Search

In order to make more effective use of space in the electric machine, a novel field modulated permanent magnet gear (FMPMG) machine is proposed. The key design aspect of the proposed FMPMG machine is adopting an inner rotor configuration and a specially designed stator core, with fixed modulator pieces, to improve the fabrication feasibility and rotation reliability. Harmonic analysis with detailed theoretical derivation is performed and reveals that the modulator pieces fixed to the stator can modulate the flux density distribution as well and realise a magnetic-gearing effect. The major parameters are designed and modelled by the 2D finite element method (FEM). First, based on the simulation analyses of the radial flux density distribution in the ‘pseudo’ outer and inner air gap, the specially designed stator is verified to create a nested magnetic-gearing effect together with the rotor PMs. The back electromagnetic force and electromagnetic torque waveform are further analysed, and the results indicate that the proposed FMPMG machine is more suitable for use as a sinusoidal excitation drive. Finally, a prototype is fabricated for experimentation to further verify the theoretical and FEM simulative validity of the proposed FMPMG machine.

Energy recovery by means of braking with the electric machine helps to extend the cruising range of electric vehicles. By optimising the recuperation system, the total energy efficiency is increased. In general, using a usual brake force distribution the capability for recuperation is not equal for every type of drive train. Usually, four-wheel drives are advantageous in comparison to drive trains with only one driven axle. Regarding the single drives, generally, a front-wheel drive provides a higher capability for recuperation than a rear-wheel drive. This is mainly due to the dynamic axle load shift, which occurs during decelerations. In order to increase the energy recovery of electric vehicles with single drive, an adaptive brake force distribution is presented and compared with other brake force distributions in this study. It uses the knowledge of the currently available adhesion coefficient between tyre and road. The positive effect on the recuperation without impairing the braking performance on a straight road is demonstrated based on simulations. In curves, the lateral forces additionally need to be considered to guarantee the stability of the vehicle, especially relating to electric vehicles having a rear-wheel drive.

48 V electric system architecture has been recently introduced to accommodate an increasing number of electric components in mild hybrid electric vehicles. In this study, two additional DC buses are added for electric machines including 36 and 24 V AC- and DC-motor/actuator loads, in a 48/12 V power supply system. This structure can yield significant benefits, especially for electrified vehicles that have more electric machines and actuators. A single-inductor multi-output structure is employed, with a control strategy that produces linear transfer functions and is less sensitive to load variation, to accommodate multiple-bus motor/actuator drives. The proposed structure and control strategy can offer options for selecting proper bus voltages, optimising the electric machine's power density. Both simulations and experiments have been performed to verify the system structure and control schemes using 36 and 24 V electric machines connected to the additional dual-output buses on top of the existing 12/48 V bus structure.

This study addresses double-sided stationary stators in a linear electric generator with similar operation to a linear oscillatory motion electric machine. The plate mover in the linear electric generator's double-sided stationary stators is powered by the acoustic power output of the thermoacoustic engine's acoustic line. The main originality of this structure resides in the linear guidance used for the moving part. The mover is made up of a plate of sandwich magnets between two stators. This plate is in magnetic balance between the two stators through a controlled friction device.

A hybrid power train system (HPTS) is considered as a promising technology to improve the efficiency of excavators. In the hybrid hydraulic excavator, due to the drastic changes in the load, ultra-capacitors are employed to realise fast charge and discharge. Accordingly, a wide change of the DC voltage will happen. When the DC voltage is small, the maximum torque that the electric machine can output will be limited. The electric machine cannot fully compensate for the engine torque. Therefore, a novel direct torque control (DTC) is proposed for a permanent magnet synchronous machine (PMSM) in HPTS to reduce the influence of DC voltage on the maximum output torque of the PMSM. In this study, the mutual influence between the torque of the PMSM and DC voltage is analysed. The traditional DTC is briefly introduced. A novel DTC for the PMSM is presented. To validate the performance of the novel DTC, simulations and experiments are carried out. The results demonstrate that the proposed DTC can reduce the effect of DC voltage variation on the maximum output torque of the PMSM. Moreover, working points of the engine in HPTS can be better stabilised.

