Dynamic Current Reference Determination of Electrically Excited Synchronous Machines Based on Torque Gradients of Copper Losses

Electrically excited synchronous machines (EESMs) have become an attractive solution to electric vehicles. The excitation of the machine can be regulated by adjusting the field current. This introduces one additional degree of control freedom. The control of the armature current in the stator and field current in the rotor is expected to not only reduce the total losses but also dynamically redistribute the load on both windings so that neither of them overheats. To realize this function, an algorithm is proposed in this study to dynamically determine the current references in EESM torque control. A cost function is introduced by placing weights on stator and rotor copper losses separately. The weights can be adjusted dynamically. The cost function is minimized by moving the current reference vector. The moving of the vector is orthogonally decomposed into a torque-related component and a cost-related component. When current or voltage limits are reached, a cancellation technique is activated to constrain the reference vector. It is shown in experimental results that expected performance is achieved in all operating conditions.