The electromagnetic noise due to the PWM switching is generated with narrow band high frequency, which causes communication obstacle and unpleasant high frequency audible noise. As a result of psychological study, it is known that the narrow band noise is more unpleasant than the broad band one.The RPWM technique is a new and effective method to let the inverter fed ac motor drive possess low acoustic noise and mechanical vibration when lower switching frequency is chosen. The chief reason is that the harmonic spectrum of a RPWM inverter can be dispersedly and continuously distributed. Many approaches can be applied to achieve this goal. Roughly, the existing RPWM techniques can be classified into three groups, namely randomized switching frequency (RSF-PWM)[4-6], randomized pulse position (RPP-PWM)[2,7-8] and random switching(RS-PWM). Although many RPWM schemes have been reported, RSF-PWM is the most popular technique. RSF-PWM can be achieved through randomly varying the switching number in a cycle , the slope of the triangular wave or the radian angle of the space vector PWM. Especially, RSF-PWM scheme proposed by Habetler and Divan is very easy and simple to implement. But, in this approach the band limited RPWM generator is implemented using a lookup table, whose contents have been generated by off line. That is, a large quantity of periodic random numbers can be computer generated and stored in some type of ROM.
Thus, to solve the above-mentioned problems, in this study a DSP controlled random switching frequency PWM inverter drive system is implemented. Real-time RPWM along with the speed control was achieved by high speed DSP TMS320C31. Since RPWM is being generated using a real time DSP, a lookup table is no need and the band unlimited RPWM generator is implemented. Using randomly changed switching frequency of the inverter, the voltage and current harmonics are spread to a wide band area. Also, the power spectrum of the audible acoustic switching noise was spread to create a more appealing, less annoying sound.
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