Aspects of brushless doubly-fed induction machines
This dissertation, composed of two parts, studies the brushless doubly-fed induction machine (BDFM)- an electric machine with two stator windings, a modified cage-rotor, and, when run synchronously, the electrical properties of a synchronous machine, i.e. precise speed and power-factor control. The first part of this dissertation is dedicated to the development of a two axis (dq) model to aid in design of the BDMF. It improves on the properties of similar models by allowing for the determination of the individual rotor-bar currents and the components of electromagnetic torque produced by each of the two stator windings. The model has the ability to predict both dynamic and steady-state behaviour, the latter verified by experiment and by comparison to predicted results of a different model of the BDFM. The second part of this dissertation, which is the primary focus of this dissertation, is concerned with the phenomenon of inter-bar rotor currents, those which flow between the rotor bars through the rotor iron. Experiments have shown that these currents, present in virtually all die-cast rotors for all types of induction-based machines, seriously degrade the performance of the BDFM by causing increased rotor losses, reduced efficiency, increased stator input power, and increased stator currents, especially when used as a motor. To understand this phenomenon and predict its effect on machine performance, a coupled-circuit model is developed based on harmonic analysis. It is verified through experiments using a specially fabricated rotor in which the rotor bar to rotor iron contact has been promoted. Investigations are conducted using the model to determine the minimum level of insulation needed to obtain satisfactory performance. Results show that the rotor bars must be insulated to an order of magnitude greater than that achieved in the normal die-casting process.