In this study, two formulations for the robust optimisation of the size of the permanent magnet in a synchronous machine are discussed. The optimisation is constrained by a partial differential equation to describe the electromagnetic behaviour of the machine. The need for a robust optimisation procedure originates from the fact that optimisation parameters have deviations. The first approach, i.e. worst-case optimisation, makes use of local sensitivities. The second approach takes into account expectation values and standard deviations. The latter are associated with global sensitivities. The geometry parametrisation is elegantly handled thanks to the introduction of an affine decomposition. Since the stochastic quantities are determined by tools from uncertainty quantification (UQ) and thus require a lot of finite element evaluations, model order reduction is used in order to increase the efficiency of the procedure. It is shown that both approaches are equivalent if a linearisation is carried out. This finding is supported by the application on an electric machine. The optimisation algorithms used are sequential quadratic programming, particle swarm optimisation and genetic algorithm. While both formulations reduce the size of the magnets, the UQ-based optimisation approach is less pessimistic with respect to deviations and yields smaller magnets.

Supercapacitors present an attractive energy storage alternative for high-performance applications due to their compact size and high-power density. Therefore, the supercapacitors have broad prospects for the development in the field of electric vehicles and renewable energy. To describe the output characteristics of the supercapacitors with high accuracy for the simulation research and practical application, a dynamic modelling method is proposed for the supercapacitors based on support vector machine (SVM) and particle swarm optimisation (PSO) algorithm. In this study, the SVM is used to predict the output voltage of the supercapacitors with the key parameters (temperature, current and initial voltage). The PSO algorithm is adopted to optimise the parameters of the SVM to improve the performance of the dynamic modelling. An experimental platform is established, where an electric machine drive system powered by the supercapacitors is controlled to operate at frequent acceleration and deceleration modes, thus leading to the frequent charging and discharging of the supercapacitors. The experimental data is collected to validate the effectiveness of the proposed method. The results show that the proposed method can effectively predict the output voltage of the supercapacitors.

Owing to the definite merits of high-efficiency and high-power density, the permanent-magnet (PM) brushless machines have been dominating the high-performance industrial applications. However, the problems of high PM material cost, fluctuating supply of rare-Earth materials and ineffective PM flux control have hindered the widespread applications of the PM brushless machines. In recent years, many attentions have been shifted to the magnetless candidates, while the focuses are mainly placed on the classical candidates. In this study, an overview of magnetless brushless machines including both the classical and advanced types is presented, with emphasis on their machine topologies, features and performances. In addition, the upcoming trend of these machines is reviewed and discussed.

In this study, a review on high-frequency modelling in electric machines is conducted. Pulse-width modulation (PWM) creates common-mode-voltage (CMV) and common-mode-current (CMC) in electric drives with high-frequency impacts such as shaft voltage and bearing current which in turn cause bearing failures. To predict and reduce these failures in an electric machine, transitional modelling of high-frequency pulses is necessary. At the first step, several methods for high-frequency modelling in an electric drive are categorised. At the second step, several types of bearing currents discussed and CMV and CMC in electric drives are introduced. At the third step, CMV, CMC and bearing currents are simulated for six types of PWM methods and experimental verification is reported. Finally, root-mean-square value of a distorted parasitic current is introduced for comparing the electromagnetic effects of PWM methods.

In external-rotor electric machine configurations, the rotor can be the major source of noise. This study analyses the distribution and the effects of radial forces and vibration in a 6/10 external-rotor switched reluctance motor designed for an E-bike application. The distribution of the electromagnetic force vector depends on the switching sequence, the geometry of the motor and the direction of rotation. Therefore, the effect of the switching strategy on the rotor radial force waveform has been analysed. It is shown that adjusting the turn-on angle can reduce the high rate of change of radial force waveform at the pole corners. The effect of switching angles on the reduction of rotor vibration has also been examined.

A single-phase electric machine invokes a great reactive power demand in operation, resulting in a large voltage fluctuation in the supply system. Furthermore, the frequent and rapid operating characteristics of an electric welding machine cause a serious voltage flicker problem. The operation of welding equipment also generates significant harmonic current distortion, and distorts the voltage waveform of the supply system. Hence, this study focused on designing and applying a thyristor switched capacitor (TSC) bank for voltage flicker improvement. However, the single-phase TSC should also satisfy the reactive power demand and compensation speed, considering proper ratings of TSC to bear high harmonic distortion. This study shows the calculation results and discusses the impact of different capacities and different step numbers on TSC design. In addition, thyristor switches and a single-phase power factor controller were designed and implemented. The experimental results are also presented.

