Copper-Loss-Minimizing Field Current Control Scheme for Wound Synchronous Machines

A copper-loss-minimizing torque control method is considered for wound synchronous machines (WSMs) in the field-weakening region. In general, the current-minimizing solutions are often found at the intersection of the torque and voltage curves. However, those curves change depending on the rotor field current in WSM. This complicates the problem of obtaining an analytic loss-minimizing solution. In this study, a hybrid approach is suggested: analytic method, iterative computation, and curve fitting. The Ferrari's method is repeatedly applied for each field current to find intersections between the voltage limit and torque curves. Then, a loss-minimizing current set is found at the minimum of a second-order function fitted to three Ferrari's solutions. At each step, inductance changes are reflected. In the cost function, the field-winding copper loss is included along with the stator copper loss. The results show that the total loss is minimized at a point where the power factor is slightly lower than unity. Simulations and experiments were carried out for a 60-kW WSM to show the usefulness of the algorithm.