Single Inverter Based Novel Stator Winding Scheme for Brushless Operation of Wound-Field Synchronous Machine with Improved Starting Torque

Abstract

Efficient, torque-dense, and cost-effective electrical machines are preferred to be used in most of the applications of electrical-mechanical energy conversion processes. Permanent magnet synchronous machine (PMSM) is one of these machines meant to be highly efficient and with high torque density. However, on the factor of cost-effectiveness it has significantly changed over the past few decades. Price fluctuation in the high energy magnetic material used for manufacturing the permanent magnet has let to alternatives such as wound-field synchronous machine (WFSM) and PM-assisted synchronous reluctance machine (PMa-SynRM). Both WFSM and PMa-SynRM are cost-effective with comparable performance to a PMSM. However, WFSM is generally preferred due to its permanent magnet (PM) less rotor structure which uses field winding for field flux control through direct current (DC) source through slip rings/brushes. WFSMs have more degrees of freedom for machine control (Id, Iq, and If), no PM losses, improved safety through direct field control during inverter fault conditions, and no de-magnetization problem. However, conventional WFSMs have an inherent problem associated with the rotor brushes and slip rings. In the long run, the brushes usually wear out from continuous operation of the machine, creating a maintenance problem. In this regard, several brushless excitation topologies have been investigated which are proposed in the literature. Brushless operation can be carried out by means of a direct wireless power transfer to the field winding of the machine, by additional exciter mounted on the machine system or by transferring harmonic power from the stator of the machine. In this thesis, a novel stator winding scheme is proposed to generate an additional sub-harmonic magneto-motive force (SH-MMF) component for brushless operation of a WFSM. The existing sub-harmonic generation schemes for brushless operation use a dual inverter, or a single inverter with asymmetrical winding and low slot fill factor in half of the stator rendering the machine with low power density. To counter this problem, the proposed scheme is designed to work with single inverter with symmetrical stator winding distribution and the same fill factor for all stator slots. The generated additional SH-MMF component in the proposed scheme is induced in the harmonic winding wound on the machine rotor. A rotating bridge rectifier mounted on the rotor periphery connects the rotor harmonic winding with the rotor field winding, and rectified DC is supplied to the rotor field winding. With the proposed topology, different machine structures were used for variable speed analysis with and without PM-assisted rotor structure. To reduce the usage of PM volume, consequent-pole rotors of different PM shapes were investigated. Additionally, a fault-tolerant scheme was also analyzed to investigate the working of the machine in case of total loss of excitation in the brushless operation of the machine. 2-D finite-element analysis (FEA) was performed on all the aforementioned machine structures to analyze the proposed idea. The FEA results were validated by experiments based on a 1-kW prototype. The results were also compared with the conventional BL-WFSM topologies. It is obtained that the proposed machine topology exhibits greater efficiency, improved starting torque, high average torque, and improved torque density compared to the previous sub-harmonic excitation schemes available in literature.