Showing papers in "Iet Electric Power Applications in 2009"

Vector control design and experimental evaluation for the brushless doubly fed machine

Abstract: A new vector control algorithm for the brushless doubly fed machine (BDFM) has been developed. The goal of BDFM control is to achieve a similar dynamic performance to the doubly fed induction machine (DFIM), exploiting the well-known induction motor vector control philosophy. For this purpose, a recently developed unified reference-frame model has been used to develop the vector control strategy. The theoretical considerations for the vector control design, control loops tuning and limits are shown. In addition, practical considerations for the experimental implementation of vector control for the BDFM are shown. Experimental results in a machine prototype confirm the good dynamic performances of this type of control.

Sensored and sensorless speed control methods for brushless doubly fed reluctance motors

Abstract: The study considers aspects of scalar V/f control, vector control and direct torque (and flux) control (DTC) of the brushless doubly fed reluctance machine (BDFRM) as a promising cost-effective alternative to the existing technological solutions for applications with restricted variable speed capability such as large pumps and wind turbine generators. Apart from providing a comprehensive literature review and analysis of these control methods, the development and results of experimental verification, of an angular velocity observer-based DTC scheme for sensorless speed control of the BDFRM which, unlike most of the other DTC-concept applications, can perform well down to zero supply frequency of the inverter-fed winding, have also been presented in the study.

Very low speed performance of active flux based sensorless control: interior permanent magnet synchronous motor vector control versus direct torque and flux control

Abstract: This study is focused on very low speed performance comparison between two sensorless control systems based on the novel `active flux` concept, that is, the current/voltage vector control versus direct torque and flux control (DTFC) for interior permanent magnet synchronous motor (IPMSM) drives with space vector modulation (SVM), without signal injection. The active flux, defined as the flux that multiplies iq current in the dq -model torque expression of all ac machines, is easily obtained from the stator-flux vector and has the rotor position orientation. Therefore notable simplification in the rotor position and speed estimation is obtained. For IPMSM, a stator-flux observer is employed based on combined current and voltage models, with speed-dependent smooth transition between them using a PI compensator of flux error. Comparative experimental results using both sensorless control systems are presented to verify the principles and to demonstrate the effectiveness of the active flux observer at very low speeds from 20 to 2 rpm (1 0.1 Hz).

Index of rotor losses in three-phase fractional-slot permanent magnet machines

Abstract: The aim of here is to define an index of the rotor losses induced by the magnetomotive force (MMF) space harmonics in fractional-slot permanent magnet (PM) machines. Such an index facilitates a rapid discrimination of the various fractional-slot PM machines, based on the numbers of slots and poles. For the sake of generality, a simple model of the rotor losses is adopted to compute such an index of rotor losses. However, the index behaviour follows that of rotor losses computed by means of more complex models.

Direct instantaneous torque control of switched reluctance machines using 4-level converters

Abstract: Direct instantaneous torque control (DITC) of switched reluctance machines (SRMs) using a novel 4-level converter is presented. The described DITC control strategy proposes detailed torque control regions and suitable control schemes. Using the 4-level converter, DITC can overcome voltage limitation of asymmetric converters and has a fast magnetisation and demagnetisation to improve dynamic performance and efficiency. For integrated advantages of DITC and 4-level converters, a suitable control scheme is described and analysed. Finally, the proposed DITC method of SRM drive systems using the 4-level converter is verified by simulation and comparative experiments.

