Influence of slot wedge insulation material on bearing currents

Abstract

As the use of variable frequency drive systems has become widely used, bearing currents due to the high-frequency common-mode voltage has become a major reliability issue in electric motors. The bearing voltage ratio (BVR) is an important parameter used in determining the risk of such capacitive bearing currents which is ultimately determined by the network of parasitic capacitances within the motor. This thesis attempts to examine the effects of the stator slot insulation material, namely its dielectric property, on the BVR in an electrostatic view. In this work, a series of motor models with the same geometry and different insulation permittivity is simulated using two-dimensional finite element analysis (FEM) to isolate important parasitic capacitances, including the winding-to-rotor capacitance. This would then be used to calculate the BVR across varying dielectric constants to determine a systematic impact of insulation material characteristics. The findings illustrate that the permittivity of the slot insulation makes the distribution of parasitic capacitance to vary, resulting in the measurable changes in BVR. It is a quantitative study of the correlation between the properties of insulation materials and BVR and the discussion of technical feasibility of reducing bearing currents by choosing materials. Moreover, the practical implications in relation to the design of the motor and the possible material-based optimization approaches are discussed. The study offers a material-based view of the perception and suppression of bearing current risks, which has a theoretical perspective of the design and design recommendations on making motor-reliable during the design stage.