Alex Van den Bossche

Autonomous renewable energy conversion system

This paper briefly reviews the need for renewable power generation and describes a medium-power Autonomous Renewable Energy Conversion System (ARECS), integrating conversion of wind and solar energy sources. The objectives of the paper are to extract maximum power from the proposed wind energy conversion scheme and to transfer this power and the power derived by the photovoltaic system in a high efficiency way to a local isolated load. The wind energy conversion operates at variable shaft speed yielding an improved annual energy production over constant speed systems. An induction generator (IG) has been used because of its reduced cost, robustness, absence of separate DC source for excitation, easier dismounting and maintenance. The maximum energy transfer of the wind energy is assured by a simple and reliable control strategy adjusting the stator frequency of the IG so that the power drawn is equal to the peak power production of the wind turbine at any wind speed. The presented control strategy also provides an optimal efficiency operation of the IG by applying a quadratic dependence between the IG terminal voltage and frequency V∼f2. For improving the total system efficiency, high efficiency converters have been designed and implemented. The modular principle of the proposed DC/DC conversion provides the possibility for modifying the system structure depending on different conditions. The configuration of the presented ARECS and the implementation of the proposed control algorithm for optimal power transfer are fully discussed. The stability and dynamic performance as well as the different operation modes of the proposed control and the operation of the converters are illustrated and verified on an experimental prototype.

Rotor-Position Estimation of Switched Reluctance Motors Based on Damped Voltage Resonance

This paper proposes a method to obtain the rotor position of switched reluctance motors (SRMs) by means of voltage measurements. It is shown that the combination of a motor and a power-electronic converter defines a resonant circuit, comprising the motor phase inductances and the parasitic capacitance of converter switches, power cables, and motor phase windings. For salient machines, in general, the associated resonance frequency of the circuit depends on the rotor position. In the position-estimation method, an initial voltage distribution is imposed over the impedances of the resonant circuit after which the circuit is let to oscillate freely. During this phase of free oscillation, the induced voltage over a phase winding exhibits a damped oscillatory behavior, from which position information can be retrieved. An overview is given of the different possibilities to trigger the voltage resonance. It is shown that the proposed position-estimation method has favorable characteristics such as measurement of large-amplitude voltages, robustness against temperature deviations of motor and power semiconductors, very high update rates for the estimated position, and absence of sound and disturbance torque. Experimental results are given for a sensorless commutation scheme of an SRM under small load.

Influence of the Amount of Permanent-Magnet Material in Fractional-Slot Permanent-Magnet Synchronous Machines

The efficiency of permanent-magnet (PM) synchronous machines with outer rotor and concentrated windings is investigated as a function of the mass of magnets used, keeping the power, volume, and mechanical air-gap thickness constant. In order to be useful for electric vehicle motors and wind turbine generators, the efficiency is computed in wide speed and torque ranges, including overload. For a given type and amount of magnets, the geometry of the machine and the efficiency map are computed by analytical models and finite-element models, taken into account the iron loss, copper loss, magnet loss, and pulsewidth-modulation loss. The models are validated by experiments. Furthermore, the demagnetization risk and torque ripple are studied as functions of the mass of magnets in the machine. The effect of the mass of magnets is investigated for several soft magnetic materials, for several combinations of number of poles and number of stator slots, and for both rare earth (NdFeB) magnets and ferrite magnets. It is observed that the amount of PM material can vary in a wide range with a minor influence on the efficiency, torque density, and torque ripple and with a limited demagnetization risk.

Axial-Flux PM Machines With Variable Air Gap

Laminated soft magnetic steel is very often used to manufacture the stator cores of axial-flux PM machines. However, as the magnetic flux typically has main components parallel to the lamination plane, different magnetic flux density levels may occur over the radial direction: High flux densities near the saturation level are found at the inner radius, while the laminations at the outer radius are used inefficiently. To obtain a leveled magnetic flux density, this paper introduces a radially varying air gap: At the inner radius, the air gap is increased, while at the outer radius, the air gap remains unchanged. This results in equal flux densities in the different lamination layers. As the total flux in the stator cores is decreased due to the variable air gap, the permanent-magnet thickness should be increased to compensate for this. The effect of a variable air gap is tested for both a low-grade non-oriented and a high-grade grain-oriented material. For both materials, the redistribution of the magnetic flux due to the variable air gap results in a significant decrease of the iron losses. In the presented prototype machine, the iron losses are reduced up to 8% by introducing a variable air gap. Finally, a prototype machine is constructed using an efficient manufacturing procedure to construct the laminated magnetic stator cores with variable air gap.

