Research On Adverse Effects Of Modern PWM Adjustable Speed Drive System And The Associated Countermeasures

With miniaturization, multi-function and high-performance of inverters,especially total digitalization of control scheme, the flexibility and adaptability ofinverters are improved continuously. Therefore, inverters are used in modernindustry more widely. Now, almost all inverters use PWM control technologies.Comparing with other inverter using different controlling method, it can improvethe inverter output waveforms, reduce motor harmonic loss and torque fluctuation,predigest inverter configuration, quicken adjusting speed, improve systemdynamic performance.However, some notable adverse effects are found in the wide applications ofPWM inverters. Voltage source PWM inverters generate high frequencycommon-mode voltage, which induces high shaft voltage, leads to bearing current,and results in premature bearing failure; High-speed switching power electronicdevices generate strong EMI. Conducted EMI and radiated EMI will disturb othercontrol systems or electronic equipment, even result in malfunction. Long cablebetween PWM inverters and motors leads to over-voltage at motor terminals,which not only worsens the above adverse effects, but also stresses on windinginsulations. These adverse effects decrease reliability, increase failures, but alwayswere ignored by users. However, the damages due to the adverse effects may belarger than the cost of the AC variable frequency drive system. Therefore,investigating the adverse effects of PWM inverters and its counter measures hasimportant significance. In order to suppress the adverse effects and improve thereliability, inverter output filters and correlative techniques of AC variablefrequency drive system are investigated in this thesis.Using transmission line theory, inverter output PWM voltages are consideredas traveling waves on the long cables. Due to the distributed parameters and highdv/dt, voltage traveling waves are reflected at motor terminals, reflected wavesadd incident waves, which results in voltage-doubling at motor terminals, leads toover-voltage and rings, and stresses on winding insulations. The over-voltage isdependent on the PWM rise-time of switching devices and cable length, shorterrise-time, longer cable, higher over-voltage. Common-mode voltage generated bya voltage source PWM inverter and the essence of its adverse effects is investigated, and common-mode voltage reducing methods are also posed. A novelinverter output filter which can not only filter the differential-mode voltage butalso suppress the common-mode voltage is proposed.It takes out thedifferential-mode voltage as main purpose. Through adding additive coil on theinductor of the differential-mode RLC passive filter, the common-mode voltagecan also be suppressed. The main advantages are the combination of two kinds offilter and simplicity in structure. The results of experiment indicate that about 60ï¼…RMS of the common-mode voltage are suppressed under meeting the demands todifferential-mode voltage filtering. The basic structure and the differential-modeequivalent circuit and the common-mode equivalent circuit of the filter areillustrated. The voltage transfer functions of the differential-mode circuit and thecommon-mode equivalent circuit are analyzed. An engineering design method ofthe filter parameters is also presented. Some experiment results are used tovalidate the filter.Three-level technology is investigated for common-mode voltage output bythe two-level inverter is only reduced by a filter. With effective on-off states inthree level inverter, common-mode voltage output by a three level inverter isanalyzed, then a conclusion is made that using software can reduce common-mode voltage in odd level inverter. Reducing and canceling common-modevoltage methods are analyzed respectively using SPWM and SVM. Simulationresults validate its correctness. A comparison is made among their effects onmotor performance, and then usability of the proposed methods is validated.The local averaged value and instantaneous value of the matrix converter'scommon-mode voltage are analyzed, the two kinds of approaches are discussedrespectively which change the position of zero output switch state to reducecommon-mode voltage's instantaneous value, the two kinds of methods reducerespectively the common-mode voltage's maximum value to 86.67ï¼…and 57.73ï¼….At the same time, the paper analyses the influence for the output voltage, the inputcurrent and the voltage gain due to the change of the position of zero outputswitch state. It is presented that the method of the zero voltage's commutation inthe paper, characteristics of the method include: the maximum voltage gain intheory; the sinusoidal symmetrical output voltage; the common-mode voltage'smaximum value is the half of the input phase voltage magnitude.