Investigation of suitable vector control techniques for low voltage IPM machine in 42 V system

Higher efficiency and smaller size of the IPM (interior type permanent magnet) machine, make it very attractive for various automotive applications including vehicle propulsion and integrated starter alternator (ISA). Although the design of IPM machine for extended speed operation itself poses many design challenges, the success of the complete drive system depends on the availability of a control algorithm that can extract the most efficient performance from the machine. In the 42 V power-net system of future automobiles, the IPM machine may have rated voltage as low as 20 V. On the other hand, because of the practical constraint of some applications such as ISA, the IPM machines can have characteristic current (the ratio of magnet flux linkage to d axis inductance) less than its peak current rating. In such a machine, available vector control techniques fail to extract the best performance capability of the machine. In this paper a comparative study has been done between two most common vector control techniques for a low voltage IPM machine that have characteristic current less than its peak rated current in 42 V system. The difficulties and shortcomings of both methods are studied using current trajectory in d-q plane.