Unified winding function approach for dynamic simulation of different kinds of eccentricity faults in cage induction machines

Abstract: The multiple coupled circuit model of a three-phase cage induction machine and the uses of a modified winding function theory to calculate the time-dependent inductances of the motor with a general eccentricity fault, including static, dynamic and mixed eccentricities, in a unified manner are considered. By defining the inverse air gap function of the eccentric machine, determining its indefinite integral and assuming step variations for the turn functions at the centres of the slots, precise analytic equations are obtained for the inductances. Differentiating these analytic equations against the rotor angular position also gives precise analytic equations to calculate the derivatives of the inductances, which may be used to determine the electromagnetic torque. Consequently, dynamic simulations of an induction machine with any eccentricity type and degree as well as a healthy one is made possible in the frame of a single Simulink program. At the beginning of the simulation process, the eccentricity components' degree and position are introduced and the program is then executed accordingly. A simple technique was used to make an industrial induction motor temporarily eccentric, with varying degrees, and experiments were conducted on it. By defining and applying effective air gap length and effective eccentricity components' degrees, good agreements are achieved between the simulated and the experimental results, which implies that the whole unified simulation program is effective.

Dynamic model for doubly-fed induction generators with unbalanced excitation, both with and without winding faults

Abstract: A detailed analytical model designed for doubly-fed induction generator (DFIG) dynamic analysis is presented. The method employs a coupled-circuit approach and uses the principles of generalised harmonic analysis and complex conductor distribution to develop a systematic method for calculating a circuit's self-inductance and mutual inductance for windings with any distribution of conductors. As such, it is capable of representing the effects that winding and excitation asymmetries have on machine performance. The accuracy of the model is verified in both the time and frequency domains against experimental results obtained from a specially constructed DFIG test rig, which can be configured to introduce a range of open-circuit and short-circuit faults. Model predictions are compared with experimental measurements for a balanced machine and for the case in which one stator phase suffers an open-circuit fault. The experimental and predicted data are shown to be in good agreement and demonstrate the effectiveness of the technique developed.

Position sensorless control for interior permanent magnet synchronous motor using adaptive flux observer with inductance identification

Abstract: A position sensorless control of interior permanent magnet synchronous motors (IPMSMs) is proposed using the adaptive full-order observer with inductance identification. First, mathematical models are discussed for robustness improvement to inductance variation and inductance identification in IPMSMs, the models show that a high-frequency signal needs to be injected to identify the inductance. Next, novel adaptive schemes for inductance identification are proposed here. Finally, identification experiments under IPMSM position sensorless control are carried out with the proposed methods, in which the feasibility and effectiveness of the methods are shown in terms of inductance identification performance and the convergence of position estimation error.

Comparative thermal analysis of different rotor types for a high-speed permanent-magnet electrical machine

Abstract: A thermal analysis of three different rotor constructions for a high-speed permanent-magnet electrical machine is performed. The first type of rotor has a carbon-fibre sleeve for retaining the magnets against the centrifugal forces and an aluminium shield for eddy currents. The other two rotors have retaining sleeves made of different titanium alloys and do not have additional eddy-current shields. The thermal analysis of the rotor types is performed using two completely different approaches. The first implemented method is a numerical-multiphysics one that couples computational fluid dynamics equations with heat-transfer equations. For better reliability of the analysis, a traditional thermal-network method is also implemented for estimation of the temperature distribution in the examined rotors. The accuracy of the aforementioned methods is verified using experimental results for the average temperature rise of the permanent magnets. The results obtained from the implemented methods show that although the rotor with a retaining sleeve made of titanium alloy Ti-6%Al-6%V-2%Sn does not have any eddy-current shield, it fulfils the thermal constraints and can be implemented for high-speed applications.

Modelling the brushless excitation system for a synchronous machine

Abstract: The structure and the operation of the model for the brushless excitation system for a synchronous machine are presented. The nonlinear model including the excitation machine, the AC-AC converter supplying the excitation machine and the rectifier diode bridge, mounted on the rotor, is based on a state machine. The states are defined by current commutation in the power electric devices. The operation of the excitation system model is verified by measurements with a slip-ring machine imitating the excitation machine. The excitation system model is integrated and simulated as a part of a synchronous machine simulator.

