Implementation strategies for concurrent flux weakening and torque control of the PM synchronous motor

Abstract

Two implementation strategies for the concurrent control of the mutual flux linkage and torque of a PM synchronous motor is discussed. A vector control system is utilized to provide independent control over the stator current and its angle. The main objective is to maintain a 1 p.u. stator voltage at higher than 1 p.u. speeds by accurately weakening the mutual flux linkage. The maximum permissible torque command for such a system is derived analytically and is shown to be a function of the commanded mutual flux linkage and machine parameters. The first strategy is based on two lookup tables that provide the stator current magnitude and its respective angle for every viable mutual flux linkage and torque command pair. The second method is based on replacing the lookup tables with two-dimensional polynomials derived using the least squares fit algorithm. The two methods are compared in terms of implementation complexity and accuracy. It is shown, through system analysis and dynamic simulation, that the table lookup approach provides a high degree of accuracy in terms of meeting the main objective of maintaining a 1 p.u. voltage requirement at high speeds. At the same time the desired torque is also enforced on the motor. Utilizing and implementing the tables require the presence of storage devices on board, while the two-dimensional polynomial approach does not have any auxiliary storage requirement. The tradeoff comes in terms of lower performance for the polynomial approach. The effect of parameter variations on the system performance is briefly discussed.