Georg Ofner

Validation of a Comprehensive Analytic Noise Computation Method for Induction Machines

The validation of an analytical approach to determine the noise behavior of induction machines is presented. Analytical electromagnetic, structural, and acoustical computations are performed. Data obtained by numerical computations as well as by vibration and noise measurements are compared with the analytical results.

Calculation of load-dependent equivalent circuit parameters of squirrel cage induction motors using time-harmonic FEM

Two methods to determine the equivalent circuit parameters of squirrel cage induction motors (IM) are presented and compared. The first one is based on the time-harmonic finite element method (FEM) simulation of the measurement process (short-circuit and no-load test), yielding an equivalent circuit with constant parameters. In addition, the time-harmonic FEM is applied in the second method to calculate the load-dependent equivalent circuit parameters of squirrel cage IMs. Here, the material properties in a two-dimensional finite element model of the induction machine are linearized in each operating point and the superposition principle for the magnetic flux is applied to define the leakage inductances. Hence, the parameters are calculated for every operating point separately, thus the variations of the parameters due to skin effect in the rotor bars and due to material saturation under arbitrary load condition and rotor speed can be taken into account.

3-D Finite Element Analysis of Additional Eddy Current Losses in Induction Motors

In this paper, an approach based on the three-dimensional finite element analysis is proposed to evaluate additional eddy current losses in induction motors of several MW. Losses produced by induced eddy currents in the end-region of the machine (stator clamping-plates and clamping-fingers) and losses provoked by parasitic leakage fields in the housing and in the metal parts of the cooling-ducts are investigated. Furthermore, excess rotor copper losses produced by higher harmonic fields in the rotor squirrel cage are taken into consideration as well. Comprehensive three-dimensional finite element models have been prepared in order to take these effects precisely into account. A numerical procedure based on nonconforming finite element meshes has been used in order to simplify the modeling procedure and the solving process has been carried out in the frequency domain with the aim of reducing the computational time.

Determination of the starting and operational characteristics of a large squirrel cage induction motor using harmonic and transient FEM

The harmonic and transient finite element method (FEM) are used to determine the starting and operational characteristics of a large induction motor (IM). The starting properties of the initially proposed motor are strongly degraded by parasitic asynchronous crawling tendencies and the motor is unable to achieve the nominal rotor velocity. It is shown in this paper that, when analyzing the IM using the time-harmonic procedure only, these parasitic effects are not detectible. In order to enable the detection of all the effects provoked by higher harmonics the designing process has been expanded by the use of a transient numerical analysis coupled to the equations of motion. The acceleration torque, position and rotational speed of the rotor are recalculated at every time step. This way, the estimation of the newly proposed motors design is realizable, based on the simulation results only. The numerical results are compared with measurements.

Finite-Element-investigations of the transient behaviour of salient pole synchronous machines with different damper end connections

The transient behaviour of synchronous machines is largely determined by the configuration of their damper windings. In this paper, the influence of damper end connections on the induced voltage in the open phase of a machine during a two-phase sudden short circuit is investigated using a commercial Finite Element package. It can be shown that a substantial increase of the resistance between the damper grids of successive poles leads to a significantly higher voltage in the open phase. Moreover, the ratio of the subtransient reactances in the direct and quadrature axes can be estimated from the magnitude of the induced voltage. This method is compared against other approaches known from literature.

Anisotropic Generalization of Vector Preisach Hysteresis Models for Nonoriented Steels

A Preisach-type vector hysteresis model is developed to represent anisotropic material characteristics under static and dynamic conditions. Bi- and uniaxial anisotropy are incorporated by generalizing the input projection of the vector model. Frequency-dependent eddy current and excess effects are considered by an additional field strength component. The model is applied to simulate the magnetic properties of weakly anisotropic nonoriented magnetic steel. The simulation results are successfully validated against measurements under alternating and circular flux carried out on a rotational single-sheet tester.

Three-Dimensional Eddy Current Loss Modeling in Steel Laminations of Skewed Induction Machines

The effects of skewing on the loss behavior of a megawatt rated slip-ring induction machine are studied. The iron loss evaluation is performed by postprocessing the field solution obtained from a multi-slice finite element model. A novel method to compute the three-dimensional eddy current distribution in the steel sheets is employed for the eddy current analysis. The iron loss models are validated against no-load iron loss measurements. Simulation results under load show that skewing leads to a highly non-uniform iron loss distribution along the machine length. The sum of the iron and copper losses increases slightly compared to a machine with straight rotor bars.

