis. The control system of the levitation force is designed and experimental setup is built. The test results prove the correctness of the mathematical analysis and feasibility of the designed control system.

A PM synchronous motor and an induction motor both having dual air-gap, axial flux configurations have been designed. Both of these two type motors have twin stators and double-side single rotor sandwiched between these two stators. On the double surfaces of both disk-type rotors, the PMs are mounted and copper rotor-bars are filled in the slots respectively. The mathematical models of these two type motors have been derived, and the performances of the motors are compared mathematically. Three-dimensional finite element method (FEM) is implemented to analyze the characteristics of the motors and make comparison between both of them. The analysis results support the feasibility of the motor structures and prove that the PM motor has much higher power density, torque density and efficiency when the same dimension is assigned to both motors.

This paper introduces the use of a three-phase matrix converter as an alternative to the DC-link back-to-back converter, for doubly-fed induction generator used in wind turbine system. A new dynamic model of matrix converter is proposed together with d-q dynamic representation of doubly fed induction generator. Consequently, the complicated three-phase coupling system is transformed into two independent direct and quadrature axes. The developed model can be used for design of system controller as well as dynamic power system studies. The accuracy of the developed model is validated by comparing the simulation results produced by the developed model with real-switch based model.

A new geometrical design of a transcutaneous transformer for an artificial heart system is presented in this paper. The primary and secondary cores are made of ferrite and amorphous magnetic materials respectively. The experimental results indicate that it is possible to transmit an electrical power of 20 W which is sufficient to drive the heart pump. With a gap of 5 mm between the coils, its transmission efficiency with the inverter can be up to 90% with a switching frequency 102.5 kHz. Some characteristics of the proposed transformer are also shown and discussed in this paper.

This paper presents an experimental method and theoretical analysis to study the dielectric effect in Mn-Zn ferrite cores. Experimental results show that dielectric effect becomes very pronounced at high frequency. This is caused by the high permittivity of the ferrite core. Theoretical analysis is then introduced to study the influence of high permittivity on the permeability measurement. Numerical results show that significant aberrations of measured permeability from its intrinsic value could occur due to the high permittivity in the Mn-Zn ferrite cores.

The pump in an artificial heart system generally requires a power of 12-35 W to operate. Electrical energy can be transmitted transcutaneously by means of the inductive coupling between the primary coil and secondary coil of the transcutaneous transformer. The primary part of the transcutaneous transformer is located on the skin and the secondary is implanted inside the body. However, the energy transmission efficiency is low because of the large gap (generally 5/spl sim/15 mm) between the coils. In this case, the design of the transcutaneous transformer has been one of the key challenges in developing an efficient power supply system. Three types of transcutaneous transformers have been investigated. The first type is ferrite pot core transcutaneous transformer. The second type use amorphous fibers as the cores. The third type is a transcutaneous transformer with a combination core which has a ferrite primary core and an amorphous secondary core. A high frequency series resonant DC-DC converter is presented for a transcutaneous energy transmission system. By simulating with various values of coil geometry, the shape of each type of transcutaneous transformer has been studied with the aim of obtaining maximum output power. For each type of transformer, a power rating of more than 20 W can be delivered from a 12 V power source. This is sufficient for driving the heart pump. Circuit analysis on the transfer gain has also been presented, the simulation results can be. well explained by theoretical calculation. Besides all this, these three types are also compared with each other with a view of the potential for clinical application.

This paper presents a compact and novel flywheel energy storage system. The flywheel acts as the rotor of the drive system and is sandwiched between two disk type stators. Concepts of magnetic levitation and axial-flux PM synchronous machine are incorporated to assist the rotor-flywheel to spin and remain in magnetic suspension in the gravitational axis. Three-dimensional finite element method is applied for studying the flux linkage, induced voltage, levitation force and rotational torque of the system. The analysis results support the feasibility of the proposed system.

