switching modes. The selection is made to restrict the flux linkage and electromagnetic torque errors within the respective flux and torque hysteresis bands, to obtain fast torque response, low inverter switching frequency and low harmonic losses. The required optimal switching voltage vectors can be selected by using a so called optimum switching voltage vector look-up table. This can be obtained by simple physical considerations involving the position of the stator flux linkage space vector, the available switching vectors and the required torque and flux linkage. In the Fig. 2, Configuration of Conventional DTC drive is shown, in which the comparison between the reference and the actual value is taken and the errors are processed through hysteresis band controllers. The flux loop controller has two levels of digital output and the torque control loop has three levels of digital output. The feedback flux and torque are calculated from the induction machine terminal voltages and currents. The three phase terminals quantities are converted into two phase stationary d q components, which are used for estimating motortorque and stator linked flux. Based on the resultant flux position and the errors in flux magnitude and in torque, a three-dimensional look up table is referred to decide the inverter switching. The „Stator Flux and Torque Estimator‟ block shown in Fig. 2 gives the sector number S k
The two-level voltage source inverters are not suitable for med iu m and high power applications due to large dv/dt stresses and more harmonic distortion. In order to reduce these problems three -level inverters are introduced in 1980s . The detailed survey on mult ileve l inverter topologies is given in . In order to obtain controllable three phase power fro m a mu ltilevel inverter, various PWM algorith ms can be generated by using both SV and TC approaches. However, in SV approach the comple xity will be increased due to the more number of voltage vectors. Hence, in most number of applications the carrier based PWM algorithms are popular for mult ileve l inverters. Few simp lified approaches by using duty cycle and offset times have been proposed for carrier based SVPWM algorith m based multilevel inverters , .
by a hysteresis controller and the speed loop was controlled by a fuzzy-logic controllerKashif et al .  utilized a three-layer feed-forward back propagation artificial neural network for flux control of a B4 inverterfed IM drive. El Badsi et al .  used a DTC scheme for torque and flux control of a B4 inverter-fed IM drive. Unfortunately, the two capacitor voltages were assumed constant in these papers. In fact, as a result of one-phase current flows through the split dc-link voltage sources, the fluctuation will inevitably appear in the two capacitor voltages, which deteriorates the output performance of the B4 inverter (i.e., torque pulsation and unbalanced three-phase currents). More seriously, if the balanced condition of the currents flowing in the two capacitor voltages is corrupted, the two capacitor voltages will deviate in two opposite directions till shutting down of the B4 inverter. With the development of fast and powerful microprocessors, increasing attention has been dedicated to the use of model predictive control (MPC) in power electronics . The first ideas about this strategy applied to power converters started in the 1980s , . The main concept is based on calculating the system’s future behavior to obtain optimal values for the actuating variables. With this intuitive concept, predictive control can be applied to a variety of systems, in which constraints and nonlinearities can be easily included, multivariable case can be considered, and the resulting controller is easy to implement . These features render the approach very attractive and effective for the control of power electronics system , , including drive control –, especially predictive torque control (PTC; particular for a two-level converter with horizon N = 1). In the PTC, the complete model and future behavior of the inverter-fed drives are taken into account. A cost function relating to torque and flux errors reduction is defined to evaluate the effects of each voltage vector and the one minimizing the cost function is selected –. In spite of the outstanding performance of B6 inverter-fed drives based on the PTC, PTC for B4 inverter-
The vector control is still very complex to implement. As a consequence of the perseverant efforts of various research engineers, an improvised scalar method known as DirectTorque Control (DTC) was invented. This method considerably alleviates the computational burden on the control platform while giving a performance which is comparable to that of a vector controlled drive. In this paper, the DTC scheme employing a Voltage Source Inverter (VSI) is possible to control directly the stator flux linkage and the electromagnetic torque by the optimum selection of inverter switching vectors. The selection of inverter switching vector is made to restrict the flux and torque errors within the respective flux and torque hysteresis bands. This achieves a fast torque response, low inverter switching frequency and low harmonic losses. The proposed scheme is described clearly and simulation results are reported to demonstrate its effectiveness. The entire control scheme is implemented with Matlab/Simulink.
