After nearly a decade of remarkable technological advancements, a superior motor’s control technique was proposed by Isao Takahashi as DirectTorqueControl (DTC) , and Depenbrock as Direct Self Control (DSC) in 1980s . The first industrialized DTC scheme was developed by ABB in the late 1990s . Since its invention, intense research studies have widely devoted to DTC, or DSC, due to its quick dynamic response, simple structure and insensitivity to motor’s parameters. The basic block diagram of DTC, as initially proposed in , is shown in Figure 1.1. In its basic configuration, DTC consists of four main blocks which are: Voltage Source Inverter (VSI), parameters (i.e. stator flux and torque) estimator, Switching Table (ST) and a pair of hysteresis controllers.
Advanced control of electrical machines requires an independent control of magnetic flux and torque. In early days dc machine played an important role since the magnetic flux and torque are easily controlled independently by the stator and rotor currents respectively. The introduction of field oriented control (FOC) in 1970s made huge turn in the control of inductionmotor drive. FOC uses frame transformation to decouple the torque and flux components of the stator current . Therefore the performance of IM becomes similar to that of the dc motor. The implementation of this system however is complicated and is well known to be highly sensitive to parameter variations due to the feed forward structure of its control system . Later in the eighties a new control technique named DirectTorqueControl is introduced , . The DTC is characterized by its simple structure and fast dynamic response. Also the inverter is directly controlled by the algorithm, i.e. a modulation technique for inverter is not needed. The main advantages of DTC are absence of co- ordinate transformation and current regulator, absence of separate voltage modulation block . Common disadvantages of conventional DTC are sluggish response in both starts up and load changes . Recent advancements in
Torqueripples are produced because of use of hysteresis band controller for torque and flux. It also results into variable switching frequency. Also as discussed in previous section, estimation of stator flux is needed for implementation of classical DTC. Stator flux estimation can be done using voltage model estimation or current model estimation. Current model estimation requires speed sensor and hence generally not used. Voltage model estimation uses pure integration which causes problems at lowspeeds and caused flux drooping. Also at low speed, the stator resistance drop can no longer be ignored, so a boost in stator flux is required . Now when stator flux vector changes the sector, there is no active voltage vector which guarantees the increase in stator flux. As a result there is flux drop at some instances. Thus at lowspeeds and heavy loads locus of stator flux vector cannot remain circle and becomes more like hexagon, which causes harmonics in stator current. To minimized this torque ripple there is need to improved the Conventional DTC scheme.
Directtorquecontrol is becoming the industrial standards for inductionmotortorquecontrol. This paper presents switching loss minimization technique of improved directtorquecontrol (DTC) of inductionmotor. Directtorquecontrol (DTC) of an inductionmotor supplied by a voltage source inverter is a simple scheme that does not need long computation time, can be implanted without speed sensors and is insensitive to parameter variations. In principle, the motor terminal voltages and currents are used to estimate the motor flux and torque. Based on the instantaneous errors in torque and stator flux magnitude and estimates of the flux position, a voltage vector is selected to limit the flux and torque errors within their flux and torque hysteresis bands. In the conventional DTC, the selected voltage vector applies for the whole switching period, irrespective of the magnitude of the torque error. DTC drive gives variable switching frequency and high torque ripple. DTC gives torque and flux ripples because no any VSI states are capable to generate the exact voltage vector from switching table required to make zero both the torque electromagnetic error and the stator flux error. To minimize this problem, a torque hysteresis band with variable amplitude fuzzy logic controller is proposed. The fuzzy logic controller is used to reduce the flux and torqueripples and it improves performance DTC especially at low speed. A duty ratio control scheme for an inverter-fed induction machine using DTC method is presented in this article. The use of the duty ratio control gives improved steady state torque response, with less torque ripple than the conventional DTC. Fuzzy logic control (FLC) used to implement the duty ratio controller. Total harmonic distortion (THD) calculation of electromagnetic torque, rotor speed and stator current of DTC and DTC with fuzzy has done successfully in this article. With the help of FLC with duty ratio, 8% THD in torque, speed and stator current have minimized compared with DTC (Uddin and Hafeez, 2012). In this paper, switching loses minimization technique through THD minimization. Switching losses are minimized because the transistors are only switched when it is needed to keep torque and flux within their hysteresis bounds, improve efficiency & reduced losses. Directtorquecontrol with the fuzzy logic controller has verified by MATLAB SIMULINK and experimentally.
