Vibrational Analysis of an Unbalanced Brushless Doubly Fed Induction Machine

The recent increase in both the adoption and size of wind energy conversion systems (WECS) has led to the investigation of new generator topologies. The doubly fed induction generator (DFIG) remains the most popular generator to be employed in WECS due to its variable speed and partially rated converter. Frequent maintenance requirements to their slip ring and brush assemblies leads to additional downtime, resulting in financial losses. The brushless doubly fed (induction) machine (BDFM) is an alternative to the DFIG, with both its power and control windings on the stator whilst using a special rotor winding. It offers higher reliability at the expense of power density. The BDFM is not a commercialized machine, and several design aspects are still under investigation. In this paper, the magnetic forces induced in the air-gap by the two sets of windings are investigated and a 2/3 (power/control winding) BDFM is analyzed with simulation results on the vibrations and deflections within the machine are presented. Experimental vibration testing is then performed on a custom built 5.5kW BDFM to validate the analytical results. It is shown that the main vibration components are due to the unbalanced flux in the machine, causing a dynamic rotor eccentricity. The results obtained from experimental tests closely matches with the presented vibrational model.