Research And Design Of Electromagnetic Compatibility In The Vairable Frequency Speed Control System

With the rapid development of high power switching device, variable frequencyspeed regulation drive system has become the first choice for many adjustable speeddrives. It has a series of advantages such as reducing production cost, saving energy,improving human living environment, industrial automation and product quality, etc.,which makes it more and more widely applied in all walks of life. However, in recentyears, the problem of electromagnetic compatibility has become the bottleneck forfurther popularization, attrcting more and more attention to it.Most of the variable frequency speed regulation system operate in harshelectromagnetic environment. And as a power electronic device which is composed ofcomputer chips, electronic components, etc., it is vulnerable to the interference ofsome electric equipment outside. The inverter itself is also a kind of interferencesources. On the one hand, almost all inverters use the nonlinear Pulse WidthModulation technique, which will generete a great many higher harmonic componentsin the inverter output pulse wave, increasing the wastage of machine. On the otherhand, because the frequency converters adopt high-speed power switching devices,the rapid turning on and off will also generate high pulse output voltage dv/dt whichwill cause some unpredictable problems such as shaft voltage, shaft current andleakage current, etc. Especially when the long cable is used to connect inverter andmotor, the high pulse output voltage dv/dt, by the long cable transmission, willproduce twice the over-voltage in the motor terminal, which leads to motor protectionmisoperation and shorten the service life of motor. Therefore, the electromagneticcompatibility refers to operation of variable frequency speed regulation systemwithout outside interference or its interference with outside.This paper starts with the principles of system, analyzes the mechanism andrelated countermeasures of the harmonic production in the input side and output sideof frequency converter. Then, in view of the electromagnetic interference, the paperdesigns the electromagnetic compatibility from the isolation, shielding, grounding,filter, installation and other aspects, making the variable frequency speed regulationsystem achieve good electromagnetic compatibility.In addition, in order to solve electromagnetic interference caused by over-voltageproduced in the motor terminal by long cable transmission, this paper, based on the conventional RLC low-pass filter, proposes a modified passive filter which iscomposed of a band rejection filter LC and two LCR low-pass filters. The passivefilter can reduce not only the dv/dt of power switching device during switchingtransients symmetrically but also the common mode voltage and leakage currentaccordingly. Under the Simulink simulation environment, this paper bulids simulationmodels of frequency converter, long cable and asynchronous motor, and verifies theeffectiveness of the design result through the simulation analysis.