The nonlinear IM model treated in this paper is fourth order with the state variables: torque, stator flux, rotor flux and other flux-dependent state. The obtained linear IM model using FBL is of second order, with only the torque and stator flux magnitude as dissociate state variables. Thus the new linear IM model is obtained spontaneously, very simple, and it substantially simplifies the controller design. The flux and torque are controlled by the new DTC scheme and the proposed controllers include SMC to maintain robust sensorless operation of IM drive. This technique based on the torque-flux linearization and control is different from existing methods discussed in -, which are depending on current control. The combination of FBL and SMC techniques preserves the fast and robust response of conventional DTC while entirely eliminating the torque and flux ripple.
In this paper a high performance advanced discontinuous pulse width modulation (ADPWM) based directtorquecontrolled (DTC) inductionmotor drive operating at high line side voltages is proposed. The proposed ADPWM based IM drive uses a special category of sequences which not only reduces the switching losses but also reduces the line current distortion during high speed operations. However analysis in this paper is limited to harmonic ripple in line current and comparison of the proposed method is done with the conventional DTC (CDTC), conventional space vector pulse width modulation (CSVPWM) based DTC and clamping sequences based DTC. The proposed method uses a special category of DPWM sequences, 0121 and 7212. This category of DPWM sequences are referred as double switching clamping sequences as they not only clamp one of the phase to either of the buses but also switches one of the remaining two phases twice in every sub cycle. In this paper it is shown that, utilizing DPWM sequences and by changing the zero state at any spatial angle where is between 00 and 600an infinite number of ADPWM methods can be generated which are categorized as “continual clamping” and “split clamping” sequences. It will be shown that steady state line current distortion at higher line side voltages is reduced significantly compared with the CDTC, CSVPWM based DTC as well as the ADPWM based DTC using clamping sequences.
This review paper studies the most commonly used electric driving methods of inductionmotor. Conventionally twolevel inverters are used in directtorque control method for controlling torque of inductionmotor which produces torque ripple. Therefore our main objective is to reduce the torque ripples which are produced by twolevelinverter. The new technique is proposed to minimize the torque ripples using three levelinverter. DirectTorque Control (DTC) method is simple method and having excellent robustness of torque control for the drive system. The three-level neutral point clamped (NPC) inverters have been mostly used in applications. SQIM used three level NPC for controlling torque of inductionmotor
3.2 DTC strategy for the DFIM connected to two 3LVSIs The development of speed control and DTC of doubly fed induction motors has favored the use of three-level inverters. The increase in levels number of the latter proves to be a better solution in high power drives. The inverter is made up of switching cells, generally with transistors or GTO thyristors for large powers . In this section, we present the study DFIM associated with two 3LVSIs with neutral point camped structure con- trolled by the DTC algorithm. Figure 3 illustrates the general schema of 3LVSI with NPC structure; it is one of the structures of three-levelinverter. It has a lot of advantages, such as the higher number of voltage vectors generated, less harmonic distortion and low switching frequency . Each arm of the inverter consists of 4 switches: S k1 , S k2 , S k3 , S k4 . The S k1 and S k2 have comple-
increasingly being used in most of the industrial applications. The development of high performance control strategies for AC drives, driven by the requirement of industry, has resulted in a rapid evolution during the last two decades. The Predictive Torque Control (PTC) technique has features of precise and quick torque response. This method is gaining popularity in the industry due to its simplicity and high dynamic performance.The control strategy combines the use of classical PI controller to obtain good steady state response and a predictive controller scheme to achieve good dynamic response. The main characteristic of predictive control is the use of a model of the system for predicting the future behaviour of the controlled variables. This information is being used by the controller to obtain the optimal actuation, according to a predefined optimization criterion. In predictive control scheme, the control objectives are defined as a cost function, which is to be minimized to have greater flexibility to include constraints which results in low computational complexity compared to DirectTorque Control (DTC) scheme. PTC offers high dynamic performance, accurate speed response. The PTC based voltage source inverter fed inductionmotor drive is capable of offering four quadrants in the torque-speed plane of operation like, forward motoring, forward generating, reverse generating and reverse motoring. To validate the proposed algorithms mathematical models were developed for inductionmotor, estimation of torque and flux and control logic. These models were integrated and simulations were carried out using Matlab/Simulink. Variation in stator currents, speed, electro-magnetic torque developed and stator flux during different operating conditions such as starting, steady state, sudden change in load and speed reversal are observed with the help of waveforms and results are discussed.
