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

Doctoral thesis, 2016

The permanent magnet synchronous motor (PMSM) is an interesting alternative for an electric motor drive system in automotive applications, due to 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 a PMSM can achieve 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. It is then 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 fault-tolerant fractional slot PMSM is designed and its capability to operate 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. The fault-tolerant thesis machine design is compared with two reference machines; one existing PMSM design of the same size and a smaller prototyped PMSM, also including measurements for the later. All three machines are evaluated both during normal and during phase open circuit operation, using amethod where the remaining two phase currents are rearranged in order to get constant currents in the rotating dq reference frame. It is found that themaximum torque is reduced to approximately 58% and the maximum speed to roughly 1/2, in relation to what is achievable for each machine during normal operation. The thermal limits are investigated and shown to follow the electrical limits for the thesis design and for the prototyped IPM machine. However, when this method is used, it is noticed that the torque quality might be reduced significantly. A semi-analytical machine modeling approach is introduced and used to model the individual saturation levels of the phases. The same model is used to calculate new current wave forms in order to reduce the torque ripple during the unbalanced conditions associated with the operation during fault. Consequently, for the thesis design with reduced torque ripple, the maximum torque is reduced to 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.