Implementation of a direct torque controller using a three-level inverter for an electric vehicle application.
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
The University of Canterbury has been involved in the research and development of electric vehicles since 1974. These electric vehicles use induction motors that are designed with a low inductance such that they can run at high speed and therefore have a high power to weight ratio. The inverter switching can cause high current ripple due to the low inductance of these motors. A three-level inverter has been previously developed for electric vehicle applications that produces an improved voltage waveform and therefore has reduced current distortion. This thesis describes the development of an induction motor torque controller for a three-level inverter in an electric vehicle application. Four different induction motor torque control schemes are considered. These are field oriented control, direct torque control (DTC), minimal torque ripple DTC, and DTC using space vector modulation. All four of the control schemes are simulated using MATLAB and Simulink, and DTC using space vector modulation is chosen based on the simulation results. DTC using space vector modulation is shown to have the low steady-state torque ripple, flux ripple, and current distortion that is characteristic of space vector modulation and the fast transient performance that is characteristic of direct torque control. The implementation of DTC using space vector modulation that is used is described in detail. DTC using space vector modulation is implemented on a custom-built embedded controller based on a TMS320VC33 DSP and a XC4020XLA FPGA. The implementation of the control algorithms and a number of supporting software components are also described. The implemented torque controller is tested against a 15hp, 400Hz, 200V induction motor. Results are presented that show the performance of the torque controller and the three-level space vector modulator. The three-level space vector modulator is shown to produce less current distortion at low modulation indices. The torque produced by the controller during steady-state operation differs from the reference torque, but this error is relatively constant over the range of speeds that are tested. The estimated response to step-changes in the torque and flux references are shown to be almost instantaneous.