The Negative Impacts Of Wide-bandgap Device Based Variable Frequency Drives On Electric Machines

With the superior characteristics and commercial availability,wide-bandgap devices are gradually finding their applications.Compared to silicon counterparts,silicon-carbide and gallium-nitride devices have faster switching speeds,higher switching frequencies,higher voltage blocking capability and higher temperature performance.The adoption of wide-bandgap devices in motor drive systems can improve power density and efficiency and enable high-fundamental-frequency drives for electric machines with high-speed and high number of poles.This is favored in highefficiency and high-power-density applications such as more electric aircrafts and electric vehicles.However,under high-switching speed and high-switching frequencies,the motor experiences overvoltage and bearing current at its stator winding and its bearing,respectively.These overvoltages and bearing currents stress the motor winding insulation and bearings,which may lead to degradation and pre-mature failure.These problems reduce the lifetime of the motor.To systematically analyze the motor negative effects with wide-bandgap inverter is the objective of this dissertation.Compared with the causes of the motor negative effects with the silicon inverter,the physical mechanism of the motor overvoltage and the bearing current with the wide-bandgap inverter have been clarified.In addition,the factors which affect the motor negative effects have been explained and summarized.Based on the wave reflection,the transmission process of the voltage pulse in the cable has been analyzed.Then,the generating mechanism of the motor overvoltage has been clarified.Considering the high frequency harmonics,the skin effect of the high frequency pulse has been analyzed.Also,an equivalent model to represent the skin effect has been established.To accurately analyze the motor overvoltage in the time domain,a ladder network model of the cable-motor system has been established.Then,a simplified fitting model of the cable-motor system has been established to reduce the computational load on parameter extraction of the ladder network model.Firstly,the impedance response of the cable-motor system has been analyzed which use the frequency domain analysis method.Then,the frequency response of the impedance of the actual cable-motor system has been fitted by extracting the impedance parameters at specific frequency points.Finally,a high frequency fitting model with the same frequency response as the actual cable-motor system has been established.Experimental results verify the effectiveness of the fitted model in predicting the motor overvoltage in the time domain.This simplified fitting model provides a modeling method for engineering design to analyze the motor overvoltage in the time domain.Using the wave reflection theory,three types of motor overvoltages have been investigated,i.e.,differential mode(phase-to-phase)motor terminal overvoltage,common mode(phase-to-ground)motor terminal overvoltage and common-mode stator neutral point overvoltage(motor neutral to ground).Then,both the high switching speed effect and the high switching frequency effect on the motor overvoltage have been investigated in this dissertation.Significant overvoltage has been observed when the switching frequency or its multiples coincide with the cable or motor anti-resonant frequency.In addition,the anti-resonant behavior in the cable and motor impedance has been used to identify the overvoltage oscillation frequency and to explain the overvoltage observations for the three types of motor overvoltages.Also,it is concluded that the serious motor overvoltage caused by high switching frequency will further lead to the increased bearing current.Above conclusions have been verified by experimental results.To investigate the impact of wide-bandgap inverter on the motor bearing currents,the mechanism and modelling of bearing currents have been analyzed first.Then,a direct measurement method has been used to assess the bearing currents in the induction motor driven by a wide-bandgap inverter.In the experiment,the number of EDM current events has been counted,and the amplitude of the capacitive-type of current has also been examined under various switching speeds and frequencies of wide-bandgap devices.Furthermore,the bearing currents are also extensively tested under various bearing temperatures,motor speeds and dc-link voltages.The correlations between the bearing currents and various factors have been revealed in this dissertation.To investigate the impacts of two grounding types on the motor overvoltages,the common-mode impedance paths with the dc-link capacitor midpoint grounding type and the heatsink grounding type have been analyzed.The difference of the two grounding types is mainly that the common-mode path of the inverter-motor drive system is different.Then,the inverter output common-mode voltage has been calculated with the two common-mode impedance model.It has been found that the motor terminal common-mode voltage and the motor stator winding neutral point commonmode overvoltage have been reduced when using the heatsink grounding type.However,the voltage overshoot of the inverter output common-mode voltage has been observed with the heatsink grounding type.This overshoot will further lead to the increased motor common-mode overvoltage.Finally,to eliminate the motor common-mode overvoltage and midpoint overvoltage,a common-mode filter has been designed.The experimental results show that both the common-mode overvoltage and neutral point overvoltage with the two grounding types have been reduced after adding the common mode filter,and the EDM bearing current has also been significantly reduced.There are 105 pictures,4 tables and 177 references in this dissertation.