Harmonic distortion in PWM inverter output waveforms

It is usual to provide a short ¿dead time¿ between switching one device in an inverter leg off and switching the other device on, to prevent them conducting simultaneously. It is known that this expedient modifies the harmonic content of the output voltage waveform of a pulse-width modulation (PWM) inverter. It changes the amplitude of the fundamental component, which can be significant in variable-speed drives when the frequency ratio is changed. It also introduces low-amplitude, low-order harmonic components which are not wanted in uninterruptible power supplies when voltage waveforms of high purity are demanded. This problem has been treated quantitatively and it has been shown by a systematic set of experiments that the proposed theoretical method is reliable. It has been found that the amplitudes of the odd harmonics produced depend only upon the ratio of the dead time to the period of the switching frequency. Similarly the amplitudes of the even harmonics, which only occur with even frequency ratios, depend on the ratio of the dead time to the period of the fundamental frequency. Accordingly it has been possible to present generalised curves which allow the design engineer to estimate the amplitudes of these harmonics for any combinations of fundamental and modulating frequencies. These results have been presented in such a way that they are applicable to both single-phase and 3-phase PWM inverters.