Modelling of squirrel-cage induction motors for electromagnetic transient programs

Abstract: The authors discuss the modelling of the squirrel-cage induction motor. The experimental measurements on motors of different rated power, 1-90 kW, clearly show that the single-cage model does not fit the torque-speed curve adequately. Consequently, when the speed varies in a wide range, only the double-cage model can fit the curve well with the squirrel-cage induction motor. The equivalence between the two most commonly used double-cage models in the literature is studied, and a method to find the equivalent parameters is developed. In addition, the induction motor modelling in the PSCAD/EMTDC and EMTP-RV simulation programs is examined, and the inaccuracies resulting from the use of manufacturer data are discussed.

Multi-criteria-based design approach of multi-phase permanent magnet low-speed synchronous machines

Abstract: A design methodology dedicated to multi-phase permanent magnet synchronous machines (PMSMs) supplied by pulse width modulation voltage source inverters (PWM VSIs) is presented. First, opportunities for increasing torque density using the harmonics are considered. The specific constraints caused by the PWM supply of multi-phase machines are also taken into account during the design phase. All the defined constraints are expressed in a simple manner by using a multi-machine modelling of the multi-phase machines. This multi-machine design is then applied to meet the specifications of a marine propeller: verifying simultaneously four design constraints, an initial 60-pole three-phase machine is converted into a 58-pole five-phase machine without changing the geometry and the active volume (iron, copper and magnet). First, a specific fractional-slot winding, which yields to good characteristics for PWM supply and winding factors, is chosen. Then, using this winding, the magnet layer is designed to improve the flux focussing. According to analytical and numerical calculations, the five-phase machine provides a higher torque (about 15%) and less pulsating torque (71% lower) than the initial three-phase machine with the same copper losses.

Recursive genetic algorithm-finite element method technique for the solution of transformer manufacturing cost minimisation problem

Abstract: The transformer manufacturing cost minimisation (TMCM), also known as transformer design optimisation, is a complex constrained mixed-integer non-linear programming problem with discontinuous objective function. This paper proposes an innovative method combining genetic algorithm (GA) and finite element method (FEM) for the solution of TMCM problem. The main contributions of the proposed method are: (a) introduction of an innovative recursive GA with a novel external elitism strategy associated with variable crossover and mutation rates resulting in an improved GA, (b) adoption of two particular finite element models of increased accuracy and high computational speed for the validation of the optimal design by computing the no-load loss and impedance and (c) combination of the innovative recursive GA with the two particular finite element models resulting in a proposed GA-FEM model that finds the global optimum, as concluded after several tests on actual transformer designs, while other existing methods provided suboptimal solutions that are 3.1-5.8% more expensive than the optimal solution.

Optimisation techniques for a hysteresis current controller to minimise torque ripple in switched reluctance motors

Abstract: The switched reluctance (SR) motor has many benefits owing to its low cost, simple design, rugged construction and comparatively high torque-to-mass ratio. Unlike DC and induction motors, the SR motor is intended to operate in deep magnetic saturation to increase the output power density. Because of the saturation effect and the variation of magnetic reluctance with respect to rotor position, all the relevant characteristics of the machine are highly non-linear functions of both rotor position and phase current. The ultimate outcome of all these non-linearities is that the generated torque contains significant ripples. The non-linearities in the SR motor have been extensively studied and many control strategies to reduce the generated torque ripples have been proposed in the literature. Modulation of phase current profile for generating torque in the SR motor with minimum ripples was the focus of most of the research. However, the main challenge to minimise the torque ripple is to design a current controller that is able to track the modulated phase current. In this work, new techniques to optimise the widely used hysteresis current controller are studied, and experimental verifications under closed-loop speed control with the modulated reference current data are presented. The experimental results indicate that the torque ripple is reduced to lie within 5% of the desired steady torque using the proposed optimisation techniques.

