Constant Frequency Torque Controller Research Articles

Comparative Analysis of Torque Ripple for Direct Torque Control based Induction Motor Drive with different strategies

ABSTRACT Direct Torque Control (DTC) method is one of the most outstanding and proficient control techniques of the induction motor. The foremost drawback of DTC induction motor drive is high torque pulsation, variable frequency of inverter switching. The main reason of torque pulsation is torque and flux hysteresis band utilised in DTC. DTC method is utilising hysteresis comparators which produce high torque pulsation and unpredictable switching frequency. In this paper, torque ripple analysis of fuzzy logic controller (FLC) based Direct Torque Control (DTC) of three-phase induction motor drive has been discussed. The space vector pulse width modulation (SVPWM) based DTC technique, which operates with the regular switching frequency, helps to solve the fundamental issues of torque pulsation. In addition to this, literature proposes different solutions like a prediction scheme, fuzzy logic controller technique, global minimum torque ripple strategy, constant frequency torque controller (CFTC) Technique, optimal switching instant scheme, duty cycle control scheme, SVPWM DTC for torque ripple reduction. The fuzzy logic controller capabilities for DTC technique are applied with the induction motor (IM) drive for high-performance applications. The simulation results of Conventional DTC, SVPWM DTC, Carrier SVPWM DTC and FLC-DTC method for the induction motor are illustrated. Fuzzy-based DTC technique compared with CSVPWM DTC method and conventional DTC method are presented. The torque ripple of 7.4% for 5 HP induction motor is achieved by FLC-based DTC method. Hardware results for DTC induction motor drive are also demonstrated and discussed. This paper presents improvement in torque ripple reduction with the help of carrier space vector pulse width modulation (CSVPWM) and FLC-based DTC.

Enhanced Performance of Constant Frequency Torque Controller–Based Direct Torque Control of Induction Machines with Increased Torque-Loop Bandwidth

Constant frequency torque controller-based direct torque control (CFTC-DTC) is emerging as a powerful control strategy for a high-performance control of alternating-current motor drives. Compared to DTC, the CFTC-DTC has less torque and flux ripples with a fixed switching frequency. Nevertheless, it has a limitation of the torque-loop bandwidth, as it uses the sampling frequency from a digital signal processor to generate a triangular-based carrier. The switching frequency of the control system is determined by the carrier frequency of the CFTC. The frequency limitation of CFTC leads to sector-flux droop, which causes a distortion in the motor phase current and aggravation in the torque and speed waveforms at low speed regions. In this article, we propose a modification of the frequency carriers of the CFTC-DTC of induction motors to increase the switching frequency and torque loop bandwidth by replacing the triangular-carrier-based waveform with a ramp-carrier-based waveform. The proposed method is verified by simulation and experimental results showing its excellent performances in steady- and transient-state operations.

Fast Torque Control and Minimized Sector-Flux Droop for Constant Frequency Torque Controller Based DTC of Induction Machines

In this paper, two simple control methods are proposed for improving direct torque control (DTC) with a constant frequency torque controller (CFTC-DTC) in induction machines. The CFTC-DTC was initially introduced to reduce torque ripple and achieve constant switching frequency in inverters. However, when compared to the original DTC, the CFTC-DTC algorithm suffers from slow torque dynamic response owing to the selection of zero-voltage vectors during torque transient. In addition, when the motor operates at low speed, flux droop occurs at sector transitions, which leads to undesirable current distortions. The problem of flux droop occurs because of short-duty voltage cycles and small radial voltage components at low speeds. This paper has two main contributions-to eliminate zero-voltage vectors during torque dynamics in order to establish a fast torque response in the transient state, and to minimize the flux droop at the sector transitions by applying another triangular carrier and modifying the torque output in order to increase the duty voltage cycles at low motor speeds. A detailed analysis of the problems and proposed methods is presented. Experimental results are provided to validate the effectiveness of both proposed methods.

Constant Frequency Torque Controller for DTC with Multilevel Inverter of Induction Machines

Direct Torque Control using multilevel inverter (DTC-MLI) with hysteresis controller suffers from high torque and flux ripple and variable switching frequency. In this paper, a constant frequency torque controller is proposed to enhance the DTC-MLI performance. The operational concepts of the constant switching frequency torque controller of a DTC-MLI system followed by the simulation results and analysis are presented. The proposed system significantly improves the DTC drive in terms of dynamic performance, smaller torque and flux ripple, and retain a constant switching frequency.

Extending Switching Frequency for Torque Ripple Reduction Utilizing a Constant Frequency Torque Controller in DTC of Induction Motors

Direct torque control(DTC) of induction machines is known to offer fast instantaneous torque and flux control with a simple control structure. However, this scheme has two major disadvantageous, namely, a variable inverter switching frequency and a high torque ripple. These problems occur due to the use of hysteresis comparators in conventional DTC schemes, particularly in controlling the output torque. This paper reviews the utilization of constant frequency torque controllers (CFTC) in DTC to solve these problems while retaining the simple control structure of DTC. Some extensions of the work in utilizing a CFTC will be carried out in this paper which can further reduce the torque ripple. This is particularly useful for a system which has a limited/low sampling frequency. The feasibility of a CFTC with an extended carrier frequency in minimizing the torque ripple is verified through experimental results.

A New Fixed Switching Frequency Direct Torque Controlled PMSM Drives with Low Ripple in Flux and Torque

Direct Torque Control (DTC) has gained popularity for development of advanced motor control due to its simplicity and offers fast instantaneous torque and flux controls. However, the conventional DTC which is based on hysteresis controller has major drawbacks, namely high torque ripple and variable inverter switching frequency. This paper presents an improved switching strategy for reducing flux and torque ripples in DTC of PMSM drives; wherein the torque hysteresis controller and the look-up table used in the conventional DTC are replaced with a constant frequency torque controller (CFTC) and an optimized look-up table, respectively. It can be shown that a constant switching frequency is established due to the use of the CFTC while the reduction of torque and flux ripples is achieved mainly because of the selection of optimized voltage vector (i.e. with an optimized look-up table). This paper also will explain the construction of DTC schemes implemented using MATLAB-Simulink blocks. Simulation results were shown that a significant reduction of flux and torque ripples which is about 90% can be achieved through the proposed DTC scheme.