Brushless field-excitation method for wound-rotor synchronous machines

This paper proposes a simplified harmonic field-excitation scheme for the brushless operation of wound-rotor synchronous machines (WRSMs). The proposed scheme involves two inverters that are operating at different frequencies to provide an input current to the armature winding of the machine. One of these two inverters works at a frequency of 60 Hz and provides the regular three-phase fundamental current, whereas the second inverter works at 180 Hz frequency to supply the single-phase third-harmonic to the common point of the start-connected stator winding of the machine. The proposed method has the advantage that a conventional single-phase inverter can be used for generating the third-harmonic current which only needs to be operated at three times the frequency of the first inverter supplying the three-phase fundamental current. The fundamental three-phase current generates the main armature field, and the single-phase third-harmonic current produces a pulsating magnetomotive force, that induces electromotive force (EMF) in the harmonic winding of the rotor. The induced EMF is rectified to supply field current to the rotor and generates the rotor field. This field, when interacts with the main armature field, produces torque. The proposed brushless field-excitation scheme for WRSMs is implemented in JMAG-Designer version 19.1 using 2-D finite element analysis to realize its operation and analyze its performance.