Ari Haavisto

End-Winding Vibrations Caused by Steady-State Magnetic Forces in an Induction Machine

We conducted a 3-D electromagnetic analysis coupled with a 3-D mechanical analysis to analyze end-winding vibrations and deformation in an induction machine caused by steady-state magnetic forces on the end winding. Both the analyses were based on the finite-element method. The electromagnetic analysis was used to calculate magnetic forces. During the mechanical analysis, complex support structures in the end region were simplified. We first updated and validated the mechanical model according to a modal model obtained from a modal test, and afterward analyzed deformation, vibrations, and stresses. According to the analysis, the shape of the rotary dynamic deformation of the end winding caused by dynamic forces is similar to the most excitable mode shape though the natural frequency of that mode is much higher than the excitation frequency. The static deformation caused by static forces tends to expand the coil ends outward. Under both types of deformation, the nose portion of the coil ends experiences larger displacement, but von Mises stresses are larger mainly in the knuckle portion.

Particle Filter-Based Estimation of Instantaneous Frequency for the Diagnosis of Electrical Asymmetries in Induction Machines

Fault diagnosis of induction machines operating under variable load conditions is still an unsolved matter. Under those regimes, the application of conventional diagnostic techniques is not suitable, since they are adapted to the analysis of stationary quantities. In this context, modern transient-based methodologies become very appropriate. This paper improves a technique, based on the application of Wigner-Ville distribution as time-frequency decomposition tool, using a particle filtering method as feature extraction procedure, to diagnose and quantify electrical asymmetries in induction machines, such as wound-rotor induction generators used in wind farms. The combination of both tools allows tracking several variable frequency harmonics simultaneously and computing their energy with high accuracy, yielding magnitudes and values similar to those obtained by the application of the fast Fourier transform in stationary operation. The experimental results show the validity of the approach for rapid speed variations, independently of any speed sensor.

Analysis of Eddy-Current Loss in End Shield and Frame of a Large Induction Machine

This paper proposes an effective method of analyzing the eddy-current loss within the end shield and frame of an electric machine. The standard impedance boundary condition is applied to the conducting surfaces of complete 3-D models, and the models are solved by time-harmonic finite-element analyses. Measurement of the temperature rise is completed as a validation of the proposed method. The effect of the distances between the conducting surfaces and coil ends is studied by computing the eddy-current loss for a series of 3-D models having different distances. In addition, simplified 2-D models are used to study the influence of material nonlinearity. In brief, the proposed method is feasible in computing the eddy-current loss. The eddy-current loss within the regions close to the coil ends is larger than the other regions in the end shield and frame, but the total loss is quite small, compared, for instance, with the copper loss within the stator winding. Furthermore, the farther the end shield and frame are from the coil ends, the smaller the eddy-current loss is. Finally, it is found that the eddy-current loss is larger in the nonlinear case than in the linear case.

Diagnosis of Induction Motors Under Varying Speed Operation by Principal Slot Harmonic Tracking

A detailed study on the energy content of a principal slot harmonic (PSH) in an induction motor operating at variable slip is carried out. The machine is tested under different faulty conditions, namely, broken rotor bar, mixed eccentricity, and interturn short circuit. The use of a combined time-frequency analysis and particle filtering feature extraction procedure allows tracking the evolution of a PSH under different load profiles and integrating its energy. The proposed fault indicator provides values that are equivalent with the traditional one for stationary operation even in the case of pulsating load. The results show that the energy in the PSH reflects the overall state of the machine under these conditions. Furthermore, procedures to discern the different faults in real applications are proposed.

