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Design and evaluation of brushless doubly-fed electric machines
Botha, S. 2025. Design and Evaluation of Brushless Doubly-fed Electric Machines. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/5b524c36-22a0-496f-b2a6-9e60877898a5
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Stellenbosch : Stellenbosch University
2025
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| _version_ | 1867613893997625344 |
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| access_status_str | Open Access |
| author | Botha, Stefan |
| author2 | Gule, N. |
| author_browse | Botha, Stefan Gule, N. |
| author_facet | Gule, N. Botha, Stefan |
| author_sort | Botha, Stefan |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Botha, S. 2025. Design and Evaluation of Brushless Doubly-fed Electric Machines. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/5b524c36-22a0-496f-b2a6-9e60877898a5 |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/132081 |
| institution | Stellenbosch University (South Africa) |
| last_indexed | 2026-06-10T12:43:23.129Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2025 |
| publishDateRange | 2025 |
| publishDateSort | 2025 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/132081 Design and evaluation of brushless doubly-fed electric machines Botha, Stefan Gule, N. Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. Electric generators -- Alternating current Electric motors, Brushless Induction generators UCTD Botha, S. 2025. Design and Evaluation of Brushless Doubly-fed Electric Machines. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/5b524c36-22a0-496f-b2a6-9e60877898a5 Thesis (PhD)--Stellenbosch University, 2025. ENGLISH ABSTRACT: The doubly fed induction generator (DFIG) is the most widely used generator in wind turbines, primarily due to its advantages such as variable-speed operation and the use of a partially rated converter. However, its main drawback lies in its well-documented reliability issues, stemming from the slip ring and brush assembly used to access the rotor windings. Permanent magnet synchronous generators (PMSGs) does address these reliability concerns, but require expensive rare earth magnets and fully rated converters. Brushless doubly fed machines (BDFMs) have been proposed as a potential replacement for DFIGs, particularly in remote locations, due to their expected superior reliability from the elimination of slip ring and brush assemblies. Several types of BDFMs have been explored in the literature, including: the Brushless Doubly Fed Induction Machine (BDFIM), the brushless doubly fed reluctance machine (BDFRM) and the rotary transformer coupled DFIG (RT-DFIG). The BDFIM and BDFRM are typically medium-speed machines with both sets of windings located on the stator. In contrast, the RT-DFIG is a conventional DFIG that uses a rotary transformer to access the rotor windings, replacing the slip rings and brushes. Despite their promise, BDFMs remain at an experimental stage, and there is no consensus on the optimal design process or implementation. Key challenges persist such as rotor topology selection, potential vibrations, manufacturing difficulty and the determination of robust control strategies. Current BDFM topologies generally result in a complex drivetrain with lower power density compared to DFIGs, which presents additional challenges for their adoption in large-scale wind energy applications. In this study, a detailed vibrational analysis is introduced to address issues of unbalanced magnetic pull (UMP) and its mechanical implications. The magnetic forces within the air gap of a 2/3 (power/control winding) BDFIM are analyzed, and using 3D finite element analysis for increased accuracy in skewed machines, the inherent flux unbalance and its corresponding UMP is examined. Experimental results from a custom-built 5.5 kW 2/3 BDFIM validate the analytical and simulation findings. An alternative to the BDFMs is also explored, replacing the slip ring and brush assembly with a three phase rotary transformer (RT). The RT-coupled DFIG design is introduced and assessed in both rotor-tied and stator-tied configurations for a commercial DFIG and a custom-designed rotor-tied DFIG. While the rotor-tied configuration demonstrates significant promise, its commercial viability is hindered by several challenges such as reduced efficiency, higher costs, increased weight, and added complexity. This study presents a comprehensive approach to advancing the design and implementation of BDFMs, with a specific focus on the 4/6 BDFIM and BDFRM as viable alternatives to the DFIG. Key improvements include simplifying the design process utilizing analytical methods such as a simplified D2L approach, and using optimization techniques, such as NSGA-II, to further improve power density and reduce torque ripple. The study emphasizes the critical role of rotor selection, proposing the nested loop rotor for the BDFIM and ducted reluctance rotor for the BDFRM to enhance efficiency. This design methodology is applied to both the BDFIM and BDFRM for operation as grid-connected generators, with an emphasis on achieving unity power factor to ensure grid code compliance. Prototypes were designed to fit within a D225M frame and experimentally validated. Challenges in manufacturing the nested loop rotor resulted in sub-optimal performance for the BDFIM. However, the BDFRM, featuring a ducted reluctance rotor, exhibited promising