Mechanical Flux Weakening Methods for the Achievement of a Very Wide Constant Power Speed Range in Automotive Applications

Permanent magnet machines are widespread in the automotive industry, due to their high torque density and high efficiency. In automotive applications such as integrated starter alternators, a very wide constant power speed range is also required for electricity generation onboard. To maintain a constant voltage, flux weakening must be employed during generation. With the conventional electrical flux weakening methods, considerable amounts of current would have to be consumed to weaken the strong flux linkage from the magnets, which results in an efficiency decrease. Conversely, mechanical methods adjust the linked flux by manipulating the position of certain machine parts, avoiding the consumption of current. Thus, mechanical flux weakening might be more suitable for automotive applications, where a very wide constant power speed range is required while preserving high levels of motoring torque capabilities. In the present literature review, various implementations of mechanical flux weakening, from several authors, are analyzed and compared. The aim is to gain insight on the effectiveness of this method, in comparison with the electrical one, on fulfilling the mentioned automotive requirements.