Vibrational Analysis of an 2/3 Brushless Doubly Fed Induction Machine

The doubly fed induction generator (DFIG) remains one of the most popular generator to be employed in wind energy conversion systems due to its variable speed and partially rated converter. Howeever, frequent maintenance of the slip ring and brush assemblies leads to additional downtime, resulting in financial losses. The brushless doubly fed induction machine (BDFM), which places both power and control windings on the stator and uses a specialized rotor winding, is an alternative to the DFIG, offering higher reliability at the cost of reduced power density. In this article, the magnetic forces induced within the air-gap of the 2/3 (power/control winding) BDFM are investigated and an analytical model is presented. 3D finite element analysis is utilized to determine the vibrational characteristics of the machine resulting from its inherent flux unbalance that causes an unbalanced magnetic pull. Practical measurement results on a custom built 5.5 kW 2/3 BDFM are presented to validate the analytical and simulation results. The experimental results demonstrate close agreement with the presented vibrational model.