Paavo Rasilo

FEM for Directly Coupled Magneto-Mechanical Phenomena in Electrical Machines

A directly coupled magneto-mechanical model is proposed for simulating the effect of the magnetostriction and electromagnetic stress in iron. The model is based on the general balance laws of electromagnetism, mechanics, and continuum thermodynamics. It is implemented in 2-D by using a conforming finite element method for the magnetic vector potential and the displacement field. The method is applied to two different types of induction machines.

Importance of Iron-Loss Modeling in Simulation of Wound-Field Synchronous Machines

Effect of hysteresis, eddy-current and excess-loss modeling on the 2-D field solution of 12.5-MW and 150-kVA wound-field synchronous machines is studied. The study is performed by comparing the differences in the solutions obtained with three different finite element formulations: one with iron losses fully included, one using only single-valued material properties, and one completely neglecting the iron losses from the solution. The electrical operating points, i.e., the terminal currents and powers are found to be only little influenced by the iron-loss model. However, the rotor eddy-current losses are found to be overestimated, if the skin effect of the eddy currents is uncoupled from the solution. Using single-valued material properties instead of hysteretic ones has a smaller effect on the rotor side, but increases the hysteresis losses in the stator. The effects on the total core losses thus depend on their distribution between the stator and the rotor. It is concluded that using single-valued material properties is reasonable in order to improve the computational performance despite the slight overestimation in the computed core losses. However, for accurate modeling of the rotor losses, the skin effect of the eddy currents should be included in the solution.

Segregation of Iron Losses From Rotational Field Measurements and Application to Electrical Machine

This paper presents a methodology for identifying a novel iron loss model and segregating the different loss components from measurements on a single-sheet tester with alternating and rotating fields. The eddy-current losses are first extracted with a 1-D numerical approach and the hysteresis and excess losses are then estimated with an analytical method that allows the separation of alternating and rotational hysteresis as well as excess losses. The elaborated iron loss model can be applied in case of distorted flux density and on a wide range of frequencies. The identified model is further applied in the time-stepping computation of an induction motor in the view of better estimation and segregation of iron losses. The results of no-load simulations at different voltage levels are in good agreement with the measured ones. All of the presented computations and models were validated experimentally.

Anisotropic and Strain-Dependent Model of Magnetostriction in Electrical Steel Sheets

This paper presents an anisotropic and mechanical strain-dependent model of magnetostriction in electrical steel sheets and its application in finite-element computations. The presented model is bidirectional and the data needed for its derivation is extracted solely from unidirectional measurements under mechanical loading. The model has six parameters that describe the magnetic and strain behavior and two parameters that describe the anisotropy. The validation of the model is carried out through measurements and computations on a single-phase transformer-like device. The comparison between computation and measurement results seems to be reasonable regardless of the fact that the magnetic behavior is modeled as single valued, isotropic, and anhysteretic. Original magnetostriction measurements are also presented and the importance of magnetostriction anisotropy in a priori isotropic electrical steel sheets is demonstrated. The model is easy to implement in existing codes and the anisotropic behavior is straightforward to modify according to a specific material.

Modeling of Hysteresis Losses in Ferromagnetic Laminations Under Mechanical Stress

A novel approach for predicting magnetic hysteresis loops and losses in ferromagnetic laminations under mechanical stress is presented. The model is based on combining a Helmholtz free-energy-based anhysteretic magnetoelastic constitutive law to a vector Jiles-Atherton hysteresis model. This paper focuses only on unidirectional and parallel magnetic fields and stresses, albeit the model is developed in a full 3-D configuration in order to account also for strains perpendicular to the loading direction. The model parameters are fitted to magnetization curve measurements under compressive and tensile stresses. Both the hysteresis loops and the losses are modeled accurately for stresses ranging from -50 to 80 MPa.

Modeling the effect of inverter supply on eddy-current losses in synchronous machines

The effect of inverter supply on the eddy-current losses in the laminated core of a synchronous machine is studied. A 2D finite element model including a dynamic model for the eddy currents in the core laminations is applied to predict the machine losses by numerical simulations. A synchronous extruder motor is simulated both with sinusoidal and pulse-width modulated voltage supplies in different operating points and the eddy-current losses both in the stator and the rotor are studied. The rotor additional inverter losses are found to be load-dependent while the stator additional losses remain constant independent of the loading.

Effect of Multilevel Inverter Supply on Core Losses in Magnetic Materials and Electrical Machines

The effect of multilevel inverter supply on power losses in magnetic cores and electrical machines is studied. A dynamic numerical model for the hysteresis, eddy current, and excess losses in a core lamination is first developed. By both measurements and simulations for a ring-core inductor, we demonstrate how increasing the number of inverter voltage levels decreases the iron losses when compared with traditional two-level supply. Although the switching frequency has a significant impact on the iron losses in the case of a traditional two-level inverter, using three or five voltage levels makes the losses almost independent of the switching. Finally, finite-element simulations show that similar reductions are also possible for the core losses of 150-kVA and 12.5-MW wound-field synchronous machines, in which rather low switching frequencies are typically used. Calorimetric loss measurements are also presented for the 150-kVA machine in order to confirm the significant effect of switching frequency on the core losses with two-level inverter supply.

Coupled Magneto-Mechanical Analysis of Iron Sheets Under Biaxial Stress

A novel single sheet tester design is proposed and a directly coupled magneto-mechanical model is used to numerically analyze the behavior of iron sheets under biaxial magneto-mechanical loading applied by the tester device. magneto-mechanically coupled constitutive equations of the material derived using an energy-based approach are integrated into a finite element model of the single sheet tester device, and simulations are performed to solve for the displacement field and the magnetic vector potential in the sample. The obtained numerical results of magnetostriction evolution due to uniaxial stress and stress-induced anisotropies due to permeability variation under different magneto-mechanical loadings are presented. The simulation results are compared with the results published in the literature for the purpose of validation.

Effect of Mechanical Stress on Excess Loss of Electrical Steel Sheets

Effect of mechanical stress on the magnetic loss of electrical steel sheets is analyzed utilizing the statistical loss theory. The focus of this paper is on the variation of the excess loss component with the applied stress and its correlation with the hysteresis loss. The model and its correlation are validated by performing comprehensive measurements at various combinations of induction levels, frequencies, and stresses. It is found that the excess losses can be modeled with sufficient accuracy by their correlation with the hysteresis losses over a wide range of stresses, frequencies, and flux densities.

Analysis of 37-kW Converter-Fed Induction Motor Losses

This paper presents an energy efficiency analysis of a 37-kW standard squirrel-cage induction motor under sinusoidal and nonsinusoidal supply. The motor losses are analyzed using the conventional IEC loss segregation method and also numerically modeled using finite-element simulations. The measured and simulated loss components are compared with three different modulation methods. The overall simulated losses are in good agreement with the measured ones, but there exist differences in the loss components.