A general and intuitive approach to understand and compare the torque production capability of AC machines

Electromagnetic torque analysis is one of the key issues in the analysis of electric machines. It plays an important role in machine design and control. The common method described in most of the textbooks is to calculate the torque in the machine variables and then transform them to the dq-frame, through complicated mathematical manipulations. This is a more mathematical approach rather than explaining the physics behind torque production, which even brings a lot of difficulties to specialist. This paper introduces a general and intuitive approach to obtain the dq-frame torque equation of various AC machines. In this method, torque equation can be obtained based on the intuitive physical understanding of the mechanism behind torque production. It is then approved to be applicable for general case, including rotor saliency and various types of magnetomotive force sources. As an application example, torque equations of permanent magnet machine, synchronous reluctance machine and induction machine are obtained, where torque production capability of each machine may be easily understood and a fair comparison of these machine types may then be carried out.