Fault-Tolerant Operation of Wound Field Synchronous Machine Using Coil Switching

Abstract

This paper proposes a fault tolerant operation of a wound field synchronous machine (WFSM) under the loss of excitation (LOE) fault in the rotor field winding. A two-mode field excitation scheme is presented, where the rotor of the WFSM was modified, and an additional harmonic winding is introduced with a rotating bridge rectifier. Mode I is the conventional direct dc supply field excitation using slip-rings and brushes, whereas mode II, is the brushless field excitation under an LOE fault, resulting in an unregulated field current. During mode II, a special coil switching is performed in the stator. Consequently, the stator winding creates an additional sub-harmonic component of the magneto-motive force (MMF) in the machine air gap along with the fundamental MMF component. The additional sub-harmonic MMF is induced in the rotor harmonic winding. A rotating diode bridge rectifier mounted on the rotor periphery rectifies the harmonic winding ac, and a stable dc was supplied to the rotor field winding. The decoupling between the additional rotor harmonic winding and stator winding is analyzed. The 2-D finite-element analysis (FEA) was performed to analyze the proposed idea. The FEA results were validated by experiments based on a 1-kW prototype.