Bearing currents in inverter fed AC motors

Modern fast switching IGBT-inverters (I nsulated Gate Bipolar T ransistors) allow high dynamic operation of variable speed drives while leading at the same time to energy savings. However, due to the steep voltage surges, bearings may suffer from inverter-induced bearing currents. These bearing currents may destroy bearings - depending on the system - within short time of operation. So far, research has focused on the physical explanation of the different bearing current phenomena. Here, a thorough analysis of the qualitative and quantitative influence of different parameters under exactly the same conditions on motors from less than 1 kW up to 500 kW rated power was carried out within the frame of a research project. The system parameters studied are motor size, motor speed and bearing temperature, design of motor and inverter (components from different manufacturers), motor cable type and motor cable length, filter operation and operation with use of insulated bearings and with use of hybrid bearings. The results obtained show that the significance of the different bearing current phenomena vary with different motor sizes and grounding configurations. As a consequence, mitigation techniques applied to reduce or eliminate the bearing currents have to be chosen for different drive systems selectively. In addition, a series of tests for bearing damage assessment was carried out to obtain a better understanding of the mechanisms of damage of the bearing. Despite manifold investigations, the phenomena of the generation of corrugation cannot be explained with today’s understanding. However, the deterioration of the bearing grease was found to correlate with the stress on the bearings. A parameter “W ” that is proportional to an energy was introduced to quantify the stress. Furthermore, models that are based on the design parameters are proposed to give physical explanations for the measured correlations of the parameters of a drive system and their impact on the bearing current phenomena. The models identify the parameters that are sensitive for the occurrence and the magnitude of the bearing current phenomena. However, detailed modeling may not always be applicable with practical applications in the field. Here, many parameters may be unknown. Therefore, a flowchart to estimate the endangerment due to inverterinduced bearing currents is proposed, where detailed knowledge of the different design parameters is not available. This flowchart might serve as a tool for engineers to assess the endangerment of a drive system due to inverter-induced bearing currents. The flowchart also summarizes possible mitigation techniques to prevent bearing damage.