Setareh Gorji Ghalamestani

Magnetostriction Measurement by Using Dual Heterodyne Laser Interferometers

Electrical machines and transformers have a core built out of laminations of ferromagnetic materials. A portion of the vibrations and noise of these devices is due to magnetic forces and magnetostriction arising from the magnetic core. Magnetic forces are well known, and analytical methods are extensively used to calculate them. Magnetostriction can be defined as the deformation of the ferromagnetic material in the presence of a magnetic field. Unlike magnetic forces, magnetostriction shows a rather complex behavior. It varies for every material, and it depends on the applied magnetic field and external pressure. Therefore, magnetostrictive behavior of every material needs to be determined experimentally by means of strain measurements. Strain gauge measurement techniques have been used before at the Electrical Energy Laboratory (EELAB), Ghent University, Ghent, Belgium. In this paper, a new measurement method using dual heterodyne laser interferometers is proposed to overcome the drawbacks of the old method. The proposed measurement setup and the working principles are explained. The possibility to apply both techniques on one and the same sample can also reveal some interesting results about the quality of both techniques.

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.

Magnetostrictive deformation of a transformer: A comparison between calculation and measurement

Magnetostriction, which refers to the deformation of the core material of transformers and electrical machines, is a main contributor to the total noise. In this work, a technique is proposed to calculate the magnetostrictive deformations of the core of the aforementioned devices. This technique is based on finite element approach with an artificial neural network model of the magnetostrictive behaviour of the core material. The strain measurements are carried out by using a dual heterodyne laser interferometer setup. The proposed technique is then applied to a single-phase test transformer core. As a validation, the vibration of the magnetised core of the same setup is measured by using a laser scanning vibrometer.

Magnetostrictive vibrations model of a three-phase transformer core and the contribution of the fifth harmonic in the grid voltage

The effect of the fifth harmonic in the grid voltage (with fundamental frequency of 50 Hz) on the vibrations of a three-phase transformer core is computed, since such harmonic has the largest contribution in the European grid voltage. The computational method is a two-dimensional (2D) finite element technique. The modal vibrations under various magnetisations (viz with different fifth harmonic components) are compared with those obtained under a purely sinusoidal magnetisation and showed that the variations for the 100 Hz harmonic of the vibrations are small. However, the 200 Hz harmonic showed a significant increase when a fifth harmonic was present on the applied voltage. In fact, the presence of a fundamental component with 50 Hz frequency and a fifth harmonic on the magnetisation signal generates a 200 Hz harmonic on the magnetostriction strains (and the magnetic forces), and thus this harmonic increases significantly.

Magnetostriction and the Advantages of Using Noncontact Measurements

Magnetic noise in electrical machines and transformers are a large portion of the total noise of the device. Part of this magnetic noise is caused by the deformation of the ferromagnetic laminations due to the magnetic field. This effect is called magnetostriction, and it strongly depends on the applied magnetic field, the material properties and external pressure to the material. A strain gauge measurement setup has been applied before to measure the magnetostrictive behaviour of ferromagnetic materials. The results obtained by this setup suffered from some limitations such as the need to filter high‐frequency harmonics. Also the measurement results for excitation below 0.8 T were not easily distinguished from the present noise. Therefore, a new setup using heterodyne laser interferometers has been built. With this new setup, on the contrary to the strain gauge setup, the sample preparation is simple. This new setup and the gradual improvements toward the optimal performance of the setup are presented in...

Magnetic forces and magnetostriction in rotating electrical machines

The deformation of magnetic origin in ferromagnetic cores is due to both magnetic forces and magnetostricion. The deformation of closed magnetic circuits, such as transformers, is commonly analysed by considering magnetostriction only, while in systems with an airgap, such as rotating electrical machines, only magnetic forces are considered for the analysis of vibrations and noise of magnetic origin. Nevertheless, both phenomena, magnetic forces and magnetostriction are present in both types of magnetic cores. In this paper, both magnetic forces and magnetostriction and their resulting stresses and strains in rotating electrical machines are illustrated by means of finite element computations.

Macroscopic Description of the Magnetostrictive Behavior of Electrical Steel in the Presence of High-Order Harmonics in the Magnetization

Magnetostrictive behavior of the electrical steel cores of transformers and electrical machines has been studied under well-known excitation waveforms, e.g., purely sinusoidal. However, the magnetization may contain higher harmonics due to the grid harmonics for the application in transformers or due to pulse width modulation excitation in the case of induction machines. We have reported the presence of a single low-order harmonic in magnetization, but in reality, a superposition of several low- and high-order harmonics is present, which needs to be studied. Thus, in this paper, the magnetostrictive behavior of electrical steel is measured under a sinusoidal magnetization with high-order harmonics. A macroscopic description of the magnetostrictive behavior is presented afterward.