Model reference adaptive control-based adaptive current control scheme of a PM synchronous motor with an improved servo performance

Abstract: A model reference adaptive control (MRAC)-based current control scheme of a PM synchronous motor with an improved servo performance is presented. Although the predictive current control is known to give ideal transient and steady-state responses among various PWM inverter-fed current control schemes for a PM synchronous motor, its steady-state response may be degraded under the motor parameter variations. To overcome such a limitation, the disturbances caused by the parameter variations will be estimated using an MRAC technique and compensated by a feedforward manner. Thus, the steady-state control performance can be effectively improved, while retaining its good dynamic performance. The proposed control scheme does not require the measurement of the phase voltage unlike the conventional disturbance estimation scheme using observer. This can be an effective way considering the phase voltage contains much harmonics as well as noise. The asymptotic stability of the overall system is proved and the adaptation laws are derived by the Lyapunov stability theory. The proposed scheme is implemented using DSP TMS320C31 and the effectiveness is verified through the comparative simulations and experiments.

Characterisation of radial vibration force and vibration behaviour of a pulse-width modulation-fed fractional-slot induction machine

Abstract: The authors present the full analysis of the vibration behaviour responsible for audible magnetic noise in a pulse-width modulation (PWM)-fed fractional-slot induction machine, including theoretical predictions, numerical simulations and experimental validations (stator modal analysis and deflection shapes visualisation). Magnetic force waves caused by slotting harmonics, PWM harmonics and their interactions are characterised in terms of nodes number, rotation speed and propagation direction. It is shown in particular that some odd spatial order vibration waves appear because of fractional slotting, and that some combinations between slotting lines and PWM lines can be very noisy because of an elliptical mode natural frequency close to the switching frequency.

A current ripple reduction of a high-speed miniature brushless direct current motor using instantaneous voltage control

Abstract: High-speed miniature brushless direct current motor (BLDCM) is used in robots and medical applications because of its high-torque and high-speed characteristics. When compared with the general BLDCM, a high-speed miniature BLDCM has a low electrical time-constant. The current and torque ripple are very high when compared with the conventional pulse-width modulation (PWM) control scheme in the conduction period because of the inherent electrical characteristics. The authors propose a simple instantaneous source voltage and phase current control for torque ripple reduction of a high-speed miniature BLDCM. To reduce the switching current ripple, instantaneously controlled source voltage is supplied to the inverter system according to the motor speed and the load torque. In addition, a fast hysteresis current controller can keep the phase current within a limited band. Computer simulations and experimental results up to 40 000 rpm show the effectiveness and verification of the proposed control scheme.

Development of a superconducting fault current limiter using various high-speed circuit breakers

Abstract: The authors constructed and tested a model superconducting fault current limiter (SFCL) using a high-temperature superconducting film according to a design that includes a vacuum interrupter with an electromagnetic repulsion mechanism. The superconductor and the vacuum interrupter are connected in parallel with a bypass coil. If a fault occurs and current flows through the system, the superconductor is quenched and the current is transferred to the parallel coil because of the voltage drop in the superconductor. This large current in the parallel coil actuates the magnetic repulsion mechanism of the vacuum interrupter. On opening the vacuum interrupter, the current in the superconductor is interrupted. This model is expected to exhibit very low-energy consumption by the superconductor. The authors succeeded in interrupting the current flowing in the superconductor within a half-cycle using a prototype SFCL. An improved SFCL with higher voltage and current ranges was used to carry out current-limiting tests and to investigate the possibility of adapting our SFCL in a power system. The authors also carried out a current-limiting test using a conventional high-speed vacuum circuit breaker (HSVCB) as a new method for realising our concept.

Negative sequence compensation within fundamental positive sequence reference frame for a stiff micro-grid generation in a wind power system using slip ring induction machine

Abstract: An isolated wind power generation scheme using slip ring induction machine (SRIM) is proposed. The proposed scheme maintains constant load voltage and frequency irrespective of the wind speed or load variation. The power circuit consists of two back-to-back connected inverters with a common dc link, where one inverter is directly connected to the rotor side of SRIM and the other inverter is connected to the stator side of the SRIM through LC filter. Developing a negative sequence compensation method to ensure that, even under the presence of unbalanced load, the generator experiences almost balanced three-phase current and most of the unbalanced current is directed through the stator side converter is the focus here. The SRIM controller varies the speed of the generator with variation in the wind speed to extract maximum power. The difference of the generated power and the load power is either stored in or extracted from a battery bank, which is interfaced to the common dc link through a multiphase bidirectional fly-back dc-dc converter. The SRIM control scheme, maximum power point extraction algorithm and the fly-back converter topology are incorporated from available literature. The proposed scheme is both simulated and experimentally verified.