Axial Flux and Eddy-Current Loss in Active Region of a Large-Sized Squirrel-Cage Induction Motor

This paper analyzes the axial flux and the corresponding eddy currents inside the laminated cores of a large-sized squirrel-cage induction motor running under the steady-state operation in order for possible optimum design of large-sized induction motors used in industry. The analysis was based on a complete 3-D numerical model. In the model, the laminated cores were replaced by anisotropic solid bodies, and the eddy currents inside the end portion of the stator frame were modeled with the standard impedance boundary condition. A time-harmonic finite-element analysis based on the magnetic vector potential-electric scalar potential formulation was performed. According to the results, under steady-state no-load, the axial flux, caused mainly by the air-gap fringing flux and the end-winding leakage flux, appears in the end portion of the cores, and decays rapidly toward the middle of the cores. The corresponding planar eddy currents inside the laminations of the stator core are mainly concentrated near the edges of the stator teeth. The eddy-current loss is small, but the distribution of the loss over the laminations is definitely non-uniform.

Validation of a Time-Harmonic Numerical Model for Solving Magnetic Field in End Region of a Radial-Flux Machine

We present an analysis of the magnetic field in the end region of a radial-flux rotating machine. In numerical simulations, we used three familiar boundary conditions to replace the modeling of the end shields and frame. We made measurements for comparison, and the simulation results were quite consistent with the measurements. Our analysis shows that the eddy current in the end shields and frame influences the magnetic field in the end region slightly and that the use of a homogeneous Neumann boundary condition or a standard impedance boundary condition (SIBC) to replace the end shields and frame can solve the magnetic field in the end region more accurately than a homogeneous Dirichlet boundary condition. Validation by the measurements demonstrates that 3-D current-driven time-harmonic model with suitable boundary conditions can be used to solve the magnetic field in the end region quite accurately.

Analytical model for magnetic anisotropy of non-oriented steel sheets

Purpose – Recent investigations on magnetic properties of non-oriented (NO) steel sheets enhance the comprehension of the magnetic anisotropy behaviour of widely employed electrical sheets. The concept of energy/coenergy density can be employed to model these magnetic properties. However, it usually presents an implicit form which requires an iterative process. The purpose of this paper is to develop an analytical model to consider these magnetic properties with an explicit formulation in order to ease the computations. Design/methodology/approach – From rotational measurements, the anhysteretic curves are interpolated in order to extract the magnetic energy density for different directions and amplitudes of the magnetic flux density. Furthermore, the analytical representation of this energy is suggested based on statistical distribution which aims to minimize the intrinsic energy of the material. The model is finally validated by comparing measured and computed values of the magnetic field strength. Find...

Evolution of high order fault harmonics during a bar breakage with compensation

Under certain circumstances the widespread Motor Current Signature Analysis focused on low frequency components used to detect rotor bar breakages in induction motors cannot provide an adequate indication of this defect. This work studies the behavior of high order components during the last stage of a fatigue test designed to reproduce a natural bar breakage, in which a compensation of the indicators generally used to diagnose the fault is observed. Some of these higher frequency harmonics remain unaffected, accurately depicting the entire bar breakage both in transient and stationary operation, which may be of application for the detection of a double fault in the rotor cage.

Diagnosis of induction machines under varying speed operation by Principal Slot Harmonic tracking

A detailed study of a Principal Slot Harmonic (PSH) energy of an induction motor directly connected to the grid and operating under variable load under different faulty conditions is carried out. Broken rotor bars, mixed eccentricity and interturn short circuit faults are considered in the paper. A combined time-frequency distribution and particle filtering post processing approach is used in the work for PSH tracking even for the case of severe pulsating load operation. The proposed fault indicator provides values that are comparable with the traditional one for stationary operation. The results show that the energy in the PSH reflects the overall state of the machine under this kind of operation. Furthermore, its changing pattern is studied as a mean to discriminate between different faults.

Measurement of torque harmonics of a cage induction machine under rotor eccentricity

Eccentricity creates an unbalanced magnetic pull in the rotor of the cage induction machine which plays a vital role in the rotor dynamic stability. In this case, it is desirable to know how different quantities of the machine behave, for example torque. Numerical analysis shows an increase in the average torque of the machine with eccentricity. The amplitudes of some of the harmonic components of the torque are also affected. In such case, the measurement of the torque waveform could give a good insight to the behavior of the torque and its harmonics. Here, a novel measurement rig is presented.