The efficient coupling driving control is a key technique that affects both the fuel economy and drivability of the hybrid electric bus (HEB). However, the uncertainties resulting from the complex driving condition and the powertrain would affect the control performance. To solve this problem, this study proposes a novel control approach, which is elaborately integrated with multi-controllers under the consideration of properties of the city-bus-route and the hybrid powertrain configuration. First, a torque split strategy with the automated mechanical transmission (AMT) gear-shifting strategy is employed to adapt the driving intention quantified by fuzzy logic. Then the coordinated control mechanism is constructed through utilising the electric machine (EM) to compensate the response deviation of engine torque, meanwhile a robust controller is designed to withstand parameter perturbation and external disturbance existing in EM. Simulation results show that the operating points of the engine and EM are adjusted into the high-efficiency areas with the assistance of AMT gear-shifting, and the EM torque tracking performance especially when parameter perturbation and external disturbance appear. Moreover, the strategy adaptively distributed the driving torque in the predefined working modes for different driver's intentions. Thus, the efficient coupling driving of HEB might be implemented by the proposed method.

A dynamic analytical electromagnetic torque model for a deep-bar induction machine is presented. The model is based on the space-vector theory of electrical machines. The parameters are estimated using the data provided by the numerical impulse response test performed within the two-dimensional time-stepping finite element analysis. Two different impulse tests are studied. The impulse excitation is applied to the stator voltage and rotor position angle, and the parameter estimates are compared. The applicability of the impulse test is based on the assumption of linear behaviour in the neighbourhood of an operation point. Hence, one of the main objectives is to study the validity of the impulse test. From the results of the angle impulse test, a frequency range of negative damping is detected. The phenomenon is also studied by measurements.

A survey on fault-tolerant control is presented for variable speed drives (VSD), mainly addressing DC and induction motors. Thus, fault diagnosis, accommodation and hardware reconfiguration are reviewed with application to VSD. First, the most common fault scenarios in these electrical machines are described. These conditions are analysed, and their implications and mathematical modelling are discussed. Next, the fault diagnosis strategies based on signal processing, model-based techniques and artificial intelligence are reviewed. Moreover, the diverse hardware reconfiguration schemes of power actuators with semiconductor failures are detailed. A key piece in any VSD is the control algorithm, which can be severely affected in a fault condition. Finally, the fault accommodation schemes suggested so far in the literature are illustrated.

The author presents the indirect boundary integral equation method (IBIEM) for capacitance computation. It is shown that parallel plate approximation leads to significant errors, especially for the capacitance to ground. The method has been used to compute the capacitance of the slot and the end region of an electric machine winding. The capacitances calculated by the IBIEM have also been used in a multiconductor transmission line and scatter matrix theory to calculate the transient voltage distribution in an electric machine winding subjected to steep-fronted surge. The results of voltage distribution have been compared with experimental results. The proposed IBIEM can be applied in other engineering applications where static capacitance is acceptable. The indirect boundary integral equation method can also be used in conjunction with other numerical techniques, such as finite element or finite difference methods in hybrid formulation. It can also be used for permeance computation when analysing the magnetic circuit.

The computation of electrical machines using the characterisation of electric alloys proposed by several authors gives performances not verified by experiments on actual machines. By means of finite-element-method software, the authors show that the electromagnetic context where electrical alloys are built into rotating electrical machines is completely different from the actual characterisation system. A new testing device is proposed to take into account the phenomena highlighted.

The variations of magnetic and loss characteristics of nonoriented cobalt-iron and silicon-iron sheets have been investigated using an Epstein frame. Cobalt-iron alloys have been tested under sinusoidal magnetic flux density with respect to frequency and temperature. Both kinds of alloys have also been investigated with respect to frequency, time constant wavefront and temperature under trapezoidal flux density. The qualitative behaviour is identical under both flux density conditions for all these alloys. These investigations have made it possible to model the variations of iron losses with experimental parameters, except the temperature. These quantitative evolutions differ with alloy and flux-density conditions and do not confirm several propositions of different authors.

Individual partial discharge (PD) detection and measurement is well recognised as a methodology for providing data on which assessments of the insulation integrity of high power plant can be made. It relies, as a technique, on the accurate acquisition of PD pulse data, including pulse magnitudes. Unfortunately, due to calibration difficulties with partial discharge detectors in static or rotating machine applications, pulse magnitude measurements may be erroneous. Calibration difficulties arise when the PD pulse is not simulated by the calibration pulse. This occurs when the PD pulse is 'distorted' as it propagates from its site of origin to the machine/detector terminals. To aid in the development of a new calibration strategy, the paper reports the results of two techniques. In the first, the transfer functions of different sections of stator winding are investigated to determine whether a frequency band exists in which a PD pulse would suffer minimal distortion as a function of location, in its propagation from site of origin to detector. Such a frequency band would indicate the appropriate bandwidth to which a detector could be tuned to avoid errors which were location dependent. This work builds on previously reported investigations. In the second, a technique of detected PD pulse reconstruction is investigated. The distorted, detected pulse postpropagation is reconstructed to its original magnitude via transfer function techniques. Given that the transfer functions from different locations in a winding are known and the location of the discharge can be identified or inferred, the technique offers an empirical approach to minimise calibration errors.

