The Analysis Of Bearing Currents In Axle Box Of Electric Locomotives

The bearing current problem in the axle box of the bogie in the electric freight locomotive is researched in this thesis. Due to the high switching frequency of the power devices such as IGBT, the traction motor which supplied by power converter produced the shaft voltage and shaft current, which exerted serious effect on the lifetime of motor bearings. In order to prevent the bearings wearing, the insulated bearings have been introduced in the traction motors under the variable frequency supply in the majority circumstances. Under this condition, the shaft voltage has little influence on bearings of traction motors. However, due to the directly connecting between the gear of the motor and the axle box of the bogie, the shaft current would travel through the connection between motor chassis and axle box. If the current density of bearings in axle box surpassing the upper limit, the bearing would be damaged and the operation safety of locomotive would be broken also. Thus correct estimation to the current density of bearings in the axle box is necessary and meaningful. This thesis mainly focuses on the effects of the shaft current of traction motor on the bearings of axle box while adopting insulated bearings in traction motors.Firstly, the present research situation of the motor bearing current and bearing current density has been analyzed. Based on it, the thesis analyzes each possible circuit between the traction motor and axle box in the locomotive bogie and determines the eventual confirmable circuit in the axle box. Secondly, the couple capacitors of traction motors under high frequency in the circuit are calculated with the finite element method of electromagnetic field. Moreover, the circuit simulation model is established under the software Matlab/Simulink, and the bearing current of axle box can be obtained. The thesis also makes a profound and insight analysis of the impact of converter's operating parameters on the axle box's bearing current, such as the modulation, the carrier ratio and the dead time of the switch devices. Finally, based on the Hertz point contact theory, the thesis calculates the contact area of the bearing, and then calculates the bearing current density and bearing lifetime. Combined with the knowledge of several subjects including the electrical machine, electromagnetic fields, power electronics and FEA (Finite Element Analysis), this thesis provides the basis for the adoption of insulated bearings in axle box.