Generation and suppression of conducted EMI from inverter-fed motor drives

Abstract

This paper focuses on a systematic analysis of the conducted electromagnetic interference (EMI) from an inverter-fed motor drive system. It proposes a theory to explain how the conducted EM1 is generated. The theory states that the EM1 can be produced from the system by two different mechanisms: parasitic capacitive coupling and inductive load current switching. Each of the EM1 mechanism is discussed in details. Suppression methods are also suggested to help effectively reduce the EM1 emission. Validation of the theory and suppression methods are then performed experimentally based on a brushless motor drive.This means that no stray inductance, on-state resistance and diode reverse recovery etc. will be considered. While it is obviously a valid argument that the switching characteristics may have certain influence on EMI, their contributions are usually negligible compared to those mechanisms to be discussed. Also, in order to focus on EM1 issues associated with an inverter, the effect of a rectifier stage on EM1 is not covered in this paper. As an AC motor drive may include a rectifier stage, the results presented here have to be modified to include the influence of the rectifier. Fortunately, the rectifier produced EM1 has been better understood and it is not difficult to modify the results presented here to include its effect. I. Introduction 11. Mechanism of EM1 With the advances in switching speeds of transistorized switches (especially the IGBTs), the high dv/dt voltages produced by inverters have been found to cause electromagnetic interference (EMI), shaft voltages, and bearing currents, etc. Although the effect of dv/dt on EM1 has been well recognized, the mechanism of EM1 generation in inverter-motor systems is nevertheless somewhat ambiguous. As a result, the selection and installation of EM1 filters remain more or less a pragmatic or trial-anderror process [ 1-41. This paper is aimed at a systematic analysis of the conducted EM1 in voltage source AC motor drives. It proposes a theory to explain how the conducted EM1 is generated and addresses issues regarding how to effectively choose and install circuitdfilters to suppress EMI. This EM1 theory is based on recognition of parasitic capacitive coupling and the associated coupling currents in a motor drive system. Taking into consideration the parasitic capacitive coupling effect, a current in any motor drive contains three components: fundamental, harmonic and coupling. The approach to conducted EM1 analysis is to find out how those current components, mainly the harmonic and coupling, can flow outside the drive system and become an EM1 producing current. By identifying possible EM1 current paths, this paper then suggests EM1 suppression techniques which block EM1 currents from flowing outside or make them circulate locally inside a drive.It is necessary for the rigor of this paper to point out that ideal switching characteristics are assumed for the devices.The rapid changes in voltages and currents of a switching power converter produce EM1 in two for...

A detailed analysis of the propagation paths of high-frequency common mode currents in AC motors

“…• In [1], authors explain that common mode currents are mainly due to the stray capacitances between the stator and the armature winding [1]. Therefore, the rotor is canceled from the following analysis.…”

“…through the stator core to ground, induces a HF common mode flux which causes a shaft voltage and subsequently bearings currents. This phenomena and the cycle of generation of bearing currents are well explained in [1][2][3][4][5][6][7][8][9][10][11].…”

Behavioural models for common mode EMI filters

“…As this type of current travels through the ground and the input power cable, any electronic devices connected to the ground or the cable will be prone to interference from them. Since the common mode currents share most of their paths with other equipment, the level of EMI emission from them is usually higher than that from the differential mode coupling currents [11]. …”

“…Under such a condition, a portion of the differential mode coupling currents must flow into the AC source as indicated by the dashed line in Figure 11. This portion of differential mode coupling currents flows outside the drive system and constitutes the conducted EMI emission in differential mode [11]. …”

“…Assuming a relatively high impedance between the negative dc bus and the ground, the main path (indicated by the solid line) for common mode coupling currents in an inverter drive with a rectifier input can be depicted as shown in Figure 12. In addition, due to parasitic capacitance across the bottom rectifier diodes, there may be some common mode currents which pass through the DC link capacitor as indicated by the light dashed line [11]. Figure 12: Differential mode coupling current contributes to conducted EMI (Case 2).…”