The travelling wave excitation of semi-infinite and finite thickness steel plate is examined using a multilayer electrical circuit concentrated parameter analogue to represent the electromagnetic field equations. Normalised characteristics of surface impedance and power input for steel plate with finite resistivity and constant permeability show that the method gives reasonably accurate results. The method is their extended to represent magnetic material which includes both hysteresis and eddy current action using a digital simulation technique, previously described. Computed values of the fundamental surface impedance and power input are determined for magnetically semihard and hard materials. Field patterns inside the material are illustrated. Some experimental results on locked rotor machines validate the predicted values.

In this study, hybrid excitation doubly salient machines (HEDSMs) with parallel excitation structures are introduced. The three types of main poles in the HEDSMs are described and researched, respectively. A 30/20-pole HEDSM is analysed by the magnetic circuit means. Regulation performance of the air-gap fluxes of different main poles is presented. The expression of the total air-gap flux shows the ideal linear relationship between the total air-gap flux and the field excitation ampere-turns. The finite element analysis also verifies the perfect effect of the field control. And the flux line plots and no-load electromagnetic force (EMF) waveforms are presented. The experimental EMF waveforms and no-load characteristic curve are coincident with the simulation results.

The necessary reliability of safety-critical aerospace drive systems is often partly achieved by using fault-tolerant (FT) electrical machines. There are numerous published literatures on the design of FT machines as well as on control algorithms used to maintain drive operation with an incurred fault. This study is set to look at the rotor losses in three- and five-phase surface mount permanent magnet machines when operating in faulty mode in order to highlight the influence of the post-fault control strategy and winding configuration. Although the work presented in this study is mainly focused on FT control methodologies targeted at mitigating phase open circuit faults, the implications of short-circuit faults is also considered and discussed.

By incorporating the flux-mnemonic concept into the hybrid-field doubly-salient permanent-magnet (DSPM) motor structure, the resulting flux-mnemonic DSPM motor not only offers efficient online air-gap flux control, but can also flexibly operate as a DSPM motor when the PMs are magnetised or a switched reluctance (SR) motor when the PMs are demagnetised. The key of this study is to artfully utilise these modes of operation for the proposed flux-mnemonic DSPM motor drive. Namely, the motor drive works in the DSPM mode during open-circuit fault, and changes to the SR mode during short-circuit fault. Both computer simulation and experimental results are given to verify the validity of the proposed fault-tolerant flux-mnemonic DSPM motor drive.

This study investigates the effect of eccentric rotor on the power losses in electrical machines. The investigations are carried out for an induction machine with PWM-voltage supply and in the cases of dynamic and static eccentricity. The time-stepping two-dimensional finite element method, with coupled field and circuit equations, is used for the simulations. The iron losses are computed through a dynamic loss model. It is shown that the operation of the machine under eccentricity fault results in an increase of both resistive and iron losses. Further the eccentricity causes additional torque modulation and unbalance magnetic pull that result in additional bearing friction losses and wearing.

There is a worldwide effort in using more and more electrical-driven auxiliaries in road vehicles. The integrated starter-alternator (ISA) is an electric drive in which the functions of starting engine and generating electric power are fulfilled by only one electric machine, instead of two separate electric machines in a traditional automotive vehicle. The interior permanent magnet (IPM) synchronous machines are attractive for the ISA application thanks to their characteristics: the flux-weakening capability, the high efficiency, the high power density and the high overload capability. This study reports some analysis and test results of an ISA that has been developed using a fractional-slot IPM machine. The results confirm that the IPM machine configuration is an excellent candidate for this application.

