Investigation of rotor-current distributions in brushless doubly-fed machines

The brushless doubly-fed machine (BDFM), which is a candidate for adjustable-speed drive and variable-spaced generator systems, has a unique rotor configuration which is the enabling feature of its synchronous behavior over wide speed ranges. Operating experience with laboratory and prototype machines has indicated that, although the rotor conductors have had equal cross-section, the outer loops of the rotor nests experience higher temperatures. Hence uneven current distribution in the loops is indicated. This problem is investigated in three ways: an experimental machine with wound loops enabling measurement of the currents; a new two-axis mathematical model with multiple rotor circuits for current prediction; a three-dimensional, eddy-current, finite element simulation for corroboration. Data from these three approaches are compared and the relative value of the predictive techniques is assessed. It is shown that the rotor current distribution is a function of several operational variables including speed and torque (which are load specified), and power factor (which is adjustable in the BDFM). Recommendations for rotor-design guidelines are determined as a result of the investigation