A compact and efficient flywheel energy storage system is proposed in this paper. The system is assisted by integrated mechanical and magnetic bearings, the flywheel acts as the rotor of the drive system and is sandwiched between two disk type stators to save space. The combined use of active magnetic bearings, mechanical bearings and axial flux PM synchronous machine assists the rotor-flywheel to spin and remain in magnetic levitation in the vertical orientation, while constrains the other four degrees of freedom in radial directions mechanically. The mathematical model of the proposed system has been derived. Three-dimensional finite element method is applied for studying the performances and verifying the mathematical model of the system. The analysis results support the feasibility of the system

Axial-gap motors have flat profiles which make them suitable for use as wheel motors for electric vehicles or as shaft-less centrifugal pumps. In this paper, we present the investigation of an induction type of axial-gap motor using three-dimensional time-stepping finite element analysis. Unlike conventional radial-gap motor with circular cross-section throughout a relatively long axial length, axial-gap motors are not so suitable to be investigated by two-dimensional finite element analysis. Three-dimensional finite element analysis represents a higher order of technology, which has only now beginning to reach maturity. Time-stepping version of three-dimensional finite element software is now available, and we have applied it to study the characteristics of a dual stator large air gap axial flux induction motor and to optimize its rotor design

A compact and efficient flywheel system is proposed in this paper. The flywheel acts as the rotor of the drive system and is sandwiched between two disk-type stators. A combination of active and passive magnet bearings allows the rotor-flywheel to spin and remains in magnetic levitation. It is a multivariable, nonlinear, and sensitive system. The conventional PID controller is not effective for such a system operating at very high speed. To achieve better control performance, a novel model-based predictive control (MPC) approach is introduced. The MPC method can precalculate the system behavior, and can be robust against model-mismatching and unexpected dynamics. The simulation results have been obtained and compared with the conventional PID control. The results show that the stability, sensitivity and robustness of the MPC approach are superior to PID control for the proposed flywheel system.

This paper describes a self-bearing centrifugal blood pump, which can be used as an artificial heart. A flat miniature squirrel-cage induction motor forms the core of the pump. Both the rotor of the induction motor and the impeller of the pump are levitated by passive and active magnetic bearings. The pump structures, design considerations, analysis of the design and time-stepping finite element simulation results of drive system and magnetic bearings, are presented in this paper.

A motor proposed in this paper has a disk-type rotor sandwiched between two disk-type stators and has the capability of both rotation and levitation control. The displacement and rotation along radial directions and axial direction are restricted and controlled by passive bearings and active magnetic bearings respectively. It does not rely on mechanical bearing, so does not require lubrication and can be expected to have longer maintenance-free lifespan. The collective system design, mathematical model, control scheme and simulation results are presented in this paper.

In this study, a piezoelectric transformer operating at the thickness shear vibration mode and with dual or triple outputs is proposed. It consists of a lead zirconate titanate (PZT) ceramic plate with a high mechanical quality factor Q/sub m/ and a size of 120 /spl times/ 20 /spl times/ 4 mm/sup 3/. The PZT ceramic plate is poled along the width direction. The electrodes of input and output parts are on the top and bottom surfaces of the ceramic plate and separated by narrow gaps. A new construction of support and lead wire connection is used for the transformer. At a temperature rise less than 20/spl deg/C and efficiency of 90%, the piezoelectric transformer with dual outputs has a maximum total output power of 169.8 W, with a power of 129.5 W in one output and 40.3 W in another. The one with triple outputs has a maximum total output power of 163.1 W, with a power of 36.9 W in the first output, 13.0 W in the second output and 113.2 W in the third output. The maximum efficiency of the piezo-electric transformer with dual outputs and triple outputs is 98% and 95.7%, respectively. The voltage gains of the transformers are less than one, and different outputs have different gains. Also, there is a driving frequency range in which the load resistance of one output has little effect on the voltage gain of another output.

This paper describes the development of disk-type bearingless motor for application as a satellite momentum/reaction wheel. It does not rely on mechanical bearings, so does not require lubrication and can be expected to have longer maintenance-free lifespan that is important in space applications. The proposed motor has a shaft-less disk-type rotor sandwiched between two disk-type stators and has the capability of both rotation and levitation control. The rotor is also the flywheel of the system. The desired degree of freedom (DOF) of rotation about z-axis motion is actively controlled while the other 5 DOFs are restricted actively or passively. Moreover, it has higher energy per weight and is smaller than the structure of a conventional motor levitated by shaft-based magnetic bearings and therefore is more suitable for space application where the space and weight are of much concern. Three-dimensional finite element simulation results are presented to support the proposition.

Realistic predictions of the dynamic and steady state performances are important in motor drive system design. This paper discusses a dynamic modeling method applied to the interior permanent magnet (IPM) motor with skewed stator slots by integrating two-dimensional electromagnetic field solutions with the electric circuit model. A multi-segment technique was applied to model the skew effect. Phenomena such as saturation and rotor saliency are taken into considerations in the suggested modeling method. The IPM motor performance characteristics under various operating conditions are investigated. Particular attention is paid to the motor torque, speed and induced back EMF responses. Experimental and simulation results of the motor model are presented.