The directtorque control (DTC) is one of the actively researched control schemes of induction machines, that provides a very quick and precise torque response without the complex field-orientation block and the inner current regulation loop. In DirectTorque Control it is possible to control directly the stator flux and the torque by selecting the appropriate inverter state.. DTC is the latest AC motor control method, developed with the goal of combining the implementation of the V/f-based inductionmotordrives with the performance of those based on vector control [1-3]. It is not intended to vary amplitude and frequency of voltage supply or to emulate a DC motor, but to exploit the flux and torque producing capabilities of an inductionmotor when fed by an inverter .CSI permits easy power regeneration to the supply network under the breaking conditions, what is favorable in large power inductionmotordrives. In a directtorquecontrolledinductionmotor drive supplied by current source inverter it is possible to control directly the modulus of the rotor flux-linkage space vector through the rectifier voltage, and the electromagnetic torque by the supply frequency of the CSI. In this paper the solution based on a stator flux vector control (SFVC) scheme has been proposed . This scheme may be considered as a development of the basic DTC scheme with the aim of improving the drive performance.
ABSTRACT: Earlier studies have pointed out the limitations of conventional inverters, especially in high-voltage and high-power applications. In recent years, multilevel inverters are becoming increasingly popular for high-power applications due to their improved harmonic profile and increased power ratings. Several studies have been reported in the literature on multilevel inverters topologies, control techniques, and applications. However, there are few studies that actually discuss or evaluate the performance of inductionmotordrives associated with single-phase multilevel inverter. This paper presents then a comparison study for a cascaded H-bridge multilevel directtorque control (DTC) inductionmotor drive. In this case, symmetrical and asymmetrical arrangements of five and seven-level H-bridge inverters are compared in order to find an optimum arrangement with lower switching losses and optimized output voltage quality. The carried out experiments show that an asymmetrical configuration provides nearly sinusoidal voltages with very low distortion, using less switching devices. Moreover, torque ripples are greatly reduced
The DTC scheme consists of torque and flux comparator (hysteresis controllers), torque and flux estimator and a switching table. It is much simpler than the vector control system due to the absence of coordinate transformation between stationary frame and synchronous frame and PI regulators. DTC does not need a pulse width modulator and a position encoder, which introduce delays and requires mechanical transducers respectively. DTC based drives are controlled in the manner of a closed loop system without using the current regulation loop. DTC scheme uses a stationary d-q reference frame having its d-axis aligned with the stator q- axis. Torque and flux are controlled by the stator voltage space vector defined in this reference frame . The basic concept of DTC is to control directly both the stator flux linkage and electromagnetic torque of machine simultaneously by the selection of optimum inverter switching modes. The DTC controller consists of two hysteresis comparator (flux and torque) to select the switching voltage vector in order to maintain flux and torque between upper and lower limit. DTC explained in this paper is closed loop drive. Here flux and torque measured from the inductionmotor using proper electrical transducer. Then flux and torque errors are found out by equation (3) and (4) .
Several studies have suggested the application of the ANN technique to select the states of the voltage inverter switches used to power the DTC-controlled IM [107– 112]. The idea is always to replace the conventional switching table that determine the inverter states by neural selector capable of managing control signals in the same way. Fig. 8 shows the block diagram of DirectTorque Neural Control (DTNC). The architecture in- cludes a multilayer neural network allowing replacing both hysteresis comparators and the selection table. This neural network is composed of an input layer, two hidden layers and an output layer. The input layer is composed of three neurons, designated respectively by the torque error, the flux error and the angular position (θ) of the stator flux vector. The two hidden layers each consist of ten neu- rons. The output layer consists of three neurons that Fig. 6 Synoptic schema of DTC-Fuzzy control of the
Induction motors are integral to functionality of any industrial activities. According to Mankad & Chudasama (2014), three phase inductionmotor constitutes over 70% of motors used in the industry. Their popularity is tied to their robustness, simple construction and maintenance free operation. Inductionmotor has been applied for decades for motive power with variable speed due to its simple control. However, complex structure associated with inductionmotor makes it unsuitable for variable frequency operation. So many advancement has been made to reduce the complexity of three phase inductionmotor and give good dynamic response to inductionmotor drive, and one of such improvement is the directtorque control scheme. This scheme uses one vector voltage out of six active and zero voltages vectors generated by voltage source inverter (VSI). The selection is so done that the torque and stator flux remain limits of hysteresis band. Application of this strategy resulted in decoupled control of inductionmotor with engaging in complex co-ordinate transformation, current regulators or pulse width modulation (PWM) pulses. However, due to hysteresis band controller for torque and stator flux, there is a production of ripples in torque and stator current, as well as variable switching frequency. This problem becomes significant at low speed and heavy load. Although various technologies have invented that have shown positive impact in addressing these issues. Dither injection technique is the focus of this research. Ranjan and shyama (2011) presented three level neutral point inverter as playing significant role in the working principle of directtorque control strategy. it is shown that drive’s efficiency is improved without being affected during low switching frequency operation. According to the researcher, measurement and computational delay would lead to low switching frequency, which invariably increase torque and flux ripples. Study conducted by Sirisha and Nireekshan (2017) showed that enhanced performance of directtorque control scheme led to its replacement of field Oriented control scheme in 1980’s. in DTC algorithm, the author stated that torque and flux are controlled independently, hence in order to satisfy their demands, there is need to suitable vector according to the digitized status produced from the hysteresis controller. However, the researcher identified two basic problems with directtorque controller in spite of its simplicity: variable switching frequency and large torque ripples.
