Modified Direct Torque Control Topologies in Five-Phase Induction Motor for Reduction of Torque Ripple, Flux Ripple, and Current Harmonic Distortion

Abstract

Induction motors have become the workhorses of modern process industries, employing advanced vector control strategies like FOC, PTC, and DTC methods. In those, 5-phase induction motors offer several advantages over conventional 3-phase machines, including increased fault tolerant capability, reduced per-phase power, improved efficiency, high torque density, lightweight construction, smooth operation, and the needs a simple 2-level 5-leg inverter for control. The Direct Torque Control (DTC) scheme is a widely used control technique due to its simplicity, less parameter sensitivity, and quicker dynamics although it suffers from higher torque ripple, flux ripple, and variable switching frequency. In DTC, the electromagnetic torque and stator flux of the induction motor are controlled by selecting a suitable voltage vector from a predefined switching vector table. Existing DTC techniques for 5-phase induction motors strictly adhere to conventional DTC logic, focusing only on altering the switching table to improve the steady-state performance of the drive, still, these schemes suffer with higher torque and flux ripple and variable switching frequencies. The solution involves combining a dual inverter configuration with a 5-phase open-end winding induction motor to amalgamate features such as a simple structure, an increased number of voltage levels, enhanced fault-tolerance, reduced voltage stress across switches, more switching redundancies, and zero common-mode voltage. This thesis aims to modify DTC control techniques to reduce torque ripple and flux ripple, along with minimizing current harmonic distortion without significantly affecting the dynamic performance of the drive. The proposed Direct Torque Control (DTC) employs a constant switching torque controller, replacing the hysteresis torque controller on a 2-level inverter-controlled 5-phase induction motor drive without modifying the lookup table. The constant switching controller is implemented with a PI controller and triangular wave, comparator; and this torque controller regulates the duty cycle of selected active vectors over a sample time during variable-speed operations. The duty cycle of active voltage vectors is controlled based on speed and load conditions, leading to reduced torque ripple at low speeds. The constant switching torque (CST) controller ensures a nearly constant switching frequency by prompting the torque controller to change its status at regular intervals. However, the proposed CST-DTC exhibits slower torque and speed dynamics compared to Conventional DTC (C DTC) due to the frequent occurrence of zero vectors. These slower dynamics are improved with the vi help of the proposed Fractional Order PI (FOPI) controller-based CST-DTC (FOPI-CST-DTC) method without affecting the steady-state performance as that of PI controller-based CST-DTC. The proposed FOPI-CST-DTC control scheme shows, the reduction in torque ripple and current harmonic distortion by nearly maintaining the constant switching frequency with improved dynamic performance. In previous work, the use of a hysteresis flux controller resulted in higher flux ripple and the harmonic plane components not eliminated. Constant Switching Flux (CSF) controller along with the Constant Switching Torque (CST) controller-based Direct Torque Control (DTC) scheme is proposed to enhance both the flux and torque profiles, as well as the current profile, without disrupting the dynamic performance of the 5-phase induction motor. This approach utilizes 10 virtual voltage vectors distributed across 10 sectors. The reduction of torque ripple and stator flux ripple is achieved by regulating the duration of voltage vectors through the Constant Switching Torque and Flux controllers. To eliminate harmonic plane components, the proposed scheme employs virtual voltage vectors generated from large and medium vectors along the same axis. The slip speed/slip angle control based direct flux control method of a 5-phase induction motor with a simple sample-based reference voltage controller with a simplified lookup table has been introduced to improve the steady-state performance of the drive with reduced complexity. The proposed control scheme uses 20 virtual voltage vectors with simplified 2-lookup tables under high speed and low speeds. The main objective of the proposed control scheme is to maintain constant switching frequency for different speeds under various loading conditions along with torque ripple and flux ripple reduction w.r.t exiting DTC schemes. The proposed controller changes its state for every sample period and hence change of active vector for every sample time causes constant switching frequency throughout the drive operation. The proposed control scheme also uses the virtual voltage vectors to eliminate the harmonic plane components. Since the proposed control scheme is based on direct flux control, precise control of stator flux is possible and hence the average flux ripple and current %THD show great reduction when compared with torque ripple content. The steady-state torque ripple and stator flux ripple further can be reduced by employing an open-end winding configuration with dual 5-leg inverter feeding. The proposed 7-level torque controller under high speeds and 3-level torque controller under low speeds along with a 2-level flux controller with a modified lookup table reduces the flux ripple, torque ripple, and harmonic current distortion. The proposed method generates 30 Virtual Voltage vectors (VVV) in dual inverter vii configuration from adjacent and nonadjacent VVVs of individual inverters by maintaining 720 and 1440 phase displacement such that the resultant common mode voltage for dual inverter configuration will become zero. The zero common mode voltage allows the use of a single DC source for two inverters and eliminates bulky isolation transformers. The Proposed DTC control schemes are built with MATLAB/Simulink software and interfaced with the dSPACE-1202 real-time controller. The experimental work has been conducted to test the effectiveness of the proposed schemes on a 5-phase induction motor drive setup. In this thesis, the obtained experimental results of proposed DTC control schemes are compared with the experimental results of existing DTC schemes to know the effectiveness of the proposed methods.