The measurement and calculation of acoustic noise radiated by small electrical machines
With the increasing noise level present in a mechanised society today it is necessary to reduce unwanted sound in any economic way possible. The work done for this thesis is part of a research project, at Queen Mary College into the noise produced by electric machines. By gaining a thorough knowledge of the way in which noise is produced in machines it will be possible to calculate and minimise the noise output of machines at the design stages The first requirement of the project is that the noise produced, by a machine should be measured accurately. Methods are developed for measuring, in an anechoic chamber, the acoustic power radiated by a machine. Also studies of the vibration and acoustic radiation characteristics of several machines are made. The second requirement is to identify the sources of the noise components in a machine. This is done by calculating the resonant frequencies of the mechanical parts of the machine and then analysing the noise with the machine running at several different speeds. Examples, of the application of the methods to the identification of noise components in small induction machines are given. The third requirement is the accurate calculation of components of the noise and comparison of the magnitudes with measured values. A method for calculating the noise produced by electromagnetic sources is given. The method is basically applicable to all types of machine although certain parts of the calculation must be modified in some cases. The calculation is divided into three parts: the calculation of the forces at the iron surfaces of the air gap, the mechanical response and the acoustic radiation characteristics. Each part is considered separately. The mechanical response, radiation characteristics and overall calculation are compared with measured results and good agreement is obtained.