Brushless Wound Rotor Synchronous Machine With Third Harmonic Field Excitation Using Single Inverter

The recent fluctuating prices of rare earth magnet used in PMSM has encouraged the researchers towards the other candidates, such as, PM-assisted synchronous reluctance machine (PMa-SyRM) and wound rotor synchronous machine (WRSM). Whereas, in WRSM, the assembly of brushes and slip rings connect the machine to its excitation system on the rotor side. To get rid of brushes and slip rings, because of wear and tears in the long run, several brushless topologies have been proposed in the literature [1–3]. In [1], two inverters are used to supply different amplitude currents to two-different portion of the stator windings to generate the sub-harmonic airgap magnetomotive force (MMF). This sub-harmonic component then couples with the corresponding sub-harmonic winding (HW) on the rotor. The use of two inverters in this topology makes it less suitable for practical applications. In [2], a controllable third harmonic (TH) zero-sequence current is generated using single inverter topology, which induces the current into a dedicated rotor HW. The rotor winding current is then rectified and supplied to the main field winding. Although, this topology used single inverter, but six extra thyristor switches are needed to generate TH currents, which switches during the positive and negative half cycles to generate zero sequence currents. The switching losses and high torque ripple are the other drawbacks. In [3], a spatial TH zero sequence MMF is generated with open winding by means of two inverters. one inverter is used to supply the fundamental current and a second inverter is utilized to inject the TH current into the three-phase open stator winding terminals. A HW as well as a conventional field winding are installed on the rotor, whereas, the HW is used to induce and then rectify the magnetic field generated by the airgap TH-MMF. The field winding is used to produce the rotor main magnetic field, which then interact with stator fundamental MMF for torque production. The disadvantage of this scheme is the utilization of two inverters, which increases the cost and size of the overall system. This paper presents a new scheme to produce time generated airgap MMF having two components, i.e. fundamental and a TH, using single inverter. The time generated TH-MMF is then induced in the rotor HW. After rectification, the rotor field winding is excited to realize the brushless operation.