Copper loss minimizing field current control for wound synchronous machines

The copper loss minimizing torque control method in the field weakening region was considered for wound synchronous machines (WSMs). In general, the maximum voltage should be utilized to minimize the stator current under the assumption that the power factor is equal to unity. Therefore, the current minimizing solutions are often found at the intersection of torque and voltage curves. However, if the field current changes in WSM both voltage limit and torque curves change. It makes problem complicated to obtain an analytic minimizing solution. In this work, a hybrid approach is suggested: The Ferrari´s method offers analytic general solutions for fourth-order polynomial, and it is utilized to find the intersection between voltage limit and torque curves. In the cost function, the field winding copper loss is included, as well as the stator copper loss. A search algorithm is proposed to find a local minimum with a second-order curve fitting. The results show that the power factor changes significantly with field current and that the stator copper loss is minimized when the power factor is unity. But the total loss is minimized at a slightly lower unity power factor point. The field current control block carries a strong filter for torque command reflecting a long time constant of field current system. Time domain simulation and experiment are carried out for a 60kW WSM to show the usefulness of the algorithm.