Surface damage of rolling contacts caused by discrete current flow

Abstract

In modern energy-efficient drive systems frequency converters are increasingly used to control the motor and enable optimum and adjustable operation. In the quest for ecological generation of electric energy wind turbines are an option. Modern types, like doublefed induction generators (DFIG) utilize a frequency converter to control the generator's rotor windings. In both applications the frequency converter control offers many beneficial features, like e.g. energy-regeneration capabilities and variable rotational speeds. On the downside, the converters can also cause parasitic currents to flow through the bearings of the attached electric machines. These currents may ultimately lead to bearings failure, In order to obtain better understanding of this process, test bearings are exposed to currents of variable intensity, similar to the ones found in field application. The surfaces of the rolling elements are then analysed by an Atomic Force Microscope to give insight into the damaging mechanisms. The results are a good starting point for defining relevant physical parameters causing electrical erosion in rolling hearings.