Ratio changing in pulse-width-modulated inverters

The presence of components of a frequency lower than that of the wanted component in the output of a pulse-width-modulated inverter (PWM) is highly undesirable when the inverter is supplying an induction motor. The generation of subharmonics can be avoided by making the carrier frequency an integer multiple of the output frequency. However, this restricts the operating range of the inverter. To obtain a wide output frequency range, it is necessary to change the ratio between the carrier frequency and the output frequency when the carrier frequency becomes either too high or too low for satisfactory operation. This technique has become practical as a result of advances in microelectronics. It is shown that, although the ratio of an ideal PWM waveform may be changed without significantly affecting the magnitude of the wanted component in the waveform, ratio changing will always result in a change in the output voltage of the inverter, owing to the practical limitations of the power circuit, unless steps are taken to compensate for these limitations.