Influence of an FPGA-based switching angle dithering on acoustics in single-pulse controlled switched reluctance machines

Switched Reluctance Machines (SRM) are considered as a competitive alternative to conventional rotating field machines, especially in terms of production costs and robustness. Single-pulse operation is often employed to control the SRM above base speed, as it yields a beneficial high efficiency. A serious drawback of SRMs is their unpleasant acoustic behavior, which is hard to control in single-pulse operation. Thus, optimization of SRMs' acoustics is an important subject. This work proposes and examines dithered switching angles in order to improve the acoustic behavior of SRMs in single-pulse operation. Different strategies to randomly change the switching angles are evaluated in simulation. The influence on the acoustic behavior is characterized by means of surface normal velocity and the psychoacoustic indicators loudness and sharpness. The results are verified in measurements for a 20kW Switched Reluctance Generator (SRG) using an FPGA-based controller. The results of the proposed strategy yield slightly better acoustic behavior. The sharpness of the SRG's noise is reduced whereas the mean-square surface normal velocity is nearly kept constant. This is due to the intended cover up effect of the dominant peaks in the frequency spectrum when dithered switching angles are employed.