Spectral analysis of the sinusoidal PWM with variable switching frequency for noise reduction in inverter-fed induction motors

The first part of the paper describes the relationship between inverter output voltage and radial magnetic force on the inner surface of the stator, which generates vibrations of the stator and causes the magnetic noise. The theoretical results are verified experimentally. In the second part the authors derive the frequency components of the pulse width modulated (PWM) and pulse frequency modulated (PFM) voltage and of the magnetic noise. This is the basis for a calculable spreading of the spectrum by means of PFM, shown in the third part. The purpose is noise reduction and elimination of selected harmonics to avoid mechanical resonances