Analytical Study of Rotor Eccentricity Effects on Brushless Doubly Fed Machines Vibration

The Brushless Doubly Fed Machine (BDFM) with high reliability and robust structure demonstrates commercial and technical advantages both as a generator and motor for variable speed applications. As a generator it is particularly attractive to be used in offshore wind turbines where reliability improvement and maintenance cost reduction are the key factors in market growth. As a motor it may be utilized for adjustable speed drives. In this work, an analytical study has been performed on the BDFM's vibration due to the interaction of its fundamental magnetic fields, exerting bending forces in the back iron. The effects of rotor eccentricity on exacerbating the machine's vibration have been considered by assessing the stator back iron displacement function in the presence of rotor eccentricity. A prototype 250 kW BDFM built in frame size D400 was tested at different operating conditions in order to examine its noise and vibration levels. A set of measurements was conducted to assess the main vibration component frequencies developed by the machine at different rotor speeds. It is shown that the main vibration components are created by bending set-up in the back iron, rotor eccentricity, and the components with time and space harmonic natures. The results obtained from experimental tests agree with the analytical theory of BDFM vibration.