Novel direct field and torque control of six-phase induction machine with special phase current waveform
Thesis (PhD (Electric and Electronic Engineering))--University of Stellenbosch, 2006.
This study focuses on the drive control system of a novel direct field and torque current control applied to a six-phase induction motor. Special phase current waveforms that make it possible to have separate field and torque windings and currents in the motor are proposed. In this thesis the control of these field and torque windings to control directly the flux and torque of the motor is investigated. With the special phase current waveforms the performance of the six-phase induction motor is evaluated through theoretical and finite element analysis. In the analysis the air gap resultant field intensity and flux density produced by the stator field, stator torque and rotor currents are investigated. It is shown that with the special current waveforms a quasi-square shaped, smooth rotating air gap flux density is generated. This smooth rotating flux is important for proper induction motor operation. An equation for the electromagnetic torque is derived and used in the theoretical calculations. The ease of the torque performance calculations is conspicuous. An approximate magnetic circuit calculation method is developed to calculate the air gap flux density versus field current relationship taking magnetic saturation into account. The air gap MMF harmonics and the per phase self and mutual inductances are analysed and calculated using, amongst other things, winding functions. In the finite element analysis specific attention is given to the MMF balanced condition (zero quadrature flux condition) in the motor and the development of a per phase equivalent model. The drive system’s performance with the proposed direct control technique is verified by a developed Matlab simulation model and measurements on a small (2 kW) two-pole, six-phase induction motor drive under digital hysteresis current control. It is shown in the thesis that the calculated results from theoretical derived equations are in good agreement with finite element and measured results. This is particularly the case for the formulas of the MMF balanced constant (zero quadrature flux linkage constant) used in the control software. The results of the simulated and measured linear relationship between the torque and torque current show that MMF balance is maintained in the motor by the drive controller independent of the load condition. The direct control of the torque also explains the good measured dynamic performance found for the proposed drive.