EXPERIMENTAL INVESTIGATIONS ON VIBRATIONS AND ACOUSTIC NOISE OF AN INDUCTION MOTOR
Li, Wen Wen
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Electric motors are important machines as they are widely used in industry. Unfortunately, they produce annoying noise which is a big concern. In order to obtain better understanding of vibrations and noise produced by induction motors, extensive investigations have been conducted on a specially designed test motor. It is well known that electromagnetic forces cause vibrations of the motor, producing radiated acoustic noise. The physical interpretations drawn from the experimental results reported in this thesis are beneficial to correlate the vibrations and the radiated noise. ISO and IEEE standards stipulate the procedures for conducting acoustic measurements so that reliable, reproducible results with specified level of accuracy can be obtained. ISO and IEEE standards describe suitable methods and measuring systems in order to arrange all the investigations in such a way that a systematic analysis can be performed. Relevant discussions with reference to these standards are presented in the thesis. After discussing the noise problems of induction motors encountered in industry and the workplace, the basic definitions and terms used for sound measurements are provided. Brief descriptions of various instruments used for vibration and noise measurements are also given. Before proceeding with the actual experimental investigations, it is essential to summarize the theoretical analyses of electromagnetic fields and electromagnetic forces produced in an induction motor. Harmonic analysis of the stator current, spectral analysis of the induced voltage in stator search-coils, and noise measurements using a sound-level-meter form the preliminary investigations which bridge theoretical analyses and detailed experimental investigations. Vibration measurements are fundamental in providing information on spectral distributions of noise. Mode-shape measurements show the stator deformations under electromagnetic forces, which are created by the three-phase currents. The effects of impressed voltage and the change ofload on vibration levels are also investigated. Noise measurements are conducted in both an ordinary laboratory and an anechoic chamber. The ordinary laboratory is similar to the practical workplace, which is neither a free-field nor a reverberant-room. The measurement results obtained in the laboratory are of practical importance. The anechoic chamber provides the free-field condition, where measurements corresponding to various standards are required. The results obtained in both ordinary laboratory and anechoic chamber are compared to produce useful practical information about the noise level change caused by sound fields. Also, the directional characteristics are carefully studied in the anechoic chamber. The results obtained are very helpful in understanding the nature of the noise radiation patterns of the motor.