DC traction load flow including AC distribution network

Abstract: In the first part some information and characterisation about an AC distribution network that feeds traction substations and their possible influences on the DC traction load flow are presented. Those influences are investigated and mathematically modelled. To corroborate the mathematical model, an example is presented and their results are confronted with real measurements.

Incorporating control trajectories with the direct torque control scheme of interior permanent magnet synchronous motor drive

Abstract: Interior permanent magnet (IPM) synchronous machines have gained increased attention for applications in electric vehicle, variable speed wind turbine, industrial drives, etc., because of their high torque density, wider speed range and compact construction. The authors present a detailed analysis and modelling of control trajectories and incorporate those trajectories in the direct torque control (DTC) scheme of an IPM synchronous motor drive, for constant-torque and constant-power operating regions. The control trajectories are implemented on a real-time digital signal processor. Because the inputs to the inner torque control loop of DTC are the references for the torque and the amplitude of the stator flux linkage (λs), they are transformed in the T–λs plane, than in the id–iq plane in the indirect control. The modelling and experimental results are presented. Results show very good dynamic and steady-state performances of direct torque controller, incorporating these control trajectories.

A novel sensorless control method for brushless DC motor

Abstract: The authors present the theory and implementation of a novel sensorless control method for the interior permanent magnet (IPM) brushless DC motor (BLDCM). The proposed new sensorless technique can accurately detect the zero-cross point (ZCP) of back electromotive force (BEMF), which is based on a comparison of the terminal voltage of the unconducting phase during the first and second part of a pulse width modulation (PWM) cycle. Compared with the conventional BEMF sensorless approach, the proposed new sensorless method solves the problem of the sensorless BLDCM drives at very low speeds. Experimental results confirm the validity of the new method.

New sensorless commutation technique for brushless DC motors

Abstract: In brushless DC (BLDC) drives commutation is performed by power electronic devices forming part of an inverter bridge. Switching of the power electronic devices has to be synchronised with rotor position. Determination of position, with or without sensors is an essential requirement. The most common sensorless method is based on detection of the zero crossings of back EMF signals. This technique works only above a certain speed. BLDC systems which rely solely on back EMF signals for commutation suffer from relatively poor starting performance characterised by back rotation of up to one hundred and eighty electrical degrees and fluctuations in electromagnetic torque. The aim of this project has been to investigate the possibility of a sensorless technique which is cost effective but with a performance at start-up comparable with that obtained when Hall sensors are used. Initial investigations led to a saliency based method. Theoretical analysis is presented which shows that the method is insensitive to variations in operational parameters such as load current and speed or circuit parameters such as power device voltage drops and winding resistances. Also a starting strategy, relying on saliency related measurements, is proposed which offers starting performance as good as Hall sensor based techniques.

Analysis of a stand-alone three-phase selfexcited induction generator with unbalanced loads using a two-port network model

Abstract: This work proposes an equivalent circuit model for a stand-alone three-phase self-excited induction generator (SEIG) with unbalanced load impedances. The SEIG supplying unbalanced loads is modelled by a two-port network that allows the steady-state performance of the generator to be analysed more easily and more efficiently. The proposed model is validated by experiments on a 0.5 Hp cage-type induction motor operated as a generator.

Analysis of large synchronous machines with axial skew, part 2: inter-bar resistance, skew and losses

Abstract: Assuming knowledge of the flux density distribution due to field and armature windings in the air gap of a large synchronous machine, it is possible to calculate the impact that inter-bar resistance and skew have on the amortisseur winding currents. A theoretical approach to predict amortisseur currents is developed and then the effect on open circuit voltage harmonics is investigated. Developing theory to predict the losses in the core of the machine, the impact of skew and inter-bar resistance on the predicted core loss is also investigated.

