Direct torque control of induction machines with constant switching frequency and reduced torque ripple

Direct torque control (DTC) of induction machines is known to have a simple control structure with comparable performance to that of the field-oriented control technique. Two major problems that are usually associated with DTC drives are: switching frequency that varies with operating conditions and high torque ripple. To solve these problems, and at the same time retain the simple control structure of DTC, a constant switching frequency torque controller is proposed to replace the conventional hysteresis-based controller. In this paper, the modeling, averaging, and linearization of the torque loop containing the proposed controller followed by simulation and experimental results are presented. The proposed controller is shown to be capable of reducing the torque ripple and maintaining a constant switching frequency.     

Analysis and prediction of inverter switching frequency in directtorque control of induction machine based on hysteresis bands andmachine parameters

In this paper, the influences of the hysteresis bands on the direct torque control (DTC) of an induction motor are analytically investigated, and the switching frequency of the inverter is predicted based on the analysis. The flux and torque hysteresis bands are the only gains to be adjusted in DTC, and the inverter switching frequency and the current waveform are greatly influenced by them. Therefore, the magnitude of the hysteresis band should be determined based on reasonable guidelines which can avoid excessive inverter switching frequency and current harmonics in the whole operating region. This paper predicts the inverter switching frequency according to torque and flux hysteresis bands based on induction machine parameters and control sampling period, and investigates the effect of hysteresis bands to line current harmonics. The simulated and experimental results prove the usefulness and feasibility of the proposed method     

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.     

Torque ripple analysis and dynamic performance of a space vector modulation based control method for AC-drives

A constant switching frequency torque control method is presented in this paper, that uses flux error vector based space vector modulation (SVM) to achieve steady state and dynamic control of torque. The effect of SVM switching on torque ripple has been analyzed using "flux ripple vectors". This approach is used to develop an insight of torque ripple and to estimate it for any operating angular velocity. We propose a method of compensation to maintain steady state control of torque in the overmodulation region of operation. During torque dynamic, the optimum maximum switching state vector is selected. This gives a response similar to that obtained using direct torque control (DTC) and direct self control (DSC) methods. The proposed method is verified experimentally.     

An improved DTC-SVM method for matrix converter drives using a deadbeat scheme

In this paper, a simple direct torque control (DTC) method for sensorless matrix converter drives is proposed, which is characterized by a simple structure, minimal torque ripple and unity input power factor. Also, a good sensorless speed-control performance in the low speed operation is obtained, while maintaining constant switching frequency and fast torque dynamics. It is possible to combine the advantages of matrix converters with the advantages of the DTC strategy using space vector modulation a deadbeat algorithm in the stator flux reference frame. Experimental results are shown to illustrate the feasibility of the proposed strategy.     

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     

Predictive direct torque control for the PM synchronous machine

This paper shows that a predictive digital control combined with the principle of direct torque control (DTC) leads to an excellent dynamic behavior of the synchronous machine with surface-mounted permanent magnets and is a real alternative to the classical field-orientated control. The advantages are a DTC control scheme with constant switching frequency and a predictable torque ripple. The settling times of the torque are reduced compared to the classical field-orientated control. The application in servo drives in which the rotor position is always measured can easily be achieved by using a commercial digital signal processor. Numerous simulations and measurements confirm the theoretical work.     

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.     

A novel direct torque controlled interior permanent magnet synchronous machine drive with low ripple in flux and torque and fixed switching frequency

A modified direct torque control (DTC) scheme for interior permanent magnet synchronous machine (IPMSM) is investigated in this paper, which features in very low flux and torque ripple and almost fixed switching frequency. It is based on the compensation of the error flux linkage vector by means of space vector modulation. Modeling and experimental results show that the flux and torque ripples are greatly reduced when compared with those of the basic DTC. With the new scheme, very short sampling time is not essential. All the advantages of the basic DTC are still retained. In addition, fixed switching frequency at different operating conditions becomes possible. The field-weakening control of this drive is also studied; an IPM DTC drive with a wider operation range and lower flux and torque ripple has been achieved experimentally.     

Simple direct torque control of induction machine using space vector modulation

A new method for direct torque control (DTC) based on load angle control is developed. The use of simple equations to obtain the control algorithm makes it easier to understand and implement. Fixed switching frequency and low torque ripple are obtained using space vector modulation, overcoming the most important drawbacks of classic DTC.     

