An investigation into induction motor vector control based on reusable VHDL digital architectures and FPGA rapid prototyping

The research investigates a novel approach, based on reusable VHDL digital architectures, to induction motor vector control modelling, simulation and FPGA rapid prototyping. Several induction motor control methods are reviewed. The dq-axis vector control strategy is established as the most appropriate technique for achieving decoupled control of the flux and torque producing components of the induction motor stator current. The space phasor theory is presented and the principles of rotor flux oriented vector control are outlined. Most a.c. drives in use today adopt a microprocessorlDSP based digital control strategy, but implementation of the current loop and PWM control are still tied to analogue control circuitry. Although these control schemes have the advantage of fast dynamic response, they suffer the disadvantages of circuit complexity, limited functions and difficulties when circuit modification is required. Recent developments in the field of microelectronics have enabled complex switching strategies for transistorized inverters to be implemented by means of ASIC technology. Consequently, motor control and power conversion systems employing ASIC/FPGA technology are receiving increased attention. A comprehensive study of the modelling, simulation and design of a vector controlled induction motor using VHDL hardware description language and targeting FPGA implementation is presented. Initially, the complete drive, including the induction motor, power electronics and the controller is behaviourally modelled using VHDL and then simulated and evaluated. Speed and torque responses of the motor to various input conditions, like step or variable inertia loads are considered. The Clark_transform, Park_transform, Current_model, PI_controller, Control_unit, Inverse Park_transfonn and PWM Wavefonn Generator blocks are decribed and their hierarchical design to implement an induction motor vector control strategy is examined. The VHDL design, synthesis, timing simulation and Xilinx FPGA implementation of the induction motor digital controller is described. The universal applicability (reusability) of the control blocks to a wide range of vector control drives, with different topologies, is shown to be a major advantage. It is also shown that the employment of FPGAs for commissioning trial provides further benefits such as: rapid prototyping, simple hardware / software design and compact implementation. The complete digital controller circuit design is synthesized, implemented and comprehensively tested. The results confinn correct controller operation in conjunction with the interface circuitry, the power system and the induction motor. The new hardware description language based modelling technique, based on hierarchical design and VHDL modular development, is validated bv both simulation results and experimental tests.