Electrical Characterisations of Bearings

Doktorsavhandling, 2019

Mechanical bearings are an integral part of industry, and are used in various places in order to reduce friction between two interacting surfaces and are used to transmit power and loads. Mechanical bearings are one of the most extensively used components within the wind industry, but on the other hand they are also one of the most dominantly failing failed components. In order to increase the feasibility of wind energy, and to make the wind power more sustainable, a reduction in operation and maintenance cost of wind energy is important. The failures in bearings in the wind energy sector and other industries increased after the introduction of switched power electronic switches (Insulated Gate Bipolar Transistors, or IGBT) within the power converters. The reasons of early failures have been linked to the presence of a common mode voltage at the neutral of the converter and its coupling on the shaft, where the bearings are located. The system is also vulnerable to different types of bearing currents, which are discussed in this report.

A small voltage in range of 10's of volts could lead to large electric-field stress of 30 to 40 V/µm in a bearing depending on nominal film thickness at the operating point. The build-up of large electric field stresses in the bearing leads to ohmic electrical conduction through the bearing. Presently, the mitigation techniques mainly discharge the voltage across the bearing by providing a low resistance path for the flowing current using different methods, such as carbon brushes,  or shaft rings, but damages due to bearing current activity and early failures still exist. Another way to mitigate bearing currents is to use filters in the electrical connections, to obstruct or to reduce the amplitude of the bearing currents, but they fail to completely eliminate them. The use of insulating coating on surfaces of the bearing and ceramic rolling elements helps to provide a high resistive path for the current in case of DC voltage, but act capacitively and let the current pass through the bearing when high frequency circulating type bearing currents flow in the system. Nevertheless, to device a successful mitigation technique, it is important to fully understand the electrical breakdown and discharge activity within the bearing’s insulation (i.e., the lubricating film) along with electrical properties of the bearing during running conditions.

In our research, we have focused on understanding the electrical properties of the mechanical bearing at different operating conditions and elaborating it through an electrical circuit model. The components of this electrical circuit model are found out experimentally through different laboratory tests. The mechanical bearing is sometimes found to behave as an insulator of electricity and is hence characterizecharacterised by an impedance during the ‘Insulating state’ of the bearing in the model. The impedance in this insulating state is further categorized as a parallel combination of a resistor and a capacitor (parallel RC branch), which corresponds to the ‘real’ and ‘imaginary’ part of the measured bearing impedance. Furthermore, when the bearing enters in into a partial breakdown state, the voltage across the bearing is ‘discharged’, resulting in flow of current through the bearing until the voltage across the bearing again recovers. The Electrical electrical characterization characterisation of bearing lubricants has been performed in order to find out the relevant electrical properties, such as relative permittivity, electrical conductivity and electric breakdown strength at rather short gaps. The electrical behaviorbehaviour of the mechanical bearing at different operating conditions such as rotational speed, mechanical load along with magnitude, frequency and shape of applied voltage has been found out experimentally in order to understand and elucidate the electrical properties of a mechanical bearing in operation.