Practical Wide Frequency Approach for Calculating Eddy Current Losses in Transformer Windings

A practical method for calculating eddy current losses in transformer windings is reported. The method improves the classical loss presentation by introducing a loss coefficient, called eddy current factor kc. In this paper, eddy current losses in round conductors are discussed. A graphical approximation of kc as a function of wire diameter, frequency, layer number, copper packing factors in the direction parallel and perpendicular to the layer is provided. The graphs are obtained by analytical expressions compared with FEM simulations. To unify the approach for different cases, a reference diameter, apparent and equivalent frequency are defined. A few short examples for applying the method in transformer design are given. The method is applicable for a variety of transformers with different frequencies, wire diameters and conductor fittings. The proposed method is verified by designing several transformers. As an example, a 2.5 kW transformer is fully described. The experiments show good matching with the calculations.

A computationally efficient method to determine iron and magnet losses in VSI-PWM fed axial flux permanent magnet synchronous machines

For electrical machines with a 3-D geometry, such as axial flux permanent magnet machines, the computation of iron and magnet losses in the case of pulsewidth modulation (PWM) supply could be performed by a transient 3-D finite element model (FEM) coupled with an electrical circuit. To reduce the CPU time, in this paper, these losses are computed with acceptable accuracy without using a 3-D transient FEM. The multislice technique is used with a 2-D static FEM, combined with a state space model of the machine. A Preisach hysteresis model is considered to evaluate the iron loss during minor loops. The loss in the electrical steel and in the magnets is evaluated for several PWM frequencies as well as for different segmentations of the magnets.

Ferrite losses of cores with square wave voltage and dc bias

Ferrite material properties may vary depending on the grade and manufacturer: there are differences between batches and also the production process may change in time. On the other hand, in power applications, the usual voltage wave forms are closer to square wave than to sine wave, and moreover they contain a dc bias component that can easily more than double the ferrite losses. Therefore, in loss critical applications it is good to be able to both test and model the losses of the actual ferrite cores under the same conditions as in the actual power application. In this paper, experiments were carried out at different induction levels and dc bias currents. Losses were measured at frequencies of 20, 100, and 500 kHz on a ferrite core with Ferroxcube 3F3 material, a Mn–Zn ferrite. We observe that mainly the lower frequencies are sensitive to dc bias. A model including wide ranges in amplitude, frequency, and also dc bias is proposed. In this model, the losses are separated in three terms: a hysteresis depe...

Characterization and optimization of a permanent magnet synchronous machine

– The first purpose of this paper is to identify – by an inverse problem – the unknown material characteristics in a permanent magnet synchronous machine in order to obtain a numerical model that is a realistic representation of the machine. The second purpose is to optimize the machine geometrically – using the accurate numerical model – for a maximal torque to losses ratio. Using the optimized geometry, a new machine can be manufactured that is more efficient than the original., – A 2D finite element model of the machine is built, using a nonlinear material characteristic that contains three parameters. The parameters are identified by an inverse problem, starting from torque measurements. The validation is based on local BH‐measurements on the stator iron., – Geometrical parameters of the motor are optimized at small load (low‐stator currents) and at full load (high‐stator currents). If the optimization is carried out for a small load, the stator teeth are chosen wider in order to reduce iron loss. An optimization at full load results in a larger copper section so that the copper loss is reduced., – The identification of the material parameters is influenced by the tolerance on the air gap – shown by a sensitivity analysis in the paper – and by 3D effects, which are not taken into account in the 2D model., – The identification of the material parameters guarantees that the numerical model describes the real material properties in the machine, which may be different from the properties given by the manufacturer because of mechanical stress and material degradation., – The optimization is more accurate because the material properties, used in the numerical model, are determined by the solution of an inverse problem that uses measurements on the machine.

Improved Thermal Modelling of Magnetic Components for Power Electronics

Abstract An improved thermal modelling of convection and radiation heat transfer for magnetic components for power electronics is presented. The proposed model of convection heat transfer uses more precise values of the exponents in the presentation of the convection coefficient hc as well as a reduced surface of the component. The model for radiation includes precise values of the emissivity of the component surface and also a reduced ‘envelope’ surface. Accurate thermal measurements of two experimental models (box shape and transformer shape, size EE42) were done for four different kinds of surfaces of the models to derive and verify the proposed models. The improved models can be used in an accurate thermal design of magnetic components.

A Smart Voltage and Current Monitoring System for Three Phase Inverters Using an Android Smartphone Application

In this paper, a new smart voltage and current monitoring system (SVCMS) technique is proposed. It monitors a three phase electrical system using an Arduino platform as a microcontroller to read the voltage and current from sensors and then wirelessly send the measured data to monitor the results using a new Android application. The integrated SVCMS design uses an Arduino Nano V3.0 as the microcontroller to measure the results from three voltage and three current sensors and then send this data, after calculation, to the Android smartphone device of an end user using Bluetooth HC-05. The Arduino Nano V3.0 controller and Bluetooth HC-05 are a cheap microcontroller and wireless device, respectively. The new Android smartphone application that monitors the voltage and current measurements uses the open source MIT App Inventor 2 software. It allows for monitoring some elementary fundamental voltage power quality properties. An effort has been made to investigate what is possible using available off-the-shelf components and open source software.