| This dissertation is supported by the National Natural Science Foundation of China “Noncontact Electric Energy Transmission Mechanism of a Novel Carbon Nanotube Electrode between Rotor and Stator” under grant 51577122.The induction machines supplied by variablefrequency power source exist bearing corrosion problem due to bearing current.In view of this problem,this dissertation carries out systematic study,which includes the generation mechanism of electrical discharge machining current,the equivalent circuit and calculation scheme of bearing current,the measurement of bearing ground voltage and current,the simulation and experiment of bearing electric corrosion and the suppression of bearing current.The main research contents of the thesis include the following aspects:Firstly,the generation mechanism of bearing current of variable frequency motors is studied.Based on the lumped equivalent circuit of variable frequency speed regulation system and variable frequency motor,the generation mechanism of bearing current is investigated.The parameters of electrical discharge machining current are calculated by utilizing distributed common-mode equivalent circuit.For a specific variable frequency motor,its voltage of neutralpoint ground-to-ground voltage,voltage of shaft to ground and bearing current are measured.The above analysis and data can supply research model and theoretical basis for subsequent bearing current suppression.Secondly,the performance analysis and critical stress of bearing electric corrosion of variable frequency motor are carried out.The multi-physics coupling scheme of the electricthermal-corrosion combination is firstly proposed to analyze the electric corrosion of bearing under different common-mode voltages.The energy method is proposed to calculate discharge average electric power and discharge number.With the help of experiment device of bearing electric corrosion,the discharge average electric power and discharge number under different common-mode voltages,different switch frequencies,different temperatures of lubricating oil,and different rotating speeds are investigated.Based experiment results,the critical stress of bearing electric corrosion is ascertained.Thirdly,the field emission experiment of carbon nanotubes is carried out.Due to the disruptive discharge of bearing film caused by common-mode voltage,the non-contact electric energy transfer discharge electrode based on carbon nanotube field emission is proposed.Under air condition,the field emission model,transmission mechanism,analytical and simulation models are established.The performance analysis under different vacuity level and different distances are investigated by experiments.Finally,the suppression of bearing current of variable frequency motor is discussed.The equivalent circuit of bearing current suppression based on carbon nanotubes is constructed.The shaft to ground voltage,common-mode voltage,shaft to ground current are analyzed.The prototype of variable frequency motor with bearing current suppression is manufactured.The experiment of bearing current suppression by the discharge electrode of the variable frequency motor bearing current based on carbon nanotube field emission is carried out,and the effectiveness of the device was verified by the experimental results. |
