Antti Lehikoinen

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.

Monte Carlo Analysis of Circulating Currents in Random-Wound Electrical Machines

Electrical machines with stranded random windings often suffer from considerable circulating current losses. These losses have been poorly studied because of the difficulty and computational cost of modeling stranded windings, and the stochastic nature of the problem due to the uncertain positions of the strands. This paper proposes two methods to model random stranded windings of arbitrary complexity. First, a circuit model considering the entire main flux path is presented, and some practical implementation considerations are discussed. Second, a computationally efficient finite-element approach based on non-conforming meshing is presented. Finally, a method is proposed to model the random packing process of strands within a slot, without any remeshing or inductance recalculation required. The proposed methods are then compared with special no-rotor measurement data of a large number of high-speed induction machines, and good agreement is observed.

Efficient Finite-Element Computation of Circulating Currents in Thin Parallel Strands

Electrical machines often utilize stranded parallel conductors to reduce the skin-effect losses. This practice can lead to uneven total current distribution among the strands, increasing the resistive losses. Direct finite-element (FE) analysis of circulating current problems can be computationally costly due to the large number of nodal unknowns in the FE mesh in the conductor domains. Methods to reduce the computational burden exist for special problems only. This paper proposes two efficient FE formulations to solve the circulating current problems with arbitrary winding configurations. According to simulations, the proposed methods yield reasonably accurate results significantly faster than the traditional brute-force approach.

Homogenization Technique for Axially Laminated Rotors of Synchronous Reluctance Machines

In this paper, we propose a homogenization technique to model the axially laminated rotor of synchronous reluctance machines. Thus, the computational effort can be significantly reduced by replacing the laminated parts of the rotor by some equivalent anisotropic media. The proposed method is validated in terms of flux density and electromagnetic torque. Some small discrepancies can be noticed due to the air-gap fluctuations caused by the steel sheets and the interlaminar insulation sheets of the rotor. With the test machine, the homogenization method reduces by the number of elements to one-fourth and the computation time to one-third.

Domain Decomposition Approach for Efficient Time-Domain Finite-Element Computation of Winding Losses in Electrical Machines

Finite-element (FE) analysis of winding losses in electrical machines can be computationally uneconomical. Computationally lighter methods often place restrictions on the winding configuration or have been used for time-harmonic problems only. This paper proposes a domain decomposition-type approach for solving this problem. The slots of the machine are modeled by their impulse response functions and coupled together with the rest of the problem. The method places no restrictions on the winding and naturally includes all resistive ac loss components. The method is then evaluated on a 500-kW induction motor. According to the simulations, the method yields precise results 70–100 faster compared with the established FE approach.

Reduced basis finite element modelling of electrical machines with multi-conductor windings

Finite element analysis of electrical machines with multi-conductor windings can be computationally costly. This paper proposes a solution to this problem, using a reduced basis approach. The field-circuit problem is first solved in a single slot only, with a set of different boundary conditions. These pre-computed solutions are then used as shape functions to approximate the solution in all slots of the full problem. A polynomial interpolation method is also proposed for coupling the slot domains with the rest of the geometry, even if the geometries or meshes do not fully conform on the boundary. The method is evaluated on several test problems. According to the simulations, accurate solutions are obtained. Furthermore, a speed-up factor of 30 is observed when analysing a six-slot phase belt of a high-speed induction machine.

Spectral Stochastic Finite Element Method for Electromagnetic Problems with Random Geometry

Abstract In electromagnetic problems, the problem geometry may not always be exactly known. One example of such a case is a rotating machine with random-wound windings. While spectral stochastic finite element methods have been used to solve statistical electromagnetic problems such as this, their use has been mainly limited to problems with uncertainties in material parameters only. This paper presents a simple method to solve both static and time-harmonic magnetic field problems with source currents in random positions. By using an indicator function, the geometric uncertainties are effectively reduced to material uncertainties, and the problem can be solved using the established spectral stochastic procedures. The proposed method is used to solve a demonstrative single-conductor problem, and the results are compared to the Monte Carlo method. Based on these simulations, the method appears to yield accurate mean values and variances both for the vector potential and current, converging close to the results obtained by time-consuming Monte Carlo analysis. However, further study may be needed to use the method for more complicated multi-conductor problems and to reduce the sensitivity of the method on the mesh used.

Higher-order finite element modeling of material degradation due to cutting

Numerical modeling of energy efficient electrical machines requires accurate and fast calculation of losses. One such loss component is core losses related to magnetic material degradation due to the cutting of electrical sheets. This paper analyzes the application of higher order finite elements for precise and computationally efficient modeling of these cutting related losses. The accuracy of higher order elements is compared with highly dense first order elements. Finally, a time-harmonic model of an induction machine with presented higher order elements is simulated and effect of cutting on performance parameters has been analyzed.

Reduced Basis Finite Element Modelling of Electrical Machines with Multi-Conductor Windings

Finite element analysis of electrical machines with multi-conductor windings can be computationally costly. This paper proposes a solution to this problem, using a reduced basis approach. The field-circuit problem is first solved in a single slot only, with a set of different boundary conditions. These pre-computed solutions are then used as shape functions to approximate the solution in all slots of the full problem. A polynomial interpolation method is also proposed for coupling the slot domains with the rest of the geometry, even if the geometries or meshes do not fully conform on the boundary. The method is evaluated on several test problems. According to the simulations, accurate solutions are obtained. Furthermore, a speed-up factor of 30 is observed when analysing a six-slot phase belt of a high-speed induction machine.

Reduced Basis Finite Element Modeling of Electrical Machines with Multiconductor Windings

Finite element (FE) analysis of electrical machines with multiconductor windings can be computationally costly. This paper proposes a solution to this problem, using a reduced basis approach. The field-circuit problem is first solved in a single slot only with a set of different boundary conditions. These precomputed solutions are then used as shape functions to approximate the solution in all slots of the full problem. A polynomial interpolation method is also proposed for coupling the slot domains with the rest of the geometry, even if the geometries or meshes do not fully conform on the boundary. The method is evaluated on several test problems both in the frequency and time domains. According to the simulations, accurate solutions are obtained, 54–90 times faster as compared with the established FE approach.