In this study, a new approach for genetic algorithm (GA) is proposed and compared with conventional GA (CGA) in the weight optimisation of a 2-MVA salient pole synchronous machine. The main differences between the two algorithms are that, in the newly proposed method, individuals are paired and crossed over based on the Mendelian rules of genetics, and the mutation operator is omitted. The rules concern the segregation of Alleles and the independent assortment of Alleles. This approach is comprehensive and conceptually accurate since its framework uses Mendelian population genetics. The operation CPU time is longer in the new approach when compared to the conventional one but can be ignored in electric machine design since it is not a real-time process. The results of the analytic solution and the new and CGA implementation methods are compared in terms of weight, efficiency and temperature. The results obtained are similar to those of the conventional ones and even better in some cases. A finite element analysis (FEA) is done to realise the machine designs optimised by the new GA (NGA) and CGA for the case of a fixed 24-pole design. Hence the improvement over CGA achieved by NGA has been validated through FEA.

Condition monitoring of rotating electrical machinery has received intense research interest for more than 30 years. However, electrical machinery has been considered reliable and the application of fast-acting digital electrical protection has rather reduced the attention operators pay to the equipment. The area based upon current literature and the author's experience is reviewed. There are three restrictions: only on-line techniques for rotating machines are dealt with; specific problems of variable speed drives are not dealt with, except in passing; conventional rather than emerging brushless, reluctance and permanent magnet machines of unusual topology are concentrated upon. The art of condition monitoring is minimalist, to take minimum measurements from a machine necessary to extract a diagnosis, so that a condition can be rapidly inferred, giving a clear indication of incipient failure modes. The current state of the art is reviewed in the following ways: survey developments in condition monitoring of machines, mechanically and electrically, over the last 30 years; put that work in context alongside the known failure mechanisms; review those developments which have proved successful and identify areas of research which require attention in the future to advance the subject.

Forces due to nonuniform airgaps in rotating electrical machines have been a research topic for over 100 years. However, most research in the area of rotating electrical machines has been performed on motors. Large forces in hydropower generators can lead to expensive damage and failures. Therefore, it is of interest to calculate the forces that arise in a large synchronous generator with an eccentric rotor and study the influence these forces have on the stability of the generator rotor. A 74 MVA synchronous hydropower generator was simulated with an eccentric rotor, using a time-stepping finite-element technique. The forces were calculated using Coulomb's virtual-work method and simulations were performed for no-load and load cases. The resulting force was found to be reduced significantly when a damper winding was taken into account. An interesting effect of the rotor damper winding was that it reduced the eccentricity force and introduced a force component perpendicular to the direction of eccentricity. The results from the finite-element simulations were used to determine how the forces affect the stability of the generator rotor. Damped natural eigenfrequencies and damping ratio for load and no-load conditions are presented. When applying the forces computed in the time-dependent model, the damped natural eigenfrequencies were found to increase and the stability of the generator rotor was found to be reduced compared with when the forces were computed in a stationary model.

In the paper, a new low-order parametric force model for an electrical machine with eccentric rotor is developed and verified. The model is designed for an electrical machine equipped with both the parallel stator windings and rotor cage, and, therefore, accounts for the equalising currents in the stator and rotor windings separately and also for their influence on each other. Furthermore, the model can also be used with an electrical machine having either the parallel stator windings or the rotor cage. The presented model with parameters estimated from the numerical test results demonstrated a very good performance in a wide whirling frequency range. The proposed model provides an opportunity to combine effectively the electromagnetic and mechanical analyses of the electrical machine.

The DC step voltage experiment is an alternative and non-destructive time-domain test to extract parameters from electrical machines that follows the standstill frequency response test procedure described in IEEE standard 115A. This work presents a new method for semi-analytically determining the characteristic parameters of the d-axis synchronous machine model. The test is carried out when the machine is at rest and the rotor's position is at d-axis; a DC-controlled power source is applied suddenly at two of the three terminals of the stator machine and the field circuit is short-circuited. Test measurements of d-axis stator voltage excitation and field and d-axis stator current responses were used. Both stator and field current patterns are simultaneously fitted to exponential functions which coefficients are analytically related to the d-axis characteristic parameters of the equivalent circuit. These characteristic parameters come from the conventional two transfer functions: direct-axis operational inductance L d (s) and the armature to field transfer function sG(s). To validate the proposed methodology, test data is used in the d-axis parameter determination of a 7 kVA, 220 V, 60 Hz synchronous generator

Effective and accurate prediction of key motor parameters, such as winding flux, back electromotive force, inductance and core losses, is crucial for design of high-performance motors. Particularly, for electrical machines with new materials and nonconventional topology, traditional design approaches based on the equivalent magnetic circuit, empirical formulas and previous experiences cannot provide correct computation. The paper presents accurate determination of major parameters of a three-phase three-stack claw-pole permanent-magnet motor with a soft magnetic composite (SMC) stator core by finite-element analysis of the magnetic field. The effects of magnetic saturation and armature reaction are considered. The theoretical results by numerical analysis are validated by the experiments on the claw-pole SMC-motor prototype.

