Ways to Improve the Reliability of Bearings in Traction Induction Machines

The problem of electric rolling stock traction machinery failures caused by electrical erosion of bearings is addressed. Damages inflicted to traction machinery bearings as a result of electric current flowing through them have been known since long ago. In recent time, however, damages of this kind have become more frequent, because frequency and voltage converters for control of three-phase inductor motors are widely used in the electric rolling stock. An overview of factors causing the occurrence of stray currents generated in frequency converter driven traction motors and sources of these currents is presented. Electric current flows through rolling bearings in the area of the contact between the rolling elements with inner and outer rings, thus causing damage to the bearing metal components' rolling surfaces and degrading the quality of lubricants. To study and analyze the currents through bearings, the stray capacitances existing in traction machines are determined, and their calculation methods are defined, the necessity of which is stemming from the fact that these capacitances influence the generation of parasitic currents. The stray capacitances in traction machines include the capacitance between the stator windings and the stator core pack, the capacitance between the stator windings and the rotor, the capacitance between the rotor and the stator core pack, and the capacitances of the bearings. From the electrical point of view, bearings may also be regarded as capacitances. As long as there is a continuous lubricant film between the inner ring and outer rings and the rolling elements, the bearings behave as capacitors. A difference of potentials across the bearing elements may lead to electrostatic discharges and, hence, to discharge currents through the bearings. One of possible ways to enhance the reliability of traction machine bearing units is to use insulated bearings. The change in the impedance of bearings with ceramic coating and hybrid rolling elements depending on the applied voltage frequency and operating temperature is calculated and analyzed. Conclusions regarding the expediency of using hybrid bearings having rings made of bearing steel and rolling elements made of bearing silicon nitride (Si3N4) are drawn.