Torque ripple reduction in DTC of induction motor driven bythree-level inverter with low switching frequency

A torque ripple reduction technique of direct torque control (DTC) for high power induction motors driven by three-level inverters with the inverter switching frequency limited around 0.5-1 kHz level is presented. It is noted that conventional two-level DTC algorithms to reduce torque ripple are devised for applications with relatively high switching frequency above 2-3 kHz and cannot accomplish satisfactory torque ripple reduction for three-level inverter systems with such lower switching frequencies. A new DTC algorithm, especially for low switching frequency inverter system, illustrates quite reduced torque ripple characteristics all over the operating speed region. Simulation and experimental results show effectiveness of the proposed control algorithm     

Load-Commutated SCR Current-Source-Inverter-Fed Induction Motor Drive With Sinusoidal Motor Voltage and Current

Current source inverter (CSI) is an attractive solution in high-power drives. The conventional gate turn-off thyristor (GTO) based CSI-fed induction motor drives suffer from drawbacks such as low-frequency torque pulsation, harmonic heating, and unstable operation at low-speed ranges. These drawbacks can be overcome by connecting a current-controlled voltage source inverter (VSI) across the motor terminal replacing the bulky ac capacitors. The VSI provides the harmonic currents, which results in sinusoidal motor voltage and current even with the CSI switching at fundamental frequency. This paper proposes a CSI-fed induction motor drive scheme where GTOs are replaced by thyristors in the CSI without any external circuit to assist the turning off of the thyristors. Here, the current-controlled VSI, connected in shunt, is designed to supply the volt ampere reactive requirement of the induction motor, and the CSI is made to operate in leading power factor mode such that the thyristors in the CSI are autosequentially turned off. The resulting drive will be able to feed medium-voltage, high-power induction motors directly. A sensorless vector-controlled CSI drive based on the proposed configuration is developed. The experimental results from a 5 hp prototype are presented. Experimental results show that the proposed drive has stable operation throughout the operating range of speeds.     

Direct self control of induction machines fed by a doublethree-level inverter

For variable speed induction motor drives with very high power and extreme demand on the quality of torque, a novel concept of feeding an induction machine with open windings by two gate-turn-off (GTO) three-level inverters is investigated. For optimal operation with the restricted switching frequency of these high-power inverters, direct self control (DSC) is mostly suited, which is well proven with two-level and three-level inverters in traction, allowing a highly dynamic control of the induction machine. In comparison with two-level and three-level inverters, torque ripple and distortion of motor currents are efficiently reduced, while maintaining the excellent dynamic qualities of DSC     

Performance evaluation of a direct torque controlled drive in thecontinuous PWM-square wave transition region

This paper investigates the operation of a direct torque controlled drive when operating under transient conditions and when operating in overmodulation conditions or in the “transition region” to six-step operation. The direct torque control is a dead-beat control of the torque and flux magnitude. In the steady-state, the stator voltage vector which drives the torque and flux to the reference value is calculated during each fixed switching period. Under transient or overmodulation conditions, an alternative switching algorithm must be used since dead-beat control is no longer possible. Two alternatives are presented for operation in overmodulation. The first involves a determination of the switching state a priori, and calculating the duty cycle for each phase based on the torque and flux error. A much simpler scheme is presented which utilizes the voltage reference vector from the direct torque control algorithm. This scheme, although not resulting in dead-beat control, is shown to provide very satisfactory performance in overmodulation. The direct torque control method shows great promise for light traction applications where a large quasi-constant power region is required. The scheme operates very satisfactorily in overmodulation, compared with existing current regulated PWM-based schemes, due to the fact that the voltage space vectors are directly controlled. A complete experimental evaluation of the proposed scheme operating in the transition region is also given     

A new stator resistance tuning method for stator-flux-orientedvector-controlled induction motor drive

Field-oriented-controlled induction motor drives have been widely used over the last several years. Conventional direct stator-flux-oriented control schemes have the disadvantage of poor performance in the low-speed operating area when the stator flux is calculated using the voltage model, due to the stator resistance uncertainties and variations. In this paper, a new closed-loop stator-flux estimation method for a stator-flux-oriented vector-controlled induction motor drive is presented in which the stator resistance value is updated during operation. This method is based on a simple algorithm capable of running in a low-cost microcontroller, which is derived from the dynamic model of the induction machine. The effects of stator resistance detuning, especially in the low-speed operating region, are investigated and simulation results are shown. The motor drive system as well as the control logic and the resistance estimator are simulated and characteristic simulation results are derived. In addition, the proposed control scheme is experimentally implemented and some characteristic experimental results are shown. The simulation as well as the experimental results reveal that the proposed method is able to obtain precise flux and torque control, even for very low operating frequencies     

Two-Level VSC Based Predictive Direct Torque Control of the Doubly Fed Induction Machine With Reduced Torque and Flux Ripples at Low Constant Switching Frequency

In this paper, a new predictive direct torque control (DTC) strategy of the doubly fed induction machine (DFIM) is presented. It is especially designed to operate at a considerably low constant switching frequency, reducing the electromagnetic torque and rotor flux ripples, in order to provide good steady-state and fast dynamic performances. This control is convenient for high power drive and generator applications, with restricted switching frequency. The DFIM is connected to the grid by the stator and the rotor is fed by a two level voltage source converter. In addition, this control method allows to implement a technique that reduces the switching power losses of the converter. Finally, experimental results show that the proposed DTC method effectively reduces the torque and flux ripples at low switching frequency, even under variable speed operation conditions.     

