When the shaft current passes through the bearing under lubrication condition, the arc often occurs and the pitting can be observed on the surface of bearing. Consequently, the life of bearing is shortened. The pitting resulting from discharge is dependent upon the shaft voltage, the oil film thickness, and the insulation of lubricant. To simulate the pitting, the dynamic pitting tester is developed to investigate the effects of the kinematics parameters on the electrical pitting formation mechanism for the common material of bearing by changing the supply voltage current and the oil film thickness. Result show that in the static condition, since the arc action causes the surface melting of two specimens, and the actions of coulomb force and electrostatic force cause the specimens to attract each other, the plateau can be observed on the surfaces of specimens. The plateau is like a bridge to connect two specimens. In this moment the plateau accumulates continuously and causes two specimens to produce the repulsive force. In the dynamic condition, the formation of pitting at the initial stage is quite similar to that in the static condition. Since the effect of sliding speed, the bridge is sheared and the friction force increases. Under the actions of joule heat and friction force, the surfaces of two specimens melt and scratch continuously. When the dynamic pitting occurs, the pitting width of square specimen, the normal force and the friction force increases with increasing supply voltage, supply current, and oil film thickness. When the interface power is larger, the melting phenomenon is more obvious, and the pitting width becomes larger. Because the surface melting and the actions of Coulomb force and electrostatic force cause the material accumulates continuously, the normal force and the friction force increase with increasing the interface power. To investigate all effects of experimental parameters on the pitting width, the empirical formula for the pitting width is established in terms of supply voltage, supply current, and oil film thickness. This formula can be used to predict oil film thickness or the size of pitting width on the bearing surface for diagnosing the lubricant condition of bearing.

Induction motors are commonly used in numerous industrial applications. To maintain a reliable operation of the motor, it is important to identify the potential faults that may cause the motor to fail. Bearing failures are one of the main causes of motor breakdown. The causes of bearing damage have been studied in detail for a long time. In some cases, bearing failed due to the current passing through them. In this thesis, bearing currents in an inverter driven five-phase induction motor are studied and a new solution is proposed. First, theory of shaft voltage and bearing current are presented. The causes are identified and current solutions are discussed. Then, new switching patterns are proposed for the five-phase induction motor. The new schemes apply a modified algorithm for the space vector pulse-width-modulation (SVPWM). The system is simulated and the results of the new switching patterns are compared with the conventional switching pattern. Finally, the new schemes are experimentally tested using a digital signal processor (DSP) to drive the five-phase IGBT inverter. The experimental results verified that the new switching pattern could reduce shaft voltages and bearing current without affecting the performance.

Abstract The electrical pitting often occurs at the bearing of the ro-tating machinery due to the actions of the shaft voltage and the shaft current resulting in the arcing effect on the lubricated surface and causing the bearing failure. Since the mechanism of the electrical pitting cannot be microscopically observed in process, it is difficult to prevent the bearing damage. Hence, this study uses a static electrical pitting tester with sub -micrometer accuracy to experimentally investigate the effects of supply voltage, supply current, oil film thickness, and ad-ditive on the threshold condition of electrical pitting under the conventional bearing material pairs. Moreover, according to the SEM micrograph and EDS analysis, the mechanism of the pitted surfaces is investigated. According to the experimental results and the surface ob-servations of steel/steel pair using a paraffin base oil, three electrical pitting regimes are found under the influences of shaft voltage and oil film thickness, namely, pitting, transition, and no-pitting regimes. In the electrical pitting regime, the interface voltage, interface impedance, and interface power increases slightly with increasing oil film thickness at a certain supply current. However, the interface voltage and interface power increases with increasing supply current, and the inter-face impedance decreases with increasing supply current at a certain film thickness. Furthermore, the pitting area versus the interface power relationship is a cubic function. According to the experimental results and the surface ob-servations of babbitt alloy/steel pair using a paraffin base oil, two electrical pitting regimes are found under the influences of shaft voltage, oil film thickness, and melting point of material, namely, pitting and no-pitting regimes. The mechanism of electrical pitting on the babbitt alloy surface is significantly influenced by the interface power and the oil film thickness. At the smaller oil film thickness, the eroded surface of babbitt alloy exhibits a concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding, especially at high supply current. The polished track can be observed at the plateau. A large amount of tin element trans-fers to the steel ball surface because the molten tin contacts the ball. At the higher oil film thickness, only a little amount of metal element transfers to each other. The major pitting area of the babbitt alloy is caused at the initial stage of the arc dis-charge. With increasing arc discharge time, the pitting area increases slightly, and finally reaches a saturated value. According to the experimental results and the surface ob-servations of babbitt alloy/steel pair using an additive of MoS2 in a paraffin base oil, two electrical pitting regimes are found under the influences of shaft voltage, oil film thickness, and particle concentration of additive, namely, pitting and no-pitting regimes. The area of pitting regime increases with increasing additive concentration and supply current. Fur-thermore, the ratio of pitting area to the interface power in-creases with increasing additive concentration and supply current at the oil film thickness smaller than 6 mm. However, this ratio increases rapidly to about 10 times with increasing additive concentration and supply current as the oil film thickness increases from 6 mm to 10 mm. This results from the molten plateau that directly connects two specimens, and the interface power is mainly consumed at the heating of the pla-teau and the interfacial materials. According to the above re-sults, the growth model of the plateau on the pitting surface is proposed at the lubricated condition using an additive of MoS2 in paraffin base oil.