A Novel Field Current Estimation Method for Brushless Wound-Field Synchronous Machine

Compared with other electric machines, wound-field synchronous machines (WFSMs) have the advantage of having three control variables (stator $d$ - and $q$ -axis currents and rotor current). Determining the optimum combination of field and armature currents is necessary for ensuring the high-performance control of the WFSM. Two methods are usually employed to supply the field winding of the WFSM. The first one uses brushes and slip rings, while the second one uses a brushless excitation system. Concerning the WFSM with a brushless excitation system, a dc field current is provided by a rotating diode bridge rectifier fed by an exciter machine. Therefore, direct measurement of the field current is not possible. As a solution, field current needs to be accurately estimated. Previously, field current estimation methods proposed in the literature have been based on modeling the brushless exciter machine and the rotating rectifier. Using the brushless exciter machine model, these methods estimate electric quantities of the exciter rotor (currents and voltages); then, through the rotating rectifier model, the rectified field current is estimated. Accordingly, any inaccuracy in the models leads to estimation errors. The effectiveness of the techniques relying on the brushless excitation system model is not clearly justified, especially because the nonlinearity of the brushless system has been intensified by the existence of the rotating uncontrolled diode bridge rectifier. This article proposes an accurate online field current estimation method for WFSMs without the need neither to estimate exciter rotor voltages or currents nor to use a rotating rectifier model. The accuracy and effectiveness of the proposed field current estimation method are demonstrated by experimental validation tests.