Analysis of Grounding Current EMI in PWM Inverter-Induction Motor Drive System

In the PWM inverter-induction motor drive system, when the IGBT switch operates at a high speed, ground current flows into the ground net through the parasitic circuit. This spike current has a wide frequency band and a small peak, which can cause serious EMI problems to other equipment in the system through the grounding network. What is more serious is that the two bridge arms of the three-phase bridge may operate at almost the same time. At this time, the peak value of the interference pulse may be about twice that of the single-arm operation.

In order to better suppress EMI, the article has studied the mechanism of the generation and propagation of this noise, but most of the analysis is based on the use of parasitic circuit lumped parameter circuit, the frequency range studied does not exceed a few MHz. Some improved methods can A satisfactory EMI prediction is obtained at frequencies up to tens of MHz, but both include complex empirically-measured parameters and device models in SABER and require lengthy calculation time.

From the point of view of EMI fault diagnosis, only studying the spectrum of interference is not sufficient to solve all problems. Factors such as the intensity and width of a single interference pulse all affect the effect of interference, and these are not clearly reflected in the spectrum diagram, so in the electromagnetic field Compatibility issues also need to specifically study the temporal waveforms of interference. Most of the articles focus on discussing the EMI emission from the device to the grid side, and little mention is made of the EMI problem of ground current through ground facing other devices. Moreover, with the action of the switching elements, the topology of the main circuit changes, and how much influence this has on the EMI problem is rarely studied. This article uses a convenient system function method, through several simple tests can be more comprehensive analysis of the PWM current control system in the ground current EMI characteristics. The main communication channel. Part of the ground current flows through C, 2 out of the motor side and returned to the inverter by Csl, with the rest flowing to the grid side. The ground current on the motor side can be measured with a current probe as shown.

Inverter induction motor system EMI test schematic. 2 IGBT turn-on/turn-off waveforms are related to ground current switching characteristics of IGBT (only here dv/dO is concerned with many factors, such as DC bus voltage, load current, gate drive impedance, junction temperature, and circuit parasitic impedance, etc. .

This article examines in detail the connection between the voltage rise/fall rate (ldv/dd) and the load current during IGBT switching. The device voltage and load current waveforms on one leg of the illustrated inverter are as shown. For measurement convenience, the inverter is connected to the ground plane through a short thick copper wire. The voltage waveform shown in curve 3 in the figure is the output of an inverter bridge arm, which is generated by the switch action of the upper tube. The current waveform shown in curve 1 is the ground current on the motor side, and the current waveform shown in curve 2 is the flow back. Inverter-side ground current.

Obviously, the ground current is generated during the IGBT switching action. The larger the Idv/dfl, the greater the grounding current. Most of the ground current flows from the motor side and returns from the inverter side. The rest of the ground current flows into the grid side. Can cause serious interference to other devices through ground network coupling.

In the figure, the ordinate voltage is 250V/: Lo: the current is 2A/grid: the abscissa time is 500ns/division.

The grounding current of the IGBT on/off operation of the tube is much more than Fig.4 Groundingcurrentdueto. The EMI problem caused by the ground current is mainly determined by the turn-on action of the IGBT. Therefore, the EMI problem when the IGBT is turned on can be mainly considered.

In order to understand this point more clearly, the ground current waveform and amplitude spectrum caused by the IGBT turn-on (red line) and turn-off (blue line) under the same operating conditions are re-drawn in IGBT, which can be seen when the IGBT is turned on. The ground current EMI problem is much more serious.

(b) Ground current spectrum diagram S Ground current EMI 2.3 EMI propagation channel characteristics In the description of the EMI problem method, the equivalent noise source of the system and its coupling path are studied separately. The voltage and current of the IGBT are considered as the noise source. The propagation channel is seen as a linear network. In this paper, the voltage of each leg of the inverter is considered as the noise source, the propagation channel is equivalent to a two-port network, and the ground current is the output response. G (youth is a two-port network system function. Its various components and frequency are off.

G(fi) can be obtained by testing the sum/s waveform data under a certain operating condition and using a Fourier transform. With similar methods, each frequency component of the ground current in other operating conditions can be obtained, and its time domain waveform can be obtained through inverse Fourier transform.

4 Influence of Topology Changes on Propagation Channel Characteristics With the action of switching elements, the topology of the main circuit of the three-phase inverter is changed. When the upper tube or lower tube of a certain phase bridge switch is actuated, because the other two There are three different topologies for the on-off and off-phase of the phase leg switching transistor: two upper or two lower legs of the bridge leg or one upper leg of the bridge arm and the other lower leg of the bridge leg are in a conducting state. Considering that this phase leg is the switching action of the upper tube or the lower tube, there are a total of 6 different topologies.

