Acoustic Noise and Vibration of Permanent Magnet Brushless Machines Having a Fractional Number of Slots per Pole

More than 65% of electrical energy which is consumed in developed countries is used by electrical machines. They are being employed in increasingly varied, and evermore challenging, applications, and are often embedded as an integral part of larger systems. However, the noise which is radiated by electrical machines is recognised as of enormous importance, irrespective of whether they are high power and provide propulsion torque, medium power for closed-loop servo control or low power for drives in consumer products. Indeed, noise standards now exist in many countries, whilst the maximum permitted noise level is usually stipulated in specifications. Thus, it is important that electrical machine manufacturers know, within given limits and at the design stage, the noise spectrum which a machine will produce, and can identify optimum and cost-effective ways of meeting given noise criteria.Although there is no dearth of text-books on acoustics, very few are directed at electrical machines, and those that are only provide designers and engineers with general information regarding the electromagnetic origins of noise, the mechanical behaviour of electrical machine, and their noise radiation characteristics. They are, therefore, of limited value when addressing a specific noise problem.Design engineers need more specific information about, and a greater understanding of, noise-causing forces and their dependence on design features, the response to those forces and the resulting acoustic power. However, the prediction of acoustic noise, as well as the estimation of electromagnetic, mechanical and acoustic parameters, is very difficult, not least because only a very small fraction of the electrical energy is converted into acoustic energy.Improved electrical machine design and magnetic materials, most notably rare-earth magnets, has led to new machine technologies, of which permanent magnet brushless machines are generally regarded as being the most energy efficient and having the highest power density. Competition is also leading to smaller machines per unit output power, and, hence, increased electric and magnetic loadings, relatively thin frames, and higher flux densities, all of which aggravate noise and vibration problems. Further, new concepts for permanent magnet brushless machines are emerging for which techniques for predicting magnetic noise generation either need to be developed further or do not exist. This is the case with the type of machine which is to be researched. It offers significant performance advantages and commercial potential. However, the ratio of its stator slot number to rotor pole number differs from that of conventional permanent magnet brushless machines, and results in many more frequency components in the radial and circumferential vibration forces, different vibration modes, and possibly unacceptable noise. In addition, there are other potentially significant sources of vibratory force, due to magnetic asymmetry, for example. Further, the vibration and acoustic noise behaviour of external rotor topologies will be very different to that of internal rotor machines.Thus, it is proposed to undertake a systematic programme of work which aims to establish reliable vibration and noise and reduction techniques for this class of machine and to demonstrate their low noise capability.