PWM Inverter In an induction motor drive system, when the IGBT switch operates at high speed, a ground current is generated to flow into the grounding grid through the parasitic circuit. This sharp pulse current has a wide frequency band and no small peaks, which can cause serious EMI problems to other devices 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, and the peak of the interference pulse may be about twice that of the single-arm arm.
In order to better suppress EMI, an article has studied the generation and propagation mechanism of this noise, but most of the analysis is based on a lumped parameter circuit using parasitic circuits, the frequency range studied is not more than a few MHz. Some improved methods can Satisfactory EMI predictions are obtained at frequencies up to tens of MHz, but both include complex empirically-measured parameters and device models in SABER, which require lengthy computational time.
From the point of view of EMI fault diagnosis, only the spectrum of interference is not enough to solve all the problems. The intensity and width of a single interference pulse affect the effect of interference, and these are not clearly reflected in the spectrum diagram, so in the electromagnetic The time domain waveform of the interference is also specifically studied in the compatibility problem. Most of the articles focus on the EMI emission of the device to the grid side, and there is little mention of the EMI problem of ground currents facing other devices through the ground. Moreover, with the action of the switching elements, the topology of the main circuit is changed, and how much influence this has on EMI is rarely studied. This paper adopts a convenient system function method, which can comprehensively analyze the EMI characteristics of the grounding current in the PWM speed control system through several simple tests. The main propagation channel. Part of the grounding current flows out of the motor side through C, 2 and returns to the inverter from Csl, and the rest flows into 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 indication. 2 IGBT turn-on/turn-off waveform and ground current connection IGBT switching characteristics (here only concerned about dv/dO is determined by many factors, such as DC bus voltage, load current, gate drive impedance, junction temperature, and parasitic impedance of the circuit, etc. .
In this paper, the relationship between voltage rise/fall rate (ldv/dd) and load current during IGBT switching operation is studied in detail. The device voltage and load current / waveform on one of the bridge arms shown are as shown. For ease of measurement, the inverter is connected to the ground plane by a short thick copper wire. The voltage waveform shown by curve 3 in the figure is the output of an inverter bridge arm, which is generated by the action of the upper tube switch. The current waveform shown by curve 1 is the ground current of the motor side, and the current waveform shown by curve 2 is the flow back. Ground current on the inverter side.
Obviously, the grounding current is generated when the IGBT is switched. The larger the Idv/dfl is, the larger the grounding current is. The most ground current flows from the motor side and returns from the inverter side, and the rest flows into the grid side. It can cause serious interference to other devices through the grounding network coupling.
In the figure, the ordinate voltage is 250V/: Luo: the current is 2A/div: the abscissa time is 500ns/div.
The grounding current drawn by the IGBT upper tube open/close action Fig.4Groundingcurrentdueto, the EMI problem caused by the ground current is mainly determined by the turn-on action of the IGBT, so the EMI problem when the IGBT is turned on can be mainly considered.
In order to understand this more clearly, the ground current waveform and the amplitude spectrum caused by the IGBT turn-on (red line) and turn-off (blue line) under the same working conditions are re-rendered. It can be seen that 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 description of the EMI problem, the system's equivalent noise source and its coupling path are studied separately, the IGBT voltage and current are regarded as noise sources The propagation channel is seen as a linear network. In this paper, the voltage of each leg of the inverter is regarded as a noise source, the propagation channel is equivalent to a two-port network, and the ground current is the output response. G (young is a two-port network system function. Its various components and frequencies are off.
G(fi) can be obtained by testing the /s waveform data under a certain operating condition and using Fourier transform. In a similar way, the frequency components of the ground current in other operating conditions can be obtained, and then the time domain waveform can be obtained by inverse Fourier transform.
4 The influence of topology changes on the characteristics of the propagation channel With the action of the switching elements, the topology of the main circuit of the three-phase inverter is changed. When the upper or lower tube of a certain phase arm is switched, because the other two The phase-bridge arm switch is turned on and off, and has three different topologies: two bridge upper tubes or two bridge lower tubes or one bridge upper tube and the other bridge lower tube are in a conducting state. Consider again that this phase bridge arm is the switching action of the upper or lower tube, and there are a total of six different topologies.
