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Broadband Electromagnetic Compatibility Model of Three Phase Synchronous Motor
Rapidly switching semiconductors in modern high power inverter/motor-drive systems generate fast changing voltages and currents which may result in unwanted emissions. While models of power inverters have been built in the past to predict emissions, they are typically “black box” models where the cause of and solution to emissions problems is difficult to analyze. To improve inverter system design strategies, a detailed measurement-based SPICE model of a power inverter system was built in which there is a straightforward correlation between system geometry and parasitic circuit elements. This model was validated through measurements. The model was able to predict transfer characteristics between ports of the inverter within 4 dB from 100 kHz to 100 MHz. Once built, this model was used to identify structures responsible for resonances and to determine possible improvements of the power inverter design to reduce emissions. Measurements of S21 and radiated emissions after adding these improvements demonstrated that they were able to reduce emissions by 10-20 dB, thus confirming the accuracy of the model and its ability to improve understanding of emission mechanisms and to guide development of emissions reduction strategies.
This paper deals with conducted electromagnetic interferences (EMIs) in adjustable-speed drive (ASD) systems. For some years, the use of high-speed switching power devices in ASDs induces high-voltage ( dv / dt ) and high-current ( di / dt ) variations that excite the parasitic elements of the power circuit, inducing conducted emissions. The advent of these devices has thus generated several unexpected problems, such as premature deterioration of motor ball bearings and high increases in the EMI levels, which are caused by the circulation of the high-frequency currents. In order to evaluate the level of the common-mode (CM) and the differential-mode (DM) currents in the ASD system, it is necessary to use a precise model of the pulse width modulation (PWM) inverter, power cable, and ac motor that takes into account various phenomena, which appear when the frequency increases. First, a PWM inverter and shielded four-wire power cable model are presented. Then, a new high-frequency modeling method of the ac motor is proposed. Finally, the ASD system is simulated and the obtained results are compared to the experimental measurements in the frequency and time domains.
A building technique of high-frequency motor models by 3-D electromagnetic field analysis has been developed to establish the simulation technology for EMC estimation in the design stage of motor drive systems. Stray capacitance of each part of a motor was calculated by static electric field analysis, while inductance and resistance of its stator windings were calculated by magnetic field analysis. A motor equivalent circuit was then built with these circuit constants to compute frequency characteristics of motor impedance. In order to take eddy current in laminated iron core into account precisely, we analyzed a 3-D model of half thickness of one laminated steel sheet, and a simplified 3-D model of coil end of a motor. We also took the difference between magnetic field distribution of common mode and that of differential mode into account by setting different coil current and boundary conditions in the analyses. The frequency dependence of calculated inductance and resistance was introduced into circuit simulations. As a result, we have obtained frequency characteristics of motor impedance, whose basic features are similar to those of measured data. Possible causes of the discrepancy have also been discussed.