This paper presents a method for determining the signatures of electrical faults in the airgap force distribution and vibration pattern of induction machines. The monitoring of faults is achieved through measurement of the vibrations of the stator frame. The radial electromagnetic force distribution along the airgap, which is the main source of vibration, is calculated and developed into double Fourier series in space and time. Finite element simulations of faulty and healthy machines are performed; they show that the electromagnetic force distribution is a sensible parameter to the changes in the machine condition. The computations show the existence of low-frequency and low-order force distributions acting on the stator of the electrical machine when it is working under fault conditions. The simulation results are corroborated by vibration measurements on an induction motor with implemented broken bars and an inter-turn short circuit. The measurements and simulations show that low-frequency components of the vibrations can be used as identifiable signatures for condition monitoring of induction motors. The determination of the vibration frequency corresponding to a given electric fault in a given machine can be achieved through numerical simulations of the magnetic field and electromagnetic forces in the cross-section of the machine, without need for complex structural analysis.

Theoretical, numerical and experimental studies are described that have been carried out to develop a solid-rotor induction machine design for a particularly demanding application as an accelerator drive for a diesel engine turbocharger. In this application the turbo-motor will be required to operate at speeds of up to 130 000 rpm and in temperatures that can exceed 200°C. The results show that the equivalent circuit gives predictions that are of sufficient precision for design purposes and provides a useful design tool. It is shown that the use of a solid rotor affects the value of the stator leakage reactance and influences the motor performance through this effect as well as by presenting a high rotor resistance.

A low-order parametric model, to accurately represent the eccentricity force acting in a salient-pole synchronous machine equipped with parallel stator windings, is proposed. The force model with parameters estimated from the numerical field calculation results exhibits an excellent performance in a wide whirling frequency range. Results presented also demonstrate the applicability of the model for the eccentricity force analysis in an induction machine. The proposed model provides an opportunity to combine the electromagnetic and mechanical analyses of electrical machine effectively.

A parameter estimation of a d-axis synchronous generator is presented, in which flux decay test data from the field and armature windings are simultaneously employed. The test is carried out while the machine is at standstill and a DC current is applied to the d-axis armature winding with the field winding in short circuit. The d-axis parameter estimation is achieved by simultaneously fitting the field and d-axis armature currents. The current patterns are approximated by exponential functions, the coefficients of which are related analytically to the d-axis characteristic parameters of the synchronous machine. It is a normal practice to relate the characteristic parameters to a d-axis transfer function, which is only related to the d-axis armature current. To use the field current data on the estimation process, a novel proposal of a transfer function that includes the field current is also used in the estimation process. To validate the proposed approach, a parameter estimation of a 7 kVA, 220 V, 60 Hz synchronous generator is presented.

Most induction motor analyses that allow for the presence of inter-bar currents are based on a rotor model in which the inter-bar currents are assumed to flow between adjacent pairs of bars. An alternative model is proposed in which the inter-bar currents flow into and around the rotor core. The validity of the new model is verified by means of measurements made on 54 rotors of identical design, which have been subjected to a range of treatments during manufacture to produce bar-to-bar resistances that differ by up to three orders of magnitude. It is shown that the new rotor model may readily be incorporated into existing analyses by redefining a single inter-bar resistance parameter. An alternative viewpoint of this is that the new rotor model redefines the relationship between the resistance parameter used in analytical models and the bar-to-bar resistance measured under test.

The design and development of an axial-flux permanent-magnet air-cored generator for use as a direct drive generator with small-scale wind and water turbines is described. The generator is designed for simplicity and ease of manufacture and consists of two rotor discs each with permanent magnets located around its periphery. The stator is made of plastic and has a number of bobbin-wound armature coils located around its periphery. A three-phase prototype generator has been designed and built with 16 magnets per rotor disc, 12 armature coils (four per phase) and an overall diameter of 495 mm. The generator produces 1000 W at 300 rpm or 2000 W at 500 rpm with an electrical efficiency substantially greater than 90%. The generator performs as predicted by the design process.

