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

Abstract

The brushless doubly-fed machine (BDFM), which is a candidate for adjustable-speed drive and variable-speed 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: a n 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, f i i t e 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.

Brushless doubly fed induction machine as a variable frequency transformer

“…This problem has been investigated in Ref. [18] which cause unequal current distribution between different rotor bars yielding unequal rotor heat distribution. Equal current distribution can be tackled by using rotor bars with different cross sectional area, larger cross sections for outer loops and lower cross sections for inner loops.…”

“…The machine is modeled using conventional dq model [16][17][18][19][20]. The machine modeling commences with inductance calculation using modified winding function [15] and based on machine dimensions then the corresponding dq-model is presented.…”

“…To simplify the machine model, to reduce the number of equation, and to eliminate the inductance dependency on rotor position, dq-model is obtained from the phase model through suitable transformations [16,18]. The final dimension of the resulting inductance matrix will be 14 × 14 but with constant coefficients.…”

“…A prototype machine is then designed [13,14] and its inductances are calculated using modified winding function [15]. Based on suitable transformations, the dq-model [16][17][18][19] for this machine is then presented and used to simulate the machine in different operational modes.…”

Stable Operation of the Brushless Doubly-Fed Machine (BDFM)

“…The open-loop instability of the BDFM was investigated in [11], but it was shown in [12] that a closed-loop control system is essential for stable operation over wide speed range. Roberts et al in [13] presented two stabilizing control algorithms for the BDFM, but results were shown only in simulation.…”