A sensorless vector control system for induction motors usingq-axis flux with stator resistance identification

This paper presents a sensorless vector control system for general-purpose induction motors, which is based on the observer theory and the adaptive control theories. The proposed system includes a rotor speed estimator using a q-axis flux and stator resistance identifier using the d-axis flux. The advantages of the proposed system are simplicity and avoidance of problems caused by using only a voltage model. Since the mathematical model of this system is constructed in a synchronously rotating reference frame, a linear model is easily derived for analyzing the system stability, including the influence of the observer gain, motor operating state, and parameter variations. In order to obtain stable low-speed operation and speed control accuracy, an algorithm for compensating for the deadtime of the inverter and correcting the nonideal features of an insulated gate bipolar transistor was developed. The effectiveness of the proposed system has been verified by digital simulation and experimentation     

基于模糊神经网络的异步电机DTC研究

针对异步电机直接转矩控制在低速时脉动大、开关频率不固定等缺陷,提出了一种基于模糊神经网络的直接转矩控制方案,该方案采用模糊神经网络调节器分别对转矩和磁链进行控制。该控制方法综合了模糊控制和神经网络的优点,原理简单、无需大量专家经验、具有优良的非线性逼近和自适应能力。最后通过仿真实验证明,采用该控制器的异步电机系统动态性能良好、低速脉动小。     

基于电阻压降补偿的直接转矩控制的研究

本文分析了异步电动机直接转矩控制的基本原理,完成基于空间电压矢量控制技术(SVPWM)的异步电机直接转距控制的MATLAB/SIMULINK仿真研究,并对低速时异步电动机性能进行了简单分析,采用定子电阻压降补偿来减小磁链轨迹的畸变,得到了较好的控制效果。     

A dead-beat type digital controller for the direct torque control of an induction motor

In this paper, a dead-beat type digital controller has been introduced to overcome the problems of a conventional direct torque controller. The proposed induction motor drive with a digital dead-beat controller shows good transient response and negligible steady-state error even at a low switching frequency, which is needed for high power machines used for transportation. Including the rotor dynamics, the stability condition and steady-state error of the proposed control system have been examined in the z-plane. In addition, the good performance has been verified through the simulation and experiment. The flux and torque controllers have been designed with only stator voltage equations in the stator flux reference frame in order to take advantages of the direct torque control. Therefore, the proposed flux and torque controllers have simple forms and can be easily designed and implemented.     

Novel switching method for single-phase NPC three-level inverter with neutral-point voltage control

This paper proposes a novel switching method with the neutral-point voltage control in a single-phase neutral-point-clamped three-level inverter (SP-NPCI) used in photovoltaic systems. A proposed novel switching method for the SP-NPCI improves the efficiency. The main concept is to fix the switching state of one leg. As a result, the switching loss decreases and the total efficiency is improved. In addition, it enables the maximum power-point-tracking operation to be performed by applying the proposed neutral-point voltage control algorithm. This control is implemented by modifying the reference signal. Simulation and experimental results provide verification of the performance of a novel switching method with the neutral-point voltage control.     

Band-constrained technique for direct torque control of induction motor

In this paper, a novel technique for the direct torque control (DTC) of an induction motor is proposed, which overcomes the trouble of high torque ripple afflicting the conventional DTC technique. With the novel technique, the inverter voltage vector selected from the switching table is applied for the time interval needed by the torque to reach the upper (or the lower) limit of the band, where the time interval is calculated from a suitable modeling of the torque dynamics. By this approach, the control system emulates the operation of a torque hysteresis controller of analog type since the application time of the inverter voltage vector is dictated by the allowed torque excursion and not by the sampling period. It is shown by experimental results that the technique yields a considerable reduction of the torque ripple. A further and ultimate reduction is obtained by compensating for the delay inherent in the discrete-time operation of the control system. The outcome is that the torque ripple of the motor is constrained within the hysteresis band of the torque controller, for a band of customary value. An ancillary merit of the technique is the almost full elimination of the average torque error inherent in the conventional technique. If the hysteresis band is shrunk, the torque ripple is bound to swing out the band limits. Under this circumstance, an extension of the technique is developed, which helps keep the torque ripple at minimum. To assess the characteristics of the proposed DTC technique, the following quantities: average torque error, rms value of the torque ripple, and inverter switching frequency are measured for different stator flux angular speeds and hysteresis bands of the torque and flux controllers. As a comparison, the same quantities are given for the conventional DTC technique.     

