In order to verify the current correction method for the load-current-caused error in the active du/dtfilter output voltage, a simulation model was developed in the MATLAB SIMULINK environment. A block diagram of the developed model is presented in Figure 4.10. The modulator block forms the gate drive signals for the output stage consisting of SIMULINK SimPowerSystems IGBT/Diode components. The output stage drives the active du/dt LC filter circuit, which is connected to the SimPowerSystems asynchronous machine model.
Three-phase current measurements are carried out after the output stage and after the active du/dt filter. The motor current measurement is used to form correction pulses of the right length. A more detailed description of the simulation model structure is given in Appendix A.
Because the research on the development of the current correction method for a frequency converter was outside the scope of this dissertation, no measurement results with the current
4.2 Simulations of the error caused by the motor current 77
Figure 4.10. Block diagram of the correction pulse simulation model. The top level model and the blocks are presented in more detail in Appendix A.
correction method are presented. The simulation model applies the theory presented previ-ously in this chapter. However, the error caused by the load current is noticeable, to the extent it can be detected at the motor sizes used in the measurement, are presented later. If the filter maximum peak current and the motor current are in the same order, the filter output voltage error becomes more apparent. Simulations are carried out for a filter design ofL=7 µH and C=0.33 µF, which leads totr≈3.2 µs andIf(t)max≈113 A.
In the simulation data, part of the start-up transient of the standard SimPowerSystems Asyn-chronous machine model is shown. The model was configured to represent an induction motor of approximately 37 kW. In the SimPowerSystems IGBT model, some of the losses, for example the losses in the conducting state, are taken into account. However, for example the dead times, which are mandatory in a real inventer, were omitted in the simulation, and therefore, the output stage model is an idealized model of a real output stage.
In Figures 4.11–4.14, the operation of the active du/dt method is shown without the current correction pulse; only the active du/dtcharge and discharge pulses are used. As can be seen, the increasing instantaneous value of the motor current causes an error in the output voltage of the filter, that is, in the motor voltage, as the LC circuit resonates. The greater the load current value during the charge is, the greater is the error and the amplitude of the unwanted LC resonance. Moreover, the resonance is visible in the filter current, which is seen in the inverter output current. Time-enlarged waveforms of inverter and motor voltages are also shown at two different time instants.
In Figures 4.19–4.22, the operation of the active du/dt method is shown with the current correction pulse applied. As can be seen, there is negligible LC oscillation, or error in the filter output and in the motor voltage, and the current correction pulses applied is seen to correct the LC filter resonance in cases, where the load current is significant compared with the filter current. Time-enlarged waveforms of inverter and motor voltages are shown at two different time instants. Furthermore, the inverter current consists of the motor current and the charge and discharge current spikes of the active du/dtLC filter.
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Figure 4.11. Simulated inverter bridge output voltages. During the transients in the PWM, charge pulses are applied according to the theory presented in Chapter 3. No correction pulse is applied.
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Figure 4.12. Simulated filter output voltages. As can be seen, the LC resonance increases as the instantaneous motor current value increases. No correction pulse is applied.
4.2 Simulations of the error caused by the motor current 79
Figure 4.13. Simulated inverter bridge and motor voltages, a time-enlarged version of one time instant of Figures 4.11 and 4.12. No correction pulse is applied.
1.7 1.72 1.74 1.76 1.78 1.8
Figure 4.14. Simulated inverter bridge and motor voltages, a time-enlarged version of one time instant of Figures 4.11 and 4.12. No correction pulse is applied. As can be seen, the increasing load current, see Figure 4.16, causes LC filter resonance. The resonance does not originate from cable reflections, since a motor cable is not present in the model.
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Figure 4.15. Simulated inverter bridge output currents. As the amplitude of the motor current increases, the resonant LC filter current is seen in the inverter output current. No correction pulse is applied.
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Figure 4.16. Simulated currents of the asynchronous machine model at the beginning of the start-up transient. No correction pulse is applied.
4.2 Simulations of the error caused by the motor current 81
Figure 4.17. Simulated inverter bridge output currents. The currents consist of the sum of the motor current and the charge and discharge currents of the LC filter circuit during the transients. The correction pulses are applied as a function of the current instantaneous value.
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Figure 4.18. Simulated currents of the asynchronous machine model at the beginning of the start-up transient. The correction pulses are applied as a function of current instantaneous value.
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Figure 4.19. Simulated inverter bridge output voltages. During the transients in the PWM, charge pulses are applied according to the theory presented in Chapter 3. The correction pulses are applied as a function of current instantaneous value.
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Figure 4.20. Simulated filter output voltages. As can be seen, the LC resonance is negligible, as correction pulses are applied as a function of current instantaneous value.
4.2 Simulations of the error caused by the motor current 83
Figure 4.21. Simulated inverter bridge and motor voltages, a time-enlarged version of one time instant of Figures 4.19 and 4.20. Current correction pulse is applied.
1.7 1.72 1.74 1.76 1.78 1.8
Figure 4.22. Simulated inverter bridge and motor voltages, a time-enlarged version of one time instant of Figures 4.19 and 4.20. Current correction pulse is applied. As can be seen, the current correc-tion pulses are applied to correct the LC filter resonance in cases where the load current is significant compared with the filter current; cf. Figure 4.14.