A systematic analysis approach is presented for four- and eight-piece Halbach arrays' magnetic field and the geometric optimisation for magnetic levitation (Maglev) applications, which utilise the non-ideal array's magnetic field harmonics. The field analysis results obtained using scalar potential and Fourier series methods are confirmed with FEM results. The geometric optimisation is based on the maximisation of the ratio of the square of flux to a unit area magnet weight. The results of the systematic analysis of magnetic field and the geometric optimisation for practical Halbach arrays can be used as a technical framework for further Maglev system designs.

For reducing the magnitude of stray current, the diode-grounded system is an improvement over the solidly grounded system, but the rail potential and stray current remain large. However, since the diode device of this system provides a path for stray current, the magnitude of the stray current can be easily and conveniently monitored, and positive-pole earth faults can be inspected. A novel control scheme is proposed. The output voltage of the traction substation is controlled such that it remains constant in the diode-grounded system, thus the magnitude of rail potential and stray current can be reduced. Kaohsiung MRT systems are used as an example of simulating multiple traction substations and multiple trains. The simulation results show that this novel control scheme can greatly reduce the stray current and rail potential.

A new type of brushless synchronous machine has been described in the companion paper 'New brushless synchronous alternator' (Brown, N.L. and Haydock, L.: IEE Proc., Electr. Power Appl. 2003, 150, (6), pp. 629-635.) The machine combines permanent magnet and wound coil excitation to provide a single-stage brushless alternator. The simple construction conceals a difficult magnetic circuit that displays complex interactions between excitation sources in the presence of saturation and leakage fields. A design procedure for such a machine using equivalent circuit models is described and the theoretical results are compared with measured values.

System analysis within the traction power system is vital to the design and operation of an electrified railway. Loads in traction power systems are often characterised by their mobility, wide range of power variations, regeneration and service dependence. In addition, the feeding systems may take different forms in AC electrified railways. Comprehensive system studies are usually carried out by computer simulation. A number of traction power simulators have been available and they allow calculation of electrical interaction among trains and deterministic solutions of the power network. In the paper, a different approach is presented to enable load-flow analysis on various feeding systems and service demands in AC railways by adopting probabilistic techniques. It is intended to provide a different viewpoint to the load condition. Simulation results are given to verify the probabilistic-load-flow models.

This paper describes a method for the dynamic simulation of a cage induction machine with static and dynamic air gap eccentricity. The method is based on the winding function theory and extension of this theory to the nonuniform air gap case. A method of calculating all inductances in a machine with both static and dynamic air gap eccentricity is presented and a numerical analysis of a machine with specified parameters is presented. Stator line current spectra collected experimentally for both the static and dynamic eccentricity conditions confirm the results obtained by the proposed numerical model.

Analytical expressions obtained from circular plate, elastic beam and cylindrical shell theory are applied to calculate the structural mass of axial-flux permanent-magnet machines for a range of typical wind-turbine ratings. Finite-element models and data from existing low-speed axial-flux machines have been used to gain confidence in the models presented, such that they can be incorporated into design-office programs to rapidly provide a first-order estimate with a reasonable level of accuracy. Results confirm that the inactive mass is greater than 60% of the machine total mass, and at multimegawatt ratings it is almost 90% of the total mass. Optimisation of the machine must therefore include the link between structural and electromagnetic designs. Results presented illustrate the impact of inactive mass on geometrical design parameters and also show that multistage designs can lead to a reduction in mass.

The paper describes two techniques for reducing the cogging force associated with the finite length of the ferromagnetic armature core of a tubular permanent-magnet machine. Analytical expressions for predicting the cogging force which is associated with armature cores having skewed or stepped end faces are established, and a cogging-force-minimisation technique is employed to determine an optimal armature-core length. The validity of the analytical cogging-force predictions and the effectiveness of the cogging-force-reduction techniques are verified by three-dimensional finite-element analyses and experimental measurements.

This paper presents a method to analyse the dynamical behaviour of large-size generators due to the magnetic pull. In rotating electrical machines, the electromagnetic radial forces acting upon rotor and stator surfaces are very large, but they are balanced when the rotor is concentric with the stator. Similarly, the tangential forces produce only an axially rotating moment. If the rotor becomes eccentric, then an imbalance of these forces occurs, so that a net radial electromagnetic force, known as Unbalanced Magnetic Pull (UMP), is developed. The models traditionally proposed in the literature to study the UMP can be considered as reliable in case of small size electrical machines supported by rolling bearings. On the contrary, in case of large-size machines, such as turbo-generators supported by oil-film bearings, the approximation of circular orbit for the geometric centre of the rotor is not acceptable. Nevertheless, the authors who have dealt with UMP in large generators have disregarded that these filtered orbits of the rotor are elliptical and generally the orbit centre is not concentric with the stator. A more realistic model is introduced in this work and the actual distribution of the air-gap length during the rotation will be determined in analytical terms, by taking into account the effects produced by the actual rotor orbit. The actual UMP is calculated by using the air-gap permeance approach and the simulation of the dynamical behaviour of a 320 MVA generator is presented, showing the harmonic content of the UMP and the presence of nonlinearities.