Minimum-time minimum-loss speed control of induction motors underfield-oriented control

A new minimum-time minimum-loss speed control algorithm for induction motors is suggested to obtain high performance, as well as high efficiency, under field-oriented control with practical constraints on voltage and current. This algorithm utilizes a two-stage control. In the transient stage, a maximum torque control algorithm is utilized to get the minimum-time response. In the steady state, a minimum-loss control algorithm is applied to improve the efficiency. Simulation studies show the performance of the proposed minimum-time minimum-loss control algorithm under field-oriented control     

A switching control strategy based on output regulation subspaces for the control of induction motors using a three-level inverter

In this work we present the design of a new switching control strategy for a three-level converter aimed to improve the performance of a direct torque control (DTC) for induction machines, special attention is given to the reduction of torque ripple. As in classical DTC, the proposed technique is aimed to directly regulate the outputs: torque and flux amplitude. We show that classical DTC can be considered as a particular case of the proposed control strategy. The proposed criterion to design the switching control sequence is based on a quadratic criterion of the output errors plus a prediction in one-step ahead, and an interesting partition of the input state space generated by the output regulation subspaces (ORS) to facilitate the selection of the control vector for reducing the computational load. As a result, a control vector is selected without the requirement of an auxiliary space vector or any other modulation technique. Simulations results using the proposed control technique confirm the validity of this approach.     

异步电机DTC系统脉动最小化研究

直接转矩控制具有控制简单、动态响应迅速、对参数变化鲁棒性强的特点,因此得到了广泛的应用。在传统的异步电动机直接转矩控制系统中,存在电压空间矢量对定子磁链幅值和磁通角的影响,特别是低速时系统脉动大。针对此问题,文章提出了一种的新的控制方法,该方法将磁链区间细分控制与电压矢量合成结合在一起,并通过引入模糊控制算法进一步提高了转矩响应时间,且减小了转矩脉动。仿真结果表明,本控制方法可以大大减小转矩脉动,具有较好的动静态性能。     

Novel Direct Current-Space-Vector Controlfor Shunt Active Power Filters Basedon the Three-Level Inverter

The growing number of electric drives with non-sinusoidal line currents has given increased interest in active power filters (APF), to avoid grid problems caused by harmonic distortions. In this paper, a novel direct current-space-vector control scheme (DCSVC) is presented for a three-level, neutral-point-clamped voltage source inverter, which is employed as an active power filter. The proposed method generates the compensation current reference indirectly generating an equivalent ohmic conductance for the fundamental component by means of the APF's dc-link voltage control. Based on the fast Fourier transform the compensation of the reactive fundamental current and selectable harmonics can be cancelled, confining the operation to only harmonic compensation and thus saving the APF's apparent power. The novel DCSVC, operating in synchronously rotating coordinates is implemented in a field programmable gate array, realizing the switching states from switching tables. The proposed control reduces the average switching frequency and thus, the switching power loss significantly, compared with a previous DCSVC, operating in stationary coordinates. Simulation and experimental results validate the feasibility and highly dynamic performance of the proposed control, both for harmonic and total non-active current compensation.      

Flux observer with online tuning of stator and rotor resistancesfor induction motors

This paper proposes an adaptive flux observer for induction motors, where stator and rotor resistances are estimated in online environments. The variation of motor parameters during operation degrades the performance of the controller and the flux observer. Among the parameters of induction motors, rotor resistance is a crucial one for flux estimation, and stator resistance also becomes critical in the low-speed region. Under the persistent excitation condition, the proposed method estimates the actual values of stator and rotor resistances simultaneously, which guarantees the exact estimation of the rotor flux. The persistent excitation condition is not satisfied when the electric torque of an induction motor is absent due to the lack of rotor currents. Even in this case, the proposed method achieves the correct estimation of the rotor flux. Simulations and actual experiments show that the rotor flux is estimated in all operating conditions and that both resistances converge to their actual values when the electrical motor torque exists     

Voltage control strategy for maximum torque operation of aninduction machine in the field-weakening region

In this paper, a novel field-weakening scheme for the induction machine is presented. The proposed algorithm, based on the voltage control strategy, ensures the maximum torque operation over the entire field-weakening region without using the machine parameters. Also, by introducing the direct field-oriented (DFO) control, which is insensitive to the variation of machine parameters in the field-weakening region, the drive system can obtain robustness to parameter variations. Moreover, the speed sensorless control can be achieved in the very-high-speed range, where the utilization of the speed sensor is limited. Experimental results for the laboratory induction motor drive system confirm the validity of the proposed control algorithm     

直接转矩控制系统减小转矩脉动的仿真研究

传统的直接转矩控制系统（DTC）转矩脉动较大，本文利用双PI控制方法进行了改进，设计出一种非零电压空间矢量和零电压空间矢量控制器，改进了速度凋节器以及开关状态表，利用Matlab／simulink进行了仿真，结果表叫，所提方案能极大的减小转矩脉动和转速响应时间，同时算法简单，易于实现。     

基于模糊控制的感应电机直接转矩控制系统

根据直接转矩控制理论,在Matlab 6.5/Simulink下构造了一个感应电机直接转矩控制系统的仿真模型。为改善感应电机系统的动、静态品质,设计了模糊自适应PI速度调节器,根据速度偏差与偏差变化率,通过模糊推理在线调整PI参数,提高系统的调速性能。仿真结果表明,这种模糊控制器具有比常规PID控制器更好的控制效果。