Novel Induction Machines for Electric Vehicle Application

This thesis investigates the use of electronic pole-changing winding induction machines for an automotive starter-generator application. Induction machines offer the benefits of a low cost, rugged construction and high overload capability. A starter-generator requires the machine to have a high torque capability at low speeds for engine cranking operations and a wide-speed flux weakening region for providing a torque boost and for extending the region of regeneration. Induction machines that are designed for a fixed pole number that can meet both of these requirements have a large frame size and this results in poor torque density.
Electronic pole-changing windings utilise the inverter to change the number of poles. The machine is wound for the higher number of poles and the number of poles is reduced to half by reversing the direction of current in specific coils of the phase windings. To avoid cancellation of MMF and the associated copper losses, state of the art, three-phase, electronic pole-changing windings for induction machines use windings with a 120 degree phase belt. However, 120 degree phase belt windings have a fundamental winding factor below 0.85 and this results in poor torque density. When the number of poles is reduced, the winding factor degrades further. In this thesis, six-phase, electronic pole-changing windings to extend the flux weakening region of operation is investigated. Six-phase pole-changing windings can be wound with a 60 degree phase belt without the disadvantage of MMF cancellation and still retain the ability to perform the pole-changing operation. The fundamental winding factor with a 60 degree phase belt is improved by 15% and this proportionately increases the torque capability when operating with the higher number of poles, below the base speed. For operation with the higher number of poles, the six-phase pole-changing winding is symmetric, with a 60 degree phase separation between the phases. When the number of poles is halved, the resulting machine is an asymmetric six-phase machine with a 30 degree spatial separation between the phases. The enables the injection of 3rd harmonic currents to produce additional torque within the limits imposed by the inverter current and the DC bus voltage. The optimal 3rd harmonic current to maximise torque capability and its impact on the torque ripple is determined for the pole-changing winding induction machine. The performance of the three-phase and the proposed six-phase pole-changing winding machines are simulated using finite element analysis and the results are validated using experimental data on a prototype machine.
Finally, using vector space decomposition, the dynamic dq-model for a six-phase pole-changing winding is developed. The transient performance of the three-phase and six-phase pole-changing winding are compared under scalar control and indirect rotor field-oriented control.