Study On Bearing Current In Induction Motor Drives Based On SiC MOSFET

With the promotion and application of silicon carbide power devices,inveter variable-speed drive systems is about to usher in innovation.The high switching speed and high switching frequency of the silicon carbide power device can improve the overall efficiency and performance of variable-speed drive systems,and is more suitable for the energy saving and high efficiency requirements of the new era.However,high-speed high-frequency high dv/dt will bring a series of negative effects to the system,such as motor bearing current problems.The inverter outputs high dv/dt highfrequency common-mode voltage,and the common-mode coupling path is provided by the parasitic capacitances in the motor.The coupling of the two produces commonmode current and bearing current,which seriously endangers the safe and reliable operation of the motor.In this thesis,7.5kW inveter variable-speed drive systems based on low-voltage AC asynchronous induction motor is studied.The influence of highspeed high-frequency high dv/dt silicon carbide inverter on bearing current and the potential hazard to bearing are deeply explored.Firstly,the common mode coupling mechanism of inveter variable-speed drive systems is derived and analyzed step by step.The common mode excitation and response model is proposed.The reason for the common mode current is strictly proved theoretically.The analytical expression of common mode excitation is deduced and the harmonic analysis is carried out.The parasitic common mode coupling capacitance and its common mode coupling path and the impedance characteristics of deep groove ball rolling bearing are analyzed.Based on the common mode excitation and response model,the lumped parameter common mode coupling equivalent circuit is derived,and the formation mechanism and damage of each type of bearing current are analyzed in detail.Secondly,the bearing current measurement method is studied,and the end cover modification scheme of the embedded installation of insulation layer is proposed.A set of 7.5kW inveter variable-speed drive systems about bearing current research and test platform is designed and built.A quantitative processing method for the statistics and evaluation of bearing current measurement results is proposed,and the possible deviation between the actual measured value and the theoretical true value is analyzed.It is pointed out that the bearing current measurement value should be only about 2/3 of the true value.Finally,a comparative experiment was designed to qualitatively and quantitatively explore the influence of various factors of inveter variable-speed drive systems on the bearing current and the potential hazard to the bearing.Through a large number of tests,it is found that the grounding method has limited influence on the bearing current,and the number of breakdowns of the bearing lubricant film per minute increases sharply with the increase of bearing temperature and DC voltage,and it increases exponentially.However,the influence of the motor speed on the breakdown and discharge behavior of the bearing lubricant film is strange,and there is a sudden change at the medium speed.In addition,the study found that the high switching speed only has a negative impact on the common mode current and the dv/dt type capacitive bearing current,and has no effect on the breakdown and discharge behavior of the bearing lubricant film.Compared with the high switching speed,the high switching frequency has a more serious negative impact on the common mode current and the dv/dt type capacitive bearing current,but only slightly affects the breakdown of the oil film.The test is performed at 10~100kHz.In the range,as the switching frequency increases,the average number of oil film breakdown discharges per minute increases monotonically linearly after a monotonic linear rise,reaching a maximum at about 60 kHz,which is about 3.4 times and 1.9 times higher than 10 kHz and 100 kHz,respectively.A reasonable explanation is given for the above test results,but its accuracy is still to be further studied and verified.