In an open-winding machine, three-phase stator currents can be controlled such that the input armature currents may contain the third-harmonic current component in addition to the fundamental. Considering this attribute of open-winding patterns, a harmonic current field excitation technique for a wound rotor synchronous machine (WRSM) is proposed in this paper based on the control of time-harmonic magneto-motive force. Two inverters connected to both terminals of the stator winding are controlled so that the input armature current generates an additional third-harmonic current component. This third-harmonic component generates a vibrating magnetic field that induces a current in the specially designed rotor harmonic winding. The current is supplied as DC current to the rotor excitation winding to generate a rotor field by using a full-bridge diode rectifier in order to achieve brushless operation. The proposed dual-inverter-controlled brushless operation for a WRSM is executed in ANSYS Maxwell using 2-D finite element analysis to validate its operation and electromagnetic performance.

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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.INDEX TERMS Brushless excitation, coil switching, fault-tolerant operation, wound field synchronous machine.The associate editor coordinating the review of this manuscript and approving it for publication was Shen Yin.

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