The modelling of electrical machines by equivalent circuits requires an appropriate calculation of inductance values. The energy-perturbation and flux-linkage methods have been devised (and extensively used) to determine inductance values of electrical machines from two-dimensional finite-element solutions. That both methods are numerically equivalent and give approximately the same values of inductance is shown. However, the flux-linkage method requires less computation resources and is numerically more robust than the energy perturbation method when two-dimensional calculated fields are used. Hence, it is concluded that the flux-linkage approach should be chosen as the preferred method for two-dimensional inductance calculation of electrical machines. The numerical characteristics and equivalence of the energy-perturbation and flux-linkage methods are demonstrated by calculating the time-varying 'apparent' inductances of the stator and rotor field windings of a turbine generator running under transient conditions.

Two new finite element (FE) methods for improved electromagnetic torque computation are proposed in the paper. The first method uses an analytical filter applied to the Maxwell stress tensor calculation. The second method is based on the virtual work principle and directly provides the separation of torque in its average and pulsating components. The FE computational methods are validated on a brushless PM motor having a rotor with surface mounted ferrite arcs. The relative merits of each method are discussed, together with computational issues, such as the influence of the meshing and the convergence of the calculation.

An approach employing genetic algorithm (GA) has been evolved for the analysis of single-phase operation of three-phase self-excited induction generators. Using symmetrical components, the circuit equations have been written and an impedance matrix has been derived, which can be straightaway solved using this method. The lengthy derivations of nonlinear equations required for the unknown variables, in the earlier methods, are thus not required in the present method. Apart from single-phase AC loads, feeding of DC loads through a controlled rectifier has also been considered and the analysis has been extended for this combined loading. For maintaining a desired DC voltage on the rectifier output side, symmetrical and extinction angle control techniques have been adopted. These techniques are shown to be better compared to the conventional phase angle control, in improving voltage regulation of the generator. The suitability of each of these control schemes for specific load conditions is discussed with examples. A biomass gasifier-based diesel engine has also been used as a prime-mover and the experimental results obtained on two generators with AC as well as DC loads, closely agree with the values predetermined using the GA approach.

The paper summarises the authors' reported and ongoing research on both modelling and on sensorless control and power-factor improvement of switched-reluctance-machine (SRM) drives. The paper presents four models of SRMs: the piecewise-interpolation model using a two-dimensional (2-D) bicubic spline, an analytical model based on the 2-D least-squares technique, a cross hybrid model based on the 2-D bicubic and 2-D bilinear splines, and the coupling model taking account of mutual coupling. These four models are validated by simulations and experiments. An estimation scheme of the initial rotor position at standstill and a sensorless control scheme at low and high speeds are also developed. In addition, the authors have studied the effect of the control and output parameters on power factor in SRM drives. Consequently, a new control strategy and two real-time schemes for improving the power factor have been developed.

Finite element modelling of geometries with large dimensional differences between adjacent sub-domains or domains which have poor aspect ratios can impose significant demands on meshing algorithms and computational resources. A simple sub-domain scaling technique for electromagnetic analysis of electrical machines by finite element analysis is proposed. The technique can be used to improve aspect ratios and is useful in modelling electrical machines with small air gap lengths.

An artificial neural network based computer-aided designing system together with the manufacturing cost estimation system for industrial induction motors is presented. The aim is to contribute to the understanding of the new system. Based on the actual design data for various types of induction motors, the cascaded artificial neural networks have been trained to calculate the output of the fundamental data and parameters. After the training, the dimensions and the basic parameters have been estimated quite accurately from the specification of the individual induction motor on the cascaded three artificial neural networks. The manufacturing cost of the individual motor has also been predicted precisely by using the neural network at the last stage of design. The proposed neural network based designing system including the cost estimation system is quite efficient in reducing the manpower of the designing process and also in enabling optimal design in a restricted time period without the knowledge of experts.