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.Peer reviewe

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 1D 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 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 presented computations and models are validated experimentally too.Peer reviewe

Simple Electromagnetic Motor Model for Torsional Analysis of Variable Speed Drives with an Induction Motor

Torsional vibrations must be considered in the design of all high-power drive-trains including an induction motor. Electromagnetic (EM) field in the air gap of an induction motor generates additional magnetic stiffness and damping between the rotor and stator. The inclusion of these magnetic effects is limited by the availability of simple and portable motor models. The main aim of this paper is to introduce a motor model including the speed and torque variation. The presented model is based on the linearization of the common space-vector models of induction motors. The parameters of this model are identified for the rated operating condition. This motor model can be extended to include variable speed and torque operation. The numerical results demonstrate that this model describes accurately the magnetic effects over the large speed and torque range. In addition, the numerical results demonstrate the significance of magnetic stiffness and damping in variable speed motor-driven compressors with a soft coupling

Optimum supply for an inverter-fed cage induction motor at different load conditions

The effects of power supply on the energy efficiency of a form-wound cage induction motor are studied when the motor operates under light loads. The cage induction motor is modelled with the space and time discretized finite-element analysis. The resistive losses are taken into account accurately by modelling eddy currents in the form-wound multi-conductor stator winding and the rotor cage. The core losses are considered with conventional empirical equations. A pulse-width-modulated (PWM) voltage is used to supply the motor. The fundamental harmonic (FH) terminal voltage is decreased from its rated value and the slip is adjusted to achieve a particular load condition. The variation of the total electromagnetic as well as the stator resistive losses are analyzed to find the optimum supply.reviewe

Reduced Basis Finite Element Modeling of Electrical Machines with Multiconductor Windings

Finite element analysis of electrical machines withmulti-conductor windings can be computationally costly. Thispaper proposes a solution to this problem, using a reducedbasis approach. The field-circuit problem is first solved in asingle slot only, with a set of different boundary conditions.These pre-computed solutions are then used as shape functionsto approximate the solution in all slots of the full problem. Apolynomial interpolation method is also proposed for couplingthe slot domains with the rest of the geometry, even if thegeometries or meshes do not fully conform on the boundary.The method is evaluated on several test problems both inthe frequency- and time-domains. According to the simulations,accurate solutions are obtained, 54-90 times faster compared tothe established finite element approach.Peer reviewe

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

Finite element analysis of electrical machines withmulti-conductor windings can be computationally costly. Thispaper proposes a solution to this problem, using a reducedbasis approach. The field-circuit problem is first solved in asingle slot only, with a set of different boundary conditions.These pre-computed solutions are then used as shape functionsto approximate the solution in all slots of the full problem. Apolynomial interpolation method is also proposed for couplingthe slot domains with the rest of the geometry, even if thegeometries or meshes do not fully conform on the boundary. The method is evaluated on several test problems. Accordingto the simulations, accurate solutions are obtained. Furthermore,a speed-up factor of 30 is observed when analysing asix-slot phase belt of a high-speed induction machine.Peer reviewe

Study of thermal stresses developed during a fatigue test on an electrical motor rotor cage

© 2018 Structural defects in the rotor cage of large electrical machines significantly impact their expected operational lifetime. This work presents the results of simulating the thermal stresses developed in a rotor cage during a fatigue test in which a bar breakage was achieved. A combined model featuring electrical, thermal and mechanical stages as well as three different meshes reflecting a progressing narrowing of one of the bars in its junction to the end ring are used for this purpose. The experimentally implemented startup and plug stopping transients are reproduced as well as, for comparison, the stall operation. The resulting stress levels are in agreement with the progression of the damage and concur with the stator measurements. Based on the analysis of the simulated rotor magnitudes, a strategy to diminish the thermal stresses in a damaged cage is proposed

Modeling of Losses Due to Inter-Laminar Short-Circuit Currents in Lamination Stacks

The cores of electrical machines are generally punched and laminated to reduce the eddy current losses. These manufacturing processes such as punching and cutting deform the electrical sheets and deteriorate its magnetic properties. Burrs are formed due to plastic deformation of electrical sheets. Burr formed due to punching on the edges of laminated sheets impairs the insulation of adjacent sheet and make random galvanic contacts during the pressing of stacked sheets. The effect of circulating current occurs if the burrs occur on the opposite edges of the stacks of laminated sheets and incase of bolted or wielded sheets, induced current return through it. This induced current causes the additional losses in electrical machine. The existence of surface current on the boundary between two insulated regions causes discontinuity of tangential component of magnetic field. Hence, based on this principle, the boundary layer model was developed to study the additional losses due to galvanic contacts formed by burred edges. The boundary layer model was then coupled with 2-D finite element vector potential formulation and compared with fine mesh layer model. Fine mesh layer model consists of finely space discretized 950028 second order triangular elements. The losses were computed from two models and were obtained similar at 50 Hz. The developed boundary layer model can be further used in electrical machines to study additional losses due to galvanic contacts at the edges of stator cores.Peer reviewe

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

Finite element analysis of winding losses in electrical machines can be computationally uneconomical. Computationally lightermethods often place restrictions on the winding configuration or have been used for time-harmonic problems only. This paperproposes a domain decomposition type approach for solving this problem. The slots of the machine are modelled by their impulseresponse functions and coupled together with the rest of the problem. The method places no restrictions on the winding and naturallyincludes 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 to the established finite element approach.Peer reviewe