Shaping the spectra of the line-to-line voltage using signal injection in the common mode voltage

A drawback of pulse width modulation in electrical drives is the high harmonic content of the line to line voltages, which gives rise to electro-magnetic interference and acoustic noise. By injection of a signal into the common mode voltage, the fundamental is not affected, but new frequency components are introduced in the line to line voltage spectrum. This paper introduces a new analytical method for estimation of the line to line voltage spectrum, where sinusoidal signal is injected into the common mode voltage. Based on the analytical calculations, also a new fixed carrier frequency quasi-random PWM method is proposed. The advantage of the proposed random PWM method is that the acoustical noise generated by the drive can be improved, without exciting the electrical and mechanical resonances of the full drive system. A graphical representation is also presented, where during a fundamental period the distribution of the zero vectors can be tracked.