Evaluating Harmonic Distortions in Brushless Doubly-Fed Induction Machines

Abstract

The brushless Doubly-Fed Induction Machine (DFIM) is a special machine type that is of interest as a generator in wind turbine drive-trains. The brushless DFIM has a significant space-harmonic content compared to conventional machine types, due to its construction. This results in additional harmonic distortions, such as torque-ripple and time-harmonics. This paper studies the causes and origins of harmonic distortions in the brushless DFIM. Using time- and space-harmonic analysis an analytical evaluation method is derived to predict torque ripple frequencies and time-harmonic frequencies in stator and rotor voltages and currents. The evaluation method is applied to a prototype brushless DFIM to demonstrate the severity of harmonic related distortions that can be present in brushless DFIMs. The evaluation is validated by additional FE analysis and measurements. Furthermore, measures are proposed to reduce harmonic distortions. The insight into harmonic distortions in the brushless DFIM and the ability to predict and prevent them must lead to improved brushless DFIM designs in the future.