Effects of slotting and unipolar flux on magnetic pull in a two-pole induction motor with an extra four-pole stator winding

Anssi Sinervo

    Research output: ThesisDoctoral ThesisCollection of Articles


    This thesis is about the radial magnetic forces between the rotor and stator in twopole induction machines. The magnetic forces arise from rotor eccentricity. The asymmetric air-gap makes the flux density on one side of the rotor stronger than on the opposite side. This produces magnetic pull. The magnetic flux density distribution in the air-gap can be expressed with spatial harmonics, i.e. flux densities with different pole-pair numbers. In two-pole machines, the main part of the magnetic force is produced by the interaction of two- and fourpole flux unless the four-pole flux is damped by parallel paths in the stator winding or an extra four-pole stator winding. The rest of the force comes from the interaction of two-pole and unipolar flux and from the higher harmonics of the air-gap flux of which the slot harmonics are a major part. The force caused by the higher harmonics and the unipolar flux is studied in the case where a four-pole stator winding is used to reduce the four-pole flux. The higher harmonics are found to produce, in addition to the traditional unbalanced magnetic pull, a force similar to the effect of the unipolar flux and the two can be distinguished only by measuring the unipolar flux. In measurements at various operation points, the higher harmonics are found to produce much more force than the unipolar flux and two-pole flux but the unipolar flux is still significant. The four-pole winding also is used to actively control the four-pole flux and the magnetic forces. Designing the controller requires a low order model of the system. Such a model is derived and the effect of the slot harmonics and the unipolar flux are included in the model. Different measurements techniques and methods are presented to identify and validate the control model. The operation point dependence of the system dynamics is studied via measurements. All results are obtained from a 30 kW test motor. The rotor of the test machine has a long flexible shaft on external bearings. The shaft is bowed causing mechanical forces. The vibration caused by these forces is suppressed with the four-pole extra winding. The machine also has search coils to measure the unipolar, two-pole, and four-pole fluxes and a unipolar actuator that can be used to control the unipolar flux.
    Translated title of the contributionUrituksen ja unipolaarivuon vaikutus magneettiseen poikittaisvoimaan nelinapaisella staattorilisäkäämillä varustetussa kaksinapaisessa oikosulkumoottorissa
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    • Arkkio, Antero, Supervisor
    Print ISBNs978-952-60-5154-3
    Electronic ISBNs978-952-60-5155-0
    Publication statusPublished - 2013
    MoE publication typeG5 Doctoral dissertation (article)


    • active control
    • bearingless machine
    • higher harmonics
    • homopolar flux
    • induction machines
    • rotor vibration
    • unbalanced magnetic pull
    • unipolar flux

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