Research Of Conducted Electromagnetic Interference In PWM Inverter

With the development of power electronics, PWM inverters are getting more widelyuse, such as PWM drives, UPS etc. The fast switching devices including IGBTs andMOSFETs can increase the dynamic response speed. However, the high frequency pulseshave very large dv dt and di dt , which produce large electromagnetic interference (EMI)from several kilo to several million hertz frequency and the electromagnetic compatibility(EMC) standard is not submitted. The EMI is not only influences the load's usual work andwithdraws the load's work life but also has detrimental effects on inverter itself. Theresearch in this dissertation is mainly about the conducted EMI in PWM inverter. Theresearch works include three aspects: analysis of EMI typical characteristic and predictionof conducted EMI spectrum, conducted EMI suppression based on EMI filter, CM EMIsuppression based modulation technology. The origin and propagation path of EMI are investigated. The conclusions are thatdifferential mode (DM) EMI is due to the pulsating current from the fast switch andcommon mode (CM) EMI is due to the dv dt charging and discharging the parasiticcapacitor. The DM lumped equivalent circuit model is analyzed. a simple equivalent circuitis proposed to study the CM EMI. The parameters of passive element are measured byexperiment. Time and frequency domain analysis are conducted to obtain CM currentwaveforms and noise spectra in 10KHz-30MHz frequencies. The CM current is a decayingoscillatory waveform. The simulated CM EMI waveforms are compared with the actualexperiments to verify the validity of the proposed model. The effects of noise source and noise load impedances on the insertion loss of EMIfilter are discussed. The noise source impedance is estimated by insertion loss methodbased on experiment. A method of designing EMI filter is proposed, which simplifies thedesign of EMI filter. Practical design examples including CM and DM EMI filter indicatethat the EMI filter design method is simple and effective. Referencing the method ofinserting CM transformer between PWM inverter and motor, CM transformer inserted in dcinput port is used to damp the resonant CM current. The mechanism of the two methods iscompared in time and frequency domain. Experimental results verify the validity of theoryanalysis. The snubber circuit can decrease the interference level. The snubber circuit is IIIthought as EMI filter because the device is basic passive elements. The snubber circuit usedto reduce radio frequency EMI is investigated and design methods are given. The results areexperimental verified. The impacts of inverter modulation strategy on CM conducted emissions areaddressed. The bipolar SPWM is proposed for CM EMI reduction in single phase PWMinverter. The CM EMI is existed due to the inconsistency of diagonal drive waveforms. Intypical three-phase inverter, there exists substantial CM EMI when traditional space vectormodulation is adopted. An active topology is obtained by adding a fourth phase to a typicalthree-phase inverter. The three-dimension space vector modulation is effective for CM EMIcancellation but the modulation index is reduced. In addition a modified space vectormodulation is proposed for CM cancellation while output harmonic and modulation indexare not detrimental.