Hierarchical EMC design for inverters in motor drive systems

Abstract

Power electronics applications are usually accompanied by high voltage and current amplitudes, and steep voltage and current transients. The EMI (Electromagnetic Interference) issue is regarded as the main side effect of power electronics applications. Noise level prediction is a vital task for filter design, but many difficulties are encountered. For instance, a large amount of experience is needed to build equivalent circuits, the use of simplified models often requires sacrificing details in the high frequency range, individual approaches must be developed for various topologies, etc.. A new method for EMI prediction of power electronics applications is thus desirable.

The universal method should overcome the following common characteristics of power electronics circuits: (a) the large difference between the time constants, and (b) the long time required to reach steady state. The time domain approach is very time-consuming. The assumptions used in the frequency domain approach, namely, periodic noise sources and a fixed propagation path, are also not valid. For example, the slopes of voltage and current transients depend on the operating points, and the assumption of periodic noise sources does not apply. Because the junction capacitors of power rectifiers and switches change with the reverse voltage, the values of the components in the noise propagation path also change.

In this thesis, a hierarchical approach is proposed for the EMC design of inverters in motor drive systems. It combines the advantage of the time domain and frequency domain approach to achieve a fast, universal and accurate result. The approach is validated by observations in the time domain and the frequency domain. The proposed approach has three steps. In the first step (functional level), a simple model of switches in the system is developed. The operating points of each switching transient and all time intervals are derived and the narrowband signals of the EMI noise can be derived. The second step (transient level) results in detailed transient waveforms which take the variation of the nonlinear switching transient into account. In the third step (propagation level), the noise propagation through the system is described by the transfer ratio, and the EMC performance is evaluated.

The approach is described in detail, and then a PWM voltage source inverter feeding an induction motor is analyzed using this approach. This approach is also applied to a resonant inverter that operates under ZVS conditions. The experimental results are compared to calculated results.

Two approaches are proposed for EMI suppression in this thesis. The first approach is by adding a passive filter on the dc-bus. It can be concluded that the same noise suppression performance can be achieved using a dc-bus filter as the conventional ac side filter. The advantage of this approach is that the connections can be made very short which can significantly improve the suppression.

A new active filter called the "fourth leg compensator" is proposed in this thesis. The fourth leg inverter generates a signal to compensate the transients of other three legs. It can suppress the fast transients of common mode voltages while reducing the amplitude of the common mode voltage by 50%. A method to determine the values of the additional components is described. It is shown that the CM voltage can be compensated, even with nonideal coupling in the CM transformer and when leakage inductance is present.