Design of a fault-tolerant fractional slot PMSM for a vehicle application

Licentiate thesis, 2014

In automotive applications, the PMSM is an interesting alternative due to the high efficiency requirement; as only a limited amount of energy can be stored in the relatively expensive battery. Another advantage is the high torque and power density since it is very important to save especially space but also weight in vehicle applications. As the battery technology develops, pure electric cars are expected to become a more and more interesting alternative. In case of pure electric vehicles, it is natural that the requirement of reliability on the electric drive system becomes an aspect of utmost importance as the electric drive is the only driving force. In this thesis a three phase fractional slot PMSM is designed and its possibility to operate and to deliver an acceptable quality of performance even after a fault occurs is investigated. The faults investigated are phase open circuit and phase short circuit, and it is shown that the thesis machine design can be operated during both circumstances. During normal operation, the fault-tolerant machine design shows a similar performance as an existing design of the same size, where fault-tolerance was not considered in the design. During fault, the maximum torque is reduced by approximately 50 % and the maximum speed is reduced to roughly 1/3 or 1/4 of the maximum speed in case of a phase open circuit fault or a phase short circuit fault respectively. A semi-analytical machine modeling approach are used to model the individual saturation levels of the phases successfully. The same model is used to calculate new current waveforms that are used to reduce the torque ripple during the unbalanced conditions associated with the operation during fault. Further, a design procedure considering the machine design performance in relation to its expected material cost is used and presented.