results, highlighting its potential as a viable replacement for the DFIG. AFRIKAANSE OPSOMMING: Die dubbel gevoede induksiegenerator (DFIG) is die mees gebruikte kragopwekker in windturbines, hoofsaaklik as gevolg van sy voordele soos veranderlike spoed en die gebruik van ’n gedeeltelike gegradeerde omsetter. Die grootste nadeel daarvan lˆe egter in sy goed gedokumenteerde betroubaarheidskwessies, wat spruit uit die sleepring en borselsamestelling wat gebruik word om toegang tot die rotorwikkelings te kry. Permanente magneet sinchrone kragopwekkers (PMSG’s) spreek wel hierdie betroubaarheidskwessies aan, maar vereis duur seldsame aardmagnete en volledig gegradeerde omsetters. Met aflandige windkraginstallasies wat na verwagting aansienlik sal toeneem in die komende jare, word die aanspreek van hierdie uitdagings al hoe meer krities. Borsellose dubbelgevoede masjiene (BDFM’s) is voorgestel as ’n potensi¨ele plaasvervanger vir DFIG’s, veral in afgele¨e plekke, as gevolg van hul verwagte voortreflike betroubaarheid van die uitskakeling van sleepring- en borselsamestellings. Verskeie tipes BDFM’s is in die literatuur ondersoek, insluitend: die Borsellose Dubbel Gevoed Induksiemasjien (BDFIM), die Borsellose Dubbel Gevoed Reluksiemasjien (BDFM) en die Roterende Transformator Gekoppelde DFIG (RT-DFIG). Die BDFIM en BDFRM is tipies mediumspoed masjiene met albei stelle windings wat op die stator gele¨e is. Daarteenoor is die RT-DFIG ’n konvensionele DFIG wat ’n roterende transformator gebruik om toegang tot die rotorwikkelings te kry, wat die sleepringe en borsels vervang. Ten spyte van hul belofte, bly BDFM’s in ’n eksperimentele stadium, en daar is geen konsensus oor die optimale ontwerpproses of implementering nie. Sleuteluitdagings duur voort, soos topologiekeuse, vibrasies, vervaardiging en beheer. Huidige BDFM-topologie¨e lei gewoonlik tot ’n meer komplekse dryfstelsel en laer kragdigtheid in vergelyking met DFIG’s, wat addisionele uitdagings bied vir die aanvaarding daarvan in grootskaalse windenergietoepassings. Hierdie studie bied ’n omvattende benadering om die ontwerp en implementering van BDFM’s te bevorder, met ’n spesifieke fokus op die 4/6 BDFIM en BDFRM as lewensvatbare alternatiewe vir die DFIG. Sleutelverbeterings sluit in die vereenvoudiging van die ontwerpproses deur analitiese metodes soos ’n vereenvoudigde D2L-benadering, en die gebruik van optimeringstegnieke, soos NSGA-II, om kragdigtheid verder te verbeter en wringkragrimpeling te verminder. Die studie beklemtoon die kritieke rol van rotorseleksie, en stel oplossings voor soos die geneste lus en kanaal-reluktansie rotors om ho¨er doeltreffendheid en betroubaarheid te bereik. Hierdie ontwerpmetodologie word toegepas op beide die BDFIM en BDFRM vir werking as roostergekoppelde kragopwekkers, met die klem op die bereiking van eenheidskragfaktor om roosterkodevoldoening te verseker. Prototipes is ontwerp om binne ’n D225M raam te pas en ksperimenteel bekragtig. Uitdagings in die vervaardiging van die geneste lusrotor het gelei tot sub-optimale werkverrigting vir die BDFIM. Die BDFRM, met ’n geleide reluksierotor, het egter belowende resultate getoon, wat sy potensiaal as ’n lewensvatbare plaasvervanger vir die DFIG beklemtoon het. ’n Gedetailleerde vibrasie-analise word bekendgestel om kwessies van ongebalanseerde magnetiese trek (UMP) en die meganiese implikasies daarvan aan te spreek. Die magnetiese kragte binne die luggaping van ’n 2/3 (krag/beheer winding) BDFIMword ontleed, en met behulp van 3D eindige element analise vir verhoogde akkuraatheid in skewe masjiene, word die inherente vloed onbalans en die ooreenstemmende UMP daarvan ondersoek. Eksperimentele resultate van ’n pasgemaakte 5.5 kW 2/3 BDFIM bevestig die analitiese en simulasiebevindinge. ’n Alternatief vir die BDFM’s word ook ondersoek, wat die sleepring en borselsamestelling vervang met ’n driefase roterende transformator (RT). Die RT-gekoppelde DFIG-ontwerp word bekendgestel en beoordeel in beide rotor-gebind en stator-gebind konfigurasies vir ’n kommersi¨ele DFIG en ’n pasgemaakte rotor-gebind DFIG. Terwyl die rotor-gebind-konfigurasie aansienlike belofte toon, word die kommersi¨ele lewensvatbaarheid daarvan belemmer deur verskeie uitdagings soos verminderde doeltreffendheid, ho¨er koste, verhoogde gewig en bykomende kompleksiteit. Doctoral 2025-05-22T12:33:15Z 2025-05-22T12:33:15Z 2025-03 Thesis https://scholar.sun.ac.za/handle/10019.1/132081 Stellenbosch University xxi, 132 pages : illustrations application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Electric generators -- Alternating current Electric motors, Brushless Induction generators UCTD Botha, Stefan Design and evaluation of brushless doubly-fed electric machines |
| title | Design and evaluation of brushless doubly-fed electric machines |
| title_full | Design and evaluation of brushless doubly-fed electric machines |
| title_fullStr | Design and evaluation of brushless doubly-fed electric machines |
| title_full_unstemmed | Design and evaluation of brushless doubly-fed electric machines |
| title_short | Design and evaluation of brushless doubly-fed electric machines |
| title_sort | design and evaluation of brushless doubly fed electric machines |
| topic | Electric generators -- Alternating current Electric motors, Brushless Induction generators UCTD |
| url | https://scholar.sun.ac.za/handle/10019.1/132081 |
| work_keys_str_mv | AT bothastefan designandevaluationofbrushlessdoublyfedelectricmachines |