Investigation of torque production and control strategy for variable flux reluctance machines

• Main Author: Lee, Beomseok
• Other Authors: Zhu, Zi-Qiang
• Published: University of Sheffield 2016
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.701764

This thesis is focused on the torque production and control strategies of variable flux reluctance machines (VFRMs). Based on energy conservation, an instantaneous torque equation is developed and analysed, which reveals the contributions of harmonic currents and inductances to the torque production. By using Fourier series, all of the harmonics in the phase currents and winding inductances are considered. It was found that the average torque of a 6/4 VFRM is mainly produced by the dc, fundamental and second harmonic components of the phase currents. Based on the derived torque equation, magnitudes and phases of the current harmonics are optimised so that the average torque is maximised under a given rms current. Undesirable level of torque ripple was found, due to the inductance harmonics. The third harmonic torque ripple is dominant in the 6/4 VFRM, whilst the 6/7 VFRM has a lower magnitude sixth harmonic component. In order to reduce the torque ripple, a harmonic field current is injected. Since the torque ripple reduction method utilises the machine parameters, the influence of the parameter mismatch is investigated. The torque waveforms predicted by the derived torque equation are compared with the results directly calculated by 2D-finite element analysis (FEA). Additionally, the experimental results verify that the derived torque equations can predict the torque production of the VFRMs to a good degree of accuracy. In order to improve the machine efficiency and extend the operating speed range, an integrated field and armature current control strategy has been proposed. The field and armature currents are injected into a single coil as a sinusoidal current biased by a dc offset, rather than a separate field and armature winding. In the integrated current control scheme, a zero sequence current generates a virtual rotor flux as the field current, whilst the three-phase sinusoidal currents produce a rotating stator field. The zero sequence current is generated by adjusting the on-time of zero vectors between two inverters. In the case of the 6/4 VFRM, an open winding configuration is utilised since the field and armature windings have the same polarity in each tooth winding. In contrast, a dual three-phase winding configuration having a neutral point is adopted for the 6/7 VFRM, due to the opposite polarity between the field and armature windings. For the vector control, the voltage and torque equations are derived in the synchronous dq-axis frame. With the aid of MATLAB/Simulink, these equations are also utilised for the dynamic simulation. The simulation and experimental results validate that the proposed strategy can effectively increase the efficiency and extend the operating speed range of the VFRMs. As an extended work, a torque ripple reduction method is also applied to the integrated current control by injecting harmonic components into the zero sequence current.