Switching Harmonics in a Three-Phase PWM Inverter

This paper provides a simple but accurate method for calculating switching harmonics generated by the pulse width modulation (PWM). This method can be used to design filter for converters and to evaluate EMI impacts.

Three-phase PWM inverters have been wildly used as major power stages for a three-phase dc-ac power conversions for many years. The maturation of techniques on this type of inverter has expanded its applications from the general industry to the aerospace. One of the side effects of this type of inverters is the switching harmonics generated by the pulse-width modulation (PWM). Many papers were published regarding the harmonics analysis. However, most of them were focusing on the output ac voltages only. In addition, complex mathematics used in the analysis prevents the practical engineers from understanding and using them in the inverter design. This paper uses a simple mathematical method for calculating switching harmonics in the output voltage, as well as the switching harmonics in the dc link. The paper will introduce the relationship between the harmonic spectrum and the mathematic equation associated with individual harmonic components. The distribution of individual switching harmonic on the harmonic spectrum is illustrated. The close form equations for calculating each individual harmonics are provided. With this method, the power electronics designer can calculate the harmonics before the converters are built and tested. The proper filters can be designed based the harmonics calculation and the EMI effects can also be addressed at the same time.

The method is verified by simulation using different simulation tools. It is also validated by practical measurement in a high power converter application to be appreciated for use in the inverter design.