The inductionmotor or asynchronous is the most widely used electrical drive.  Has explained the complete analysis of electrical machinery drive system. Actually because independent control of torque and flux separately excited dc drives are simpler in control. Due to ruggedness, efficiency and simplicity the induction motors have been used in several applications for over a century. [2-3] has presented the analysis and simulation model development of inductionmotor in MATLAB / SIMULINK software. In AC drives control the directtorque control scheme is considered as one of the most advanced technology in the modern world. The directtorque control scheme is a simple technique compare to other techniques. In this technique by selecting optimum inverter switching modes the motortorque and flux are controlled independent and also direct. The primary input of the motor is stator voltage and stator current. From this the stator flux and electromagnetic torques are calculated. The torque errors and flux errors are restricted within the hysteresis band. The two important merits of this directtorque control technique is improved in steady state efficiency and quick torque response in transient operation.
provides a simple control structure. Since it was introduced in the middle of 1980’s ,  many researchers have been working in this area and several modifications and improvements have been made in order to overcome the two major disadvantages of the hysteresis-based of DTC scheme, namely the high torque ripple and variable switching frequency of the inverter. Previous proposed techniques to overcome these problems include the use of variable hysteresis band, controlled duty cycle technique and use of space vector modulation (DTC-SVM) based. All these techniques have managed to improve the performance of DTC, in the expense of loosing the simple structure of DTC. In -, a simple approach to solve the problems and at the same time retaining the simple structure of DTC was introduced. In this approach, a constant frequency torque controller was used to replace the hysteresis torque controller.
The classical directtorque control strategyis a closed loop control scheme, the important elements of the control structure being: the power supply circuit, a three phase voltage source inverter, the inductionmotor, the speed controller to generate the torque command and the DTC controller. The CDTC controller again consists of torque and flux estimation block, two hysteresis controllers and sector selection block, the output of the CDTC controller is the gating pulses for the inverter.
With the torque and flux producing components of stator current command and rotor field angle, we get d-q axis currents. d-q axis current commands in rotating frame are then converted to stationary reference frame using transformation. In case of indirect vector control, rotor flux and torque can be independently controlled by stator axis current components i ds and i qs [1,3].
Most of the faults in three-phase induction motors have relationship with air-gap eccentricity which is the condition of the unequal air-gap between the stator and the rotor. This fault can result from variety of sources such as incor- rect bearing positioning during assembly, worn bearings, a shaft deﬂection, heavy load and so on. In general, there are two forms of air-gap eccentricity: radial (where the axis of the rotor is parallel to the stator axis) and axial. Each of them can be static (where the rotor is displaced from the stator bore centre but is still turning upon its own axis) or dynamic eccentricity (where the rotor is still turning upon the stator bore centre but not on its own centre) (Siddiqui et al., 2015a; Hegde and Maruthi, 2012; Intesar et al., 2011; Sahraoui et al., 2008).
The classical DTC technique is in terms of hysteresis-loop controller with single vector switching table. Its switching frequency differs with speed and load torque, which can bring out high torque pulsation particularly in low speed due to the low switching frequency, which greatly restricts its application . Common disadvantages of conventional DTC are high torque ripple and slow transient response to the step changes in torque during start-up . Therefore, intelligent methods are used such as Artificial Neural Networks (ANN), Fuzzy Logic (FL) and Sliding mode control (SMC) theory . Majority of them are concerned with enhancement of the flux and torque estimator and combined operation of DTC with a space vector-modulation (SVM) technique .