Hybrid Cascaded H-Bridge Multilevel-Inverter Induction-Motor-Drive Direct Torque Control for Automotive Applications

This paper presents a hybrid cascaded H-bridge multilevel motor drive direct torque control (DTC) scheme for electric vehicles (EVs) or hybrid EVs. The control method is based on DTC operating principles. The stator voltage vector reference is computed from the stator flux and torque errors imposed by the flux and torque controllers. This voltage reference is then generated using a hybrid cascaded H-bridge multilevel inverter, where each phase of the inverter can be implemented using a dc source, which would be available from fuel cells, batteries, or ultracapacitors. This inverter provides nearly sinusoidal voltages with very low distortion, even without filtering, using fewer switching devices. In addition, the multilevel inverter can generate a high and fixed switching frequency output voltage with fewer switching losses, since only the small power cells of the inverter operate at a high switching rate. Therefore, a high performance and also efficient torque and flux controllers are obtained, enabling a DTC solution for multilevel-inverter-powered motor drives.      

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。     

基于双PI控制的异步电机直接转矩控制系统的研究

传统的直接转矩控制系统(DTC)转矩脉动较大,文章利用双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,改进了速度调节器以及开关状态表,利用Matlab/Simulink进行了仿真。结果表明,所提方案能极大地减小转矩脉动和转速响应时间,同时算法简单,易于实现。     

基于软开关技术的永磁同步电机直接转矩控制系统

给出了永磁同步电机直接转矩控制的系统结构,提出了直接转矩控制技术与变流器软开关技术相结合的方法.电机控制系统综合了直接转矩控制快速特性和软开关的低开关损耗特性,以提高电力电子器件开关频率从而减小永磁同步电机的转矩脉动,最后给出了控制系统的计算机仿真.     

Enhanced inverter switching for fast response direct torque control[of induction motor drives]

In classical direct torque control (DTC), an inverter switching event can occur once in each control update period. Because the nature of the inverter switching event is unconstrained, it is essential to limit the inverter switching frequency, and hence the control update period, to ensure that under no circumstances is the allowable switching frequency of any individual power device exceeded. Consequently the switching capability of individual power devices is generally under-utilized, and the control scheme produces high levels of ripple in the motor current and torque. This paper describes a new strategy for device switching and voltage vector selection in DTC. The basis of the strategy is an increase in the control update frequency, while limiting the switching rate of each inverter leg. Although the rate at which device switching events may occur is unchanged, the higher control update frequency leads to higher resolution timing of switching events. The advantages of the strategy, demonstrated by experimental results on a 3 kW induction motor drive, are a significant reduction in the steady-state torque ripple as well as a faster transient response     

基于DSP的矿用电机车DTC控制方法的研究

本文介绍矿用蓄电池电机车变频调速目前发展状态,指出采用直接转矩控制方案应用在矿用蓄电池电机车上的合理性,通过DSP硬件平台设计和软件开发编写DTC控制程序,最终由实验得出该控制方法具有牵引力大,起动电流小,调速性能好,节能等优势,具有广泛的应用意义。     

Switching frequency imposition and ripple reduction in DTC drivesby using a multilevel converter

A new strategy for direct torque control with imposed switching frequency (DiCoIF) is proposed. This strategy was specially designed to operate with a multicell (flying capacitors) inverter with any number of levels, which means it can also be used for standard two-level inverters. This approach combines the well known advantages of the multicell. inverter with those of a direct torque controlled (DTC) based strategy. It is shown that the multicell topology presents enough degrees of freedom to control both torque and flux with very low ripple and high dynamics on one hand, and to impose the switching frequency and the capacitors voltage balance on the other hand. Experimental and simulation results, obtained with a standard two-level inverter and with a four-level multicell inverter, are presented and discussed. Finally, a comparative analysis either with the classical DTC and field oriented techniques is carried out     

基于改进DTC-SVM的永磁同步电机伺服系统研究

传统的永磁同步电机直接转矩控制方法转矩响应速度快,但转矩脉动大、电流脉动大、开关频率不固定。为此,本文在分析了面贴式永磁同步电机数学模型的基础上,提出一种基于改进DTC-SVM的空间矢量脉宽调制直接转矩控制系统。提出基于Simulink的DTC-SVM伺服系统仿真模型。仿真结果表明,整个系统既保持了传统直接转矩控制的响应速度快的优点,又具有定子电流波形畸变小,转矩脉动小,系统工作稳定,抗干扰性能强的优点。     

Probabilistic voltage harmonic analysis of direct torque controlled induction motor drives

In high frequency motor-drives such as direct torque controlled (DTC) induction motor drives, the motor harmonic loss, and electromagnetic interference are largely affected by the spectrum of the motor input voltage. Nonlinear elements in the control loop of DTC drive make harmonic analysis of the drive very complex compared to classical pulsewidth modulated controlled drives. In this paper, a probabilistic method to study the harmonic contents of voltage in DTC of induction motors is presented. The DTC voltage chain is simulated with a random process. Then, the autocorrelation function of voltage vectors is calculated and its power spectrum density is obtained. The effect of flux and torque hysteresis controller bands, machine parameters, and inverter dc-link voltage on the motor voltage spectrum is investigated. Major harmonics in the DTC voltage spectrum are specified and their behaviors are described. Simulation and experimental results are presented to justify the theoretical analysis.