Abstract

Permanent magnet synchronous machines (PMSMs) with rare-earth magnets are widely used by the hybrid electric and electric vehicle industry due to their high torque density and efficiency. However the increasing fluctuating prices of the rare earth magnets have triggered the search for other alternative electrical machines such as the induction machine (IM), synchronous reluctance machine (SynRM) and a variable flux machine (VFM). The SynRM and the variable flux machine have been identified as potential PMSMs replacement. This is because the control strategy for the SynRM is closer to the PMSM and that the VFM is a PMSM. Therefore the same hardware used for the PMSM drive can be used for the SynRM and the VFM.

There has also been an increasing demand for higher dc bus voltage on the electric drive train for better performance. A three level inverter has been identified as the best candidate to meet this demand. Hence the thesis presents a three level inverter neutral point clamped SynRM drive for traction applications. A new space vector modulation scheme is also proposed for the three level inverter SynRM drive. A comparison between a three and two level inverter SynRM drives was also conducted.

Due to high cost associated with EV/HEVs, a low cost high resolution position sensor is proposed for the SynRM and the variable flux machine drives. The low cost position sensor is also compared to the position sensor used in electric power steering (EPS) machine. The EPS machines are predominantly used in vehicles and the low cost position sensor is proposed as a best alternative for the position sensor used in the EPS machine due to their simpler algorithm for position information, higher resolution and cost reduction benefits.

The effect of the three level inverter supply on the core losses of the SynRM was also conducted. The SynRM experienced lower core losses when supplied from a three level inverter. The reduction in the core losses is more significant in the stator tooth which is harder to cool as compared to the stator yoke. Hence the additional benefit of a three level inverter SynRM drive is that the burden of the cooling system will be reduced and hence a reduction in the cost associated with machine cooling.

The thesis also presents a novel control strategy for a variable flux machine (VFM) which uses low cost aluminum-nickel-cobalt (AlNiCo) permanent magnets (PMs). The strategy implements field weakening for speed extension and takes into account the demagnetization characteristics of the AlNiCo magnets. The magnet flux is reduced from the armature current pulses thus eliminating the additional copper losses associated with the flux weakening current in conventional rare earth PMSMs. The performance of the core losses on the VFM are also evaluated and quantified.