Taking one of the phases as an example, the amplitude and frequency characteristics of the propagation channel system function of the voltage from the phase-to-phase switching action to the ground current core at the time of operation of the upper tube of a phase leg arm are obtained. The transfer function is the same as it is. As can be seen from the figure, these functions are basically the same, which shows that this change in the topology of the main circuit has negligible impact on the characteristics of the EMI propagation channel studied in this paper.

The two channels on the tube pass through the tube-..., the spectral characteristics of the propagation channels of the upper and lower tubes. The effect of the combined effects of more than 2.5 noise sources The malfunction depends not only on the amplitude of the interfering pulses but also on its width. However, the interference intensity measured by the spectrum analyzer can not fully reflect these time domain information, which often masks the actual interference level.

In a three-phase inverter, the output voltages of three bridge arms can be regarded as three noise sources. According to the control strategy of three-phase inverters, IGBTs are turned on and off according to certain rules, in their switches. Energize ground current generation during operation. At some point, the switching action of the two different bridge arms will be very close in time. At this time, the ground current is the result of the interaction of the two bridge switches.

Although there is not much difference in the spectrum measured by the spectrum analyzer, the peak value of the generated interference current may be significantly larger in the time domain. In the extreme case, the two switches operate almost simultaneously, and the intensity of the interference can be approached. With twice the single-arm operation, EMI problems such as circuit malfunctions are more likely to occur.

The actual test results are shown in the figure. Curves 1 to 3 in the figure are the output voltages of the three bridge arms of the inverter, and curve 4 is the ground current at the motor side. When two bridge arm switches operate at the same time, due to the superposition, the peak value of the ground current becomes large, and the oscillation time becomes longer. For example, the peak value of the ground current in (b) is almost twice of the peak value in (a).

(b) Simultaneous operation of two bridge switches. The ordinate voltages of curves 1, 2, and 3 in the diagram are 250 V/div; the current of curve 4 ordinate is 2.0 A/div; the time of the abscissa is 5 grids under the action of two noise sources. Ground current 3 Study example A 5.5kW drive system was installed in the electromagnetic shielding room. The three-phase LISN, inverter and an asynchronous motor at the input were all fixed to a 5mm thick aluminum plate and the inverter was connected to the motor. There are three 200cm long connection cables.

By testing the bridge arm voltage and ground current when the IGBT of each bridge arm is operating and using formula (1) to calculate, the amplitude-frequency characteristics of the system function of the three-phase propagation channel can be obtained as shown. Its characteristics are similar to that of the band pass, because The three-phase circuit (including the inverter and the induction motor) is symmetrical and the three functions are basically the same.

The amplitude spectrum characteristic of the three-phase propagation channel can easily predict the magnitude of the ground current under different operating voltage and current conditions after knowing the characteristics of the ground current EMI propagation channel. Each frequency component of the ground current can be obtained by Equation (1), and then its time domain waveform can be obtained by inverse Fourier transform.

The amplitude and frequency characteristics of the ground current when the input AC voltage is 460V and the load current is 3A, for example, the time domain waveform is shown as 0, and the measured value agrees well with the predicted value.

The ground current spectrum when the IGBT is turned on 4 Conclusion The ground current can create external interference through the impedance of the ground network. The ground current is in the shape of an oscillating spike. Its appearance moment corresponds to the time when the IGBT switch jumps. The faster the voltage jump, the higher the peak ground current. For the same IGBT, the Idv/dfl is basically independent of the load current at turn-on and is much larger than Idv/drl at turn-off, and the turn-off time decreases as the load current increases. When the maximum amplitude of the ground current is mainly caused by the IGBT turning on three-phase PWM inverter, the topology of the main circuit is changed, but this change has basically no effect on the characteristics of the EMI propagation channel discussed in this paper. The amplitude-frequency characteristics of the system functions describing each phase propagation channel are similar to those of band-pass, and their values ​​are basically the same. When the two bridge arms are actuated at the same time, the external disturbance pulse of each bridge arm may be superimposed, and the amplitude may be close to 2 times of the interference pulse when the single bridge arm switches.

A linear system function method is used to describe the characteristics of the coupling path between the switching elements of the three-phase inverter main circuit and the ground plane. Practice has shown that this method is effective in analyzing and predicting the PWM inverter-induction motor drive system externally. In the case of a perturbation problem, the resulting error is acceptable.

In the above analysis, the actual nonlinearity of the system has been neglected to bring certain errors, and due to the neglect of the di/df effect, when the Idv/dfl is relatively small, the prediction will bring relatively large errors, especially the analysis. In the case of simultaneous operation of the two switches, further research is also required to consider the impact of the hill on the ground current.

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