4744 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009 Effect of Magnetostriction Anisotropy in Nonoriented Electrical Steels on Deformation of Induction Motor Stator Cores Sakda Somkun, Anthony J. Moses, and Philip I. Anderson Wolfson Centre for Magnetics, School of Engineering, Cardiff University, Cardiff CF24 3AA, United Kingdom Magnetostriction in nonoriented (NO) electrical steels is a harmful source of vibration and acoustic noise in electrical machines. It is far more anisotropic than specific power loss, which results in more complexity in prediction of the vibration and acoustic noise. An inves- tigation of the anisotropy in magnetostriction and mechanical elastic properties of a NO steel is presented in this paper. Measurements of deformation in the stator teeth of an induction motor model core were conducted in order to study the effect of the magnetostriction anisotropy. Strain due to Maxwell forces in the air gap was calculated and compared with the measured deformation. Measurement re- sults indicate that anisotropic magnetostriction has a great impact on unsymmetrical deformation of the stator teeth of induction motor cores. Index Terms—Acoustic noise, anisotropy, deformation, induction motors, magnetostriction, nonoriented (NO) electrical steels, vibra- tions. I. INTRODUCTION V IBRATION and acoustic noise of induction motors are in- creasingly of concern. The main sources of the vibration and acoustic noise are Maxwell and magnetostrictive forces [1]. The Maxwell force is principally present in the air gap, where there is a discontinuity in relative permeability of the materials [2]. The magnetostrictive force causes the dimensions of a mag- netic material to change when the material is magnetized. The magnetostrictive force has been calculated to be responsible for up to 50% of the total electromagnetic force [3]. Hence, the magnetostriction is an important source of the core vibra- tion and acoustic noise. Nonoriented (NO) electrical steel commonly used in elec- trical machines is often considered to be magnetically and me- chanically isotropic. However, there is still inherent anisotropy in its characteristics and resulting specific power losses measured along different directions in the plane of laminations, causing a significant increase in flux density harmonics and overall core losses [4], [5]. It has been reported that magnetome- chanically coupled properties such as magnetostriction in NO steel can be far more anisotropic than its specific power losses [6]. The variation of peak magnetostriction at the same flux den- sity magnetized along different direction can be up to 400%. Measured magnetostriction in electrical steels has been fed into a finite element model (FEM) for calculating vibration and deformation in electrical machines [7], [8] although the anisotropy in magnetostriction was not taken into account. In this study, investigations of magnetostriction anisotropy in a NO electrical steel were conducted. The effect of magne- tostriction anisotropy on core deformation was examined using measurements of localized strain in stator teeth of an induction motor model core, where the NO steel was subjected to alter- nating flux at different angles with respect to the rolling di- rection (RD) and the Maxwell force in the air gap. Strain due Manuscript received March 07, 2009. Current version published September 18, 2009. Corresponding author: S. Somkun (e-mail: somkuns@cardiff.ac.uk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2022320 to the Maxwell force and sketches of core deformation taking mechanical elasticity into account are also presented. II. EXPERIMENTAL APPROACH A. Study of Magnetostriction Anisotropy Epstein strips of M400-50A NO steel, 305 mm long, 30 mm wide, were cut at 10 intervals from the RD to the transverse direction (TD). Three strips cut at each angle were tested. They were magnetized singly under sinusoidal induction at peak flux density from 1.00 to 1.7 0.005 T, 50 Hz. Magnetostric- tion was obtained by double integral of the output signal of an accelerometer fixed at the free end of the strip [9]. B. Mechanical Elasticity Measurement The same grade and size of the material used in the magne- tostriction measurement were put in a tensile stress machine. One sample cut at each angle was tested. Tensile forces from 1.50 to 4.00 0.02 kN were applied to each strip causing 100.0 1.3 to 266.7 3.3 MPa of tension. Longitudinal and trans- verse strains were measured by a Rosette resistant strain gauge attached at the center of each sample. The modulus of elas- ticity along the longitudinal direction of the strip was calcu- lated by the slope between the tensile stress and the longitudinal strain. Poisson’s ratio of the material was calculated as the ratio between the transverse and longitudinal strain. Four mea- surements were carried out per sample. C. Measurement of Localized Strain in a Stack of Induction Motor Stator Laminations Twenty stator and rotor laminations of M400-50A NO steel were also used to construct an induction motor model core [10]. A four-pole double-layer stator winding with 120 fractional pitch was selected. No rotor windings were used. The 36 stator teeth are each 10 apart. Foil-type resistance strain gauges were attached to measure localized strain in the stator teeth cov- ering from 0 to 90 as shown in Fig. 1. Core dimensions are listed in Table I. 0018-9464/$26.00 © 2009 IEEE