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Analysis of Brushless Doubly-Fed Motor under Various Operating Modes

Gorgin, Hamed | 2009

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39595 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Oraee Mirzamani, Hashem
  7. Abstract:
  8. The Brushless Doubly-Fed Machine (BDFM) is a machine which incorporates the robustness of the squirrel cage induction machine and the speed and power factor control of a synchronous machine. All of these advantages are obtainable from a relatively low rating power electronic converter (e.g. 30% of the machine’s rating). The stator of a BDFM is furnished with two balanced three phase windings of conventional design (power and control winding). These windings differ in pole number and are often excited at different frequencies. Hence, there is no direct magnetic coupling between them. The power winding is connected to a constant-voltage, constant-frequency source and the control winding is excited via a fractional rated bi-directional power electronic converter. BDFM has three modes of operation depending on its winding excitation, include: simple induction mode, cascade mode and synchronous mode. The rotor of the BDFM carries a cage of special design. The essential requirement for BDFM action is that the frequency and distribution of the currents induced in the rotor by the air gap field of the first stator winding matches that induced by the second. The modeling and structural aspects of a Brushless Doubly-Fed Motor are investigated in this thesis. A dynamic model is represented to predict the machine’s behavior especially during stator faults and with unbalanced excitations. The simulation is carried out for three different faults in stator windings including, phase to neutral fault, phase to phase fault and open circuit fault. The proposed model is based on the concepts of the generalized harmonic theory of electrical machines and the modified dynamic equations of the coupled-circuit approach. To verify validation of this model, a 2-D magneto dynamic finite element model, taking into account the special rotor topology and rotation, is extracted and simulated. The simulations show good comparison between the two models results and validate the proposed dynamic model. Also, both the simulation and experimental results show the presence of fluctuations and ripples in the machine’s torque. These ripples influence the motor’s operation and its starting performance. In this research, the effects of stator or rotor slot skewing on the torque ripples is investigated by studying the cogging torque and distortions in the back electromotive force waveforms.
  9. Keywords:
  10. Finite Element Model ; Brushless Doubly Fed Machine (BDFM) ; Nested Loop Rotor ; Generalized Harmonic Theory ; Cogging Torque

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