Taking one of the phases as an example, through the test and calculation, the amplitude-frequency characteristic of the propagation channel system function from the voltage caused by the one-phase switching action to the grounding current of the one-phase bridge arm is obtained as shown in the figure. The transfer function is the same as it. It can be seen from the figure that these functions are basically consistent, which indicates that the change of the main circuit topology has negligible influence on the characteristics of the EMI propagation channel studied in this paper.
The two upper tubes pass through the two tubes -..., the amplitude spectrum characteristics of the propagation channels of the upper and lower tubes. The influence of more than 2.5 noise sources affects not only the amplitude of the interference pulse but also its width. However, the intensity of the interference measured by the spectrum analyzer does not fully reflect these time domain information, which often masks the actual interference level.
In a three-phase inverter, the output voltage of the three bridge arms can be regarded as three noise sources. According to the control strategy of the three-phase inverter, the IGBTs are turned on and off according to certain rules, in their switches. The excitation ground current is generated during operation. At some point, the switching action of two different bridge arms will be close in time, and the ground current at this time is the result of the joint action of the two bridge arms.
Although the spectrum measured by the spectrum analyzer is not much different, from the time domain, the peak value of the generated interference current may become significantly larger. In the extreme case, the two switches operate almost at the same time, and the intensity of the interference can be close. When the single-arm arm is twice as long, EMI problems such as malfunction of the circuit 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 of the motor side. When the two bridge arm switches are simultaneously operated, the ground current peak becomes larger due to the superposition, and the oscillation time becomes longer. For example, the peak value of the ground current in (b) is almost twice the peak value in (a).
(b) The simultaneous operation of the two bridge arm switches shows that the ordinate voltage of the curves 1, 2, and 3 is 250 V/div; the ordinate current of the curve 4 is 2.0 A/div; and the time of the abscissa is 5 noise cells. Grounding current 3 research example A 5.5kW drive system is installed in the electromagnetic shielding room. The three-phase LISN, inverter and an asynchronous motor at the input end are fixed on a 5mm thick aluminum plate, and the inverter is connected to the motor. There are three 200cm long connecting cables.
By testing the bridge arm voltage and ground current when each bridge arm IGBT is operated and calculating by using equation (1), the amplitude-frequency characteristic of the system function of the three-phase propagation channel can be obtained as shown, and its characteristic is similar to bandpass 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 characteristics of the three-phase propagation channel can easily predict the ground current under different operating voltage and current conditions after knowing the characteristics of the ground current EMI propagation channel. The 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.
When the input AC voltage is 460V and the load current is 3A, the ground current amplitude frequency characteristic is as shown in the figure. The time domain waveform is as shown in 0, and the measured value and the predicted value agree well.
The ground current spectrum when the IGBT is turned on 4 conclude that the current can be externally disturbed by the impedance of the grounding network. The grounding current is in the shape of an oscillating sharp pulse, and its occurrence time corresponds to the time when the voltage of the IGBT switch tube jumps. The faster the voltage jumps, the higher the peak value of the ground current. For the same IGBT, Idv/dfl is basically independent of the load current at turn-on, and is much larger than Idv/drl at turn-off, and the voltage trip time decreases with increasing load current during turn-off. The maximum amplitude of the ground current is mainly changed by the IGBT's turn-on three-phase PWM inverter. The topology of the main circuit is changed, but this change has little effect on the characteristics of the EMI propagation channel discussed in this paper. The amplitude-frequency characteristics of the system functions describing the propagation channels of each phase are similar to those of bandpass, and their values ​​are basically the same. When the switches of the two bridge arms are simultaneously operated, the external interference pulses of the bridge arms may be superimposed, and the amplitude may be close to twice the interference pulse of the single bridge arm switch action.
The linear system function method is used to describe the coupling path characteristics between the switching elements of the three-phase inverter main circuit to the ground plane. The practice shows that this method is effective in analyzing and predicting the PWM inverter-induction motor drive system. When the problem is disturbed, the error produced is acceptable.
In the above analysis, the actual nonlinearity of the system is neglected to bring about a certain error, and since the effect of di/df is neglected, when the Idv/dfl is relatively small, the prediction will bring a relatively large error, especially the analysis. In the case where the two switches operate at the same time, further research needs to consider the influence of the mountain's on the ground current.
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