Impact of inter-bar currents on the performance of the brushless doubly fed motor

The brushless doubly fed machine has been proposed as an alternative adjustable-speed drive because of its potential for reducing the size of the necessary power electronic converter. The motor's economic viability may depend upon the ability to die cast the rotor, bringing the cost of production in line with that of an induction machine. Recent industrial experience has shown that when the rotor is die cast, without any special precautions for insulating the bars from the rotor iron, the performance is degraded with respect to a fabricated rotor with insulated bars. This performance degradation is shown to be caused by the presence of inter-bar rotor currents. This paper presents a coupled-circuit model which is able to simulate the performance of the motor, including the presence of the inter-bar currents. The model describes the physical structure of the windings and the rotor in terms of complex harmonic impedances. Both measured results and predicted results are presented. In addition, this model enables investigation of the decreased performance obtained with varying values of inter-bar resistance.