Effects of pulse-width-modulated supply voltage on eddy currents in the form-wound stator winding of a cage induction motor

Abstract: The eddy currents of a form-wound multi-conductor stator winding because of the non-sinusoidal supply voltage in a cage induction motor are studied. The time and space dependence of the field, circuit variables and the motion of the rotor are modelled with the time-discretised finite-element analysis. A pulse-width-modulated voltage is used to supply the motor. The eddy-current loss distribution of the stator bars and the total eddy-current losses are studied. The radial distance of the stator bars from the air gap is re-emphasised as a design parameter because of its effect on the losses and the hot spots. The means for minimising the losses and avoiding the local hot spots are studied.

Influence of the copper ion on aging rate of oil-paper insulation in a power transformer

Abstract: To identify the influence of copper ion on the aging rate of oil–paper insulation, accelerated thermal aging experiments on oil–paper insulation were designed and performed at two significant temperatures. A set of characteristic parameters, including dissolved gases, degree of polymerisation, furfural content, water content and acid number of oil, was tested. The experiments' results reveal that copper ion has a different influence on insulation paper and oil during the aging process of the insulation system. On the one hand, it delays the degradation of insulation paper, reducing the ratio of CO2/CO by about 11% at 80°C and by more than 50% at 130°C. On the other hand, copper ion promotes the oxidation of oil and thus accelerates the aging rate.

Modelling and predicting of a switched reluctance motor drive using radial basis function network-based adaptive fuzzy system

Abstract: A novel fuzzy-neural system, which is referred to as a radial basis function network-based adaptive fuzzy system (RBFN-AFS), is presented, to model the switched reluctance machine (SRM) and predict the dynamic performances in an SRM drive system. First, we use an indirect method to measure the phase flux linkage of a 6/4 SRM and then use the co-energy method to calculate phase torque characteristics. Secondly, the RBFN-AFS is designed to learn and train the SRM in the knowledge of the electromagnetic characteristics by using the hierarchically self-organising learning algorithm. This modelling scheme does not require any prior information about the SRM system apart from the input and output signals, and has good capability of generalisation and excellent convergent speed. Then, an RBFN-AFS current-dependent inverse flux linkage model and an RBFN-AFS torque model are used to simulate the various transient and steady-state performances of the 6/4 0.55 kW SRM. The simulation and experimental results based on a DSP drive platform are reported to show that the modelling scheme has good estimation performance under different operation conditions of the SRM.

Analysis of large synchronous machines with axial skew, part 1: flux density and open-circuit voltage harmonics

Abstract: An approach to predict the air-gap flux density and voltage harmonics in large synchronous machines is presented. Combining a small number of solutions of magnetostatic finite-element analysis (FEA) with harmonic analysis, the approach allows designers to rapidly investigate the source of flux density harmonic predictions. The method provides the ability to trace flux density predictions to individual mmf-permeance combinations and applies to machines with both integral and fractional slot windings, with arbitrary skew. Predicted results are compared with that of measured values of open- and short-circuit air-gap flux density in one machine. Open-circuit voltage predictions for five machines are compared with measurements and predictions using commercial FEA.

Direct control algorithm for doubly fed induction generators in weak grids

Abstract: An algorithm utilising direct feedbacks of torque and reactive power for the control of a doubly fed induction generator (DFIG) is proposed. Although the algorithm is based on field-oriented control, the algorithm calculates the feedback and feed-forward variables in the stator reference frame to reduce the number of coordinate transformations. Since the algorithm uses a single-loop approach instead of a cascaded loop design, the tuning of the controller is easier and the response time is faster when compared to those of most existing methods. The internal model control (IMC) design method is used in the proposed scheme to formulate the controller parameters. Measures on the possible voltage variations are included to increase the robustness of the system. Computer simulation and experimental implementation are reported. The results show that the algorithm can control the torque and reactive power independently. Harmonic distortions generated by the system are not noticeable because of the use of constant switching frequency.