Numerical analysis of steady-state operation of three-phase induction machines by an approximate frequency domain technique1

ZusammenfassungEine Vorabschätzung im Frequenzbereich wird angewandt, um die notwendigen Startwerte für die transiente Analyse im Zeitschrittverfahren zu ermitteln. Die transiente Analyse erlaubt es, zeitlich veränderliche Vorgänge, wie induzierte Spannungen, Wirbelströme und Oberwellenmomente, zu erfassen. Mit den Startwerten aus der Frequenzbereichs-Analyse kann der stationäre Betriebszustand deutlich schneller erreicht werden als bei der Annahme aller Startwerte mit null. Die Anwendung der Methode wird an?einem Strom- und Spannungs-gespeisten 2-D-Asynchronmaschinen-Modell sowie an einem 2-D-Mehr-Schichten-Modell eines Käfigläufers gezeigt.SummaryAn approximate frequency domain technique is applied to determine the initial values for the transient time-stepping numerical scheme required to take time dependent phenomena like induced voltages, eddy currents and slot ripples in induction machines into account. Using the initial conditions obtained, the steady-state operation of the motor is obtained much faster than by assuming zero initial values. This procedure is employed to the current-fed and voltage-fed 2-D induction motor models and, additionally, to the 2-D multi-slice FEM model of the 1 MW squirrel cage induction motor.

Steady state modeling of doubly fed induction generators for mega watt class wind turbines

Doubly fed induction generators in conjunction with frequency converters allow the economic construction of large wind power plants with variable speed drives and a rated power output of up to several MW. Compared to fixed speed generators, the advantages of this configuration are high energy yields despite changing wind speeds, reduced negative effects on the grid and the possibility of reactive-power control. Furthermore, only a fraction of the rated power has to be carried by the frequency converter, which reduces its size and cost. It is vital to the manufacturer of these generators, to reliably predict the operating behaviour by using a suitable mathematical model. This paper elaborately describes such a model for the simulation of the steady state behaviour with a special focus on saturation and losses. Methods for modeling the nonlinearities and for solving the resulting nonlinear equations are described. Simulation results are verified by comparing them to measurement values. Results show that strong saturation of the main field increases the necessary rotor current when the wind power plant has to deliver reactive power to the grid while operating under overvoltage conditions.

Numerical simulation of the end-winding deformations in the synchronous machine under symmetrical short-circuit conditions using FEM*

ZusammenfassungIn der Arbeit wird eine neuartige Methode zur Berechnung der Wickelkopfschwingungen unter Berücksichtigung von symmetrischen Kurzschlussbedingungen vorgestellt. Zur Lösung der elektromagnetischen und strukturmechanischen Problemstellung wird ein sequentiell gekoppeltes Simulationsverfahren im Zeitbereich verwendet. Zur Berechnung des zeitlich veränderlichen elektromagnetischen Feldes wird eine T, Φ – Φ – Formulierung der Finiten Elemente herangezogen. Dies ermöglicht die Berücksichtigung von induzierten Wirbelströmen in der Pressplatte, den Zahnfingern und allen relevanten Gehäuseteilen. Die Verwendung von nicht konformen Netzen führt dabei zu einer erheblichen Reduktion des Berechnungsaufwands. In der vorgestellten Simulationsmethode werden in jedem Zeitschritt die berechneten Lorentzkräfte der strukturmechanischen Simulation bereitgestellt. Diese Kräfte werden dann im Zentrum der Wickelkopfspulen als äußere Lasten vorgeschrieben. Zur Berechnung der räumlichen Deformationen dient ein übliches Zeitschrittintegrationsverfahren.SummaryIn this paper the numerical simulation of the end-winding deformations in a synchronous machine under symmetrical short circuit conditions is presented. A sequentially coupled numerical simulation between an electromagnetic and a structural mechanical problem has been carried out. Owing to the short-circuit conditions, both problems are to be computed in the time-domain. Analysis of the time-variant magnetic field has been performed using the T, Φ – Φ formulation for the finite element method (FEM). This way, the influence of the induced eddy currents in the clamping-plates and in the housing of the machine on the magnetic field in the end-region of the generator has been taken into the account. In order to reduce the computational time, the non-conforming finite element (FE) mesh approach has been used to solve the electromagnetic problem. In each time-step the Lorentz forces in the end-winding have been calculated and carried over to the structural-mechanical model. These forces have been applied in the centre of the end-winding bars in order to provoke the transient excitation for the mechanical structure. A time-stepping scheme has been applied to calculate end-winding deformations in each time-step.