Noise Reduction Strategies Of Pmsm System Based On The Spread Spectrum Principle

In the conventional PMSM (permanent-magnet synchronism motor) drive systems, the fixed switching frequency in inverters introduces massive harmonics highly concentrated near switching frequency and its multiples to the acoustic noise spectrum, which causes annoying acoustic noise, even doing harms to human's health especially the motor operators. Although the noise could be reduced by applying comparatively higher switching frequency out of human's listening, switching loss of power electronics will become significant. Thus, noise reduction technique based on spread spectrum has drawn more and more attention. In this paper, periodic and random frequency modulation were investigated to explore their effects in terms of spectrum spreading and noise reduction.Firstly, based on the analysis about the principle of fixed frequency SVPWM(Space Vector Pulse Width Modulation), as well as its implementation in the digital control system, the output signals of the inverter under fixed switching frequency were analyzed, with the spectral distribution obtained. Moreover, simulation results by Simulink and experimental results in the DSP based plat verified the theory.Secondly, three periodic frequency modulation patterns, including the triangular, sawtooth and sinusoidal frequency modulation, were proposed to spread the spectrum of switching noise existing in PMSM system under fixed switching frequency modulation. In order to investigate the noise reduction and spectrum spreading effects under different ratios of electrical period and modulation period, we simulated and experimented under 5 ratios, whose results revealed that the bigger the ratio, the better the effects. Additionally, spectrum spreading effect under different phase angulars between switching frequency function and line voltage with the same periodic ratio were also researched. Simulation and experiment results illuminated that periodic frequency modulation could spread the harmonic energy to wider frequency ranges effectively with similar distribution under three patterns with just some differences in continuity.Besides, the spectrum under periodic frequency modulation is not as continuous as we expected, so we did some research on the random frequency modulation. In order to get the random numbers with good distribution performance, Mersenne Twister generators were used. And the final simulation and experimental results told that the random frequency modulation had the best effect compared with three periodic frequency modulation patterns mentioned before.Finally, the utilization of frequency modulation technique makes the harmonic components of output voltage and current of the inverter complex, which will cause the variation of rotor high frequency eddy current loss and inverter loss. Thus, rotor eddy current loss was calculated under different frequencies using finite element method, and than the eddy current loss and inverter loss under different frequency modulation patterns were calculated and compared in this paper. Calculation results indicated that the eddy current loss is higher under both the periodic and random frequency modulation compared with fixed frequency modulation. While, the loss of the inverters keeps almost the same.