Modeling and reduction of shaft voltages in AC motors fed by frequency converters

摘要

This thesis deals with the modeling and mitigation of shaft voltages and bearing currents in frequency converter-fed AC motors. Common-mode voltages produced by modern frequency converters can cause destructive currents in motor bearings. The distribution of the common-mode voltage between parasitic capacitances in the motor causes a capacitive shaft voltage, and high-frequency common-mode currents flowing in the stator core generate a shaft-encircling magnetic flux, which causes an induced shaft voltage. Simulation models for these phenomena are needed in the evaluation of new drive system designs that could cause shaft voltage problems. Methods for the mitigation of shaft voltages can also be investigated by such models. In the thesis, the circumferential magnetic flux that causes the induced shaft voltage is investigated experimentally and by numerical analysis. Analytical expressions are derived for the magnetic flux and the common-mode impedance of the stator core. A circuit model suitable for the shaft voltage analysis is developed for induction motors. Both the capacitive and induced shaft voltages are included in the model. A correct common-mode impedance distribution of the drive system in a wide frequency band is essential in the shaft voltage analysis. Therefore, the circuit models of all drive system components are developed, and a complete model of the electric drive is composed. The model is validated by comparing the simulations to experimental results. Some promising methods for bearing current mitigation in high-power low-voltage drives are investigated in the thesis using simulations, analytical expressions, and experiments. New design principles are proposed for mitigating the shaft voltages in induction motors.