techniques for high performance electrical drives with Induction machines in Industry applications is a novel PWM based DirectTorqueControl of Inductionmotor Drive. However its inherent fast switching frequency has among other effects, the drawback of generating high level common mode voltage variations with resulting high frequency common mode currents flowing to the ground through the parasitic capacitances between different parts of the drive and the ground. To reduce these common mode emissions the selection of Inverter switching are limited. High current and Torque ripple is observed in DTC which is mainly because of the Look Up table arrangement. This can be overcome by employing improved switching table by using adaptive control. To reduce the emissions a Optimal switching table is used instead of look up table, in which each sector consists of three vectors, the odd voltage vectors are used in odd sectors and even voltage vectors are used in even sectors therefore the zero voltage vectors are eliminated, which reduces the Torqueripples, Flux and Total Harmonic Distortions-. At the initial stage of developing the DTC method a VSI is employed to generate the voltages that control the speed and torque of the inductionmotor. In this paper, the proposed novel PWM based DTC algorithm reduces the common mode emissions of the drive.
The inductionmotor is most widely because of its high reliability, robust in operations, relatively low cost and modest maintenance requirements. But they require much more complex methods of control, more expensive and higher rated power converters than DC and permanent magnet machines. Three phase inductionmotor is widely used in industrial drive because they are reliable and rugged. Single phase induction motors are widely used for heavier loads for example in fans in household appliances. The fix speed service, induction motors are being increased with variable frequency drives. Inductionmotor achieves a quick torque response, and has been applied in various industrial applications instead of dc motors. It permits independent control of the torque and flux by decoupling the stator current into two orthogonal components FOC (Field Oriented Control). However it is very sensitive to flux, which is mainly affected by parameter variations. It depends on accurate parameter identification to achieve the expected performance. The vector control of IM drive for speed control is mainly classified into two types such as field oriented control (FOC) and directtorquecontrol (DTC). In FOC, the speed of the inductionmotor is controlled like a separately excited dc-motor with more transformations and complexity involved in the system. In order to control the inductionmotor speed in simple way without required any transformations the DTC is used. In the middle of 1980 directtorquecontrol was developed by Takahashi and Depenbrock as an alternative to field oriented control to overcome its problems. Directtorquecontrol is derived from the fact that on the basis of the errors between the reference and the estimated values of torque and flux it is possible to directly control the inverter states in order to reduce the torque and flux errors within the prefixed band limits. Directtorquecontrol is a strategy research for inductionmotor speed adjustment feeding by variable frequency converter. It controls torque on the base of keeping the flux value invariable by choosing voltage space vector.
DirectTorqueControl (DTC) strategies of Inductionmotor drives are mostly used in variable speed applications . The most prominent control objective concerning the inductionmotor is to keep the electromechanical torque within bounds around its reference; it consists of hysteresis band control for controlling of flux and torque [2, 5 and 6].DTC Strategy is used to compare error between set and estimated flux and torque values. In DTC theory, the machine model is of Stationary Reference frame, due to this demagnetization phenomenon causes directtorquecontrol to affect high flux ripple and torque ripple[15, 12]. This phenomenon commonly affects DTC strategy performance at lowspeeds as well as at low values of the dc bus voltage. The goal of DTC strategy of inductionmotor is to reduce torque ripple and flux ripple which can be achieved by bus clamped operation of inverter . Further, the conventional approach of prescribed bands of hysteresis torquecontrol in DTC can be altered with higher order hysteresis band torquecontrol for more reduction of torque ripple. The higher order hysteresis toque controller is operated with duty cycle control, which affects the active time of inverter voltage vectors . By adopting duty cycle control to the higher hysteresis band torquecontrol based bus clamped directtorquecontrol of inductionmotor makes reduction in torque ripple and flux ripple [8,11]. The proposed duty cycle control based four level torque hysteresis band directtorquecontrol of induction machine (4L THB BCDTC) has low inverter switching losses, low current harmonic distortion to the motor has and torque ripple reduction.
output voltage which gives quick torque response and is highly efficient. Here efficiency optimization in steady state operation has been considered and this proposed control circuit has the disadvantage of making some drift in extremely low frequency operation which can however be compensated easily and automatically to reduce the effect of variation of machine constant. Thomas G Habetler  proposed a DTC strategy of an induction machine based on predictive, deadbeat control of the flux and torque. Here variations in torque and flux over the switching period is analyzed by estimating the voltage behind the transient reactance and the synchronous speed. The stator voltage is calculated which is required to track the reference flux and torque. Then Space vector PWM is used to define the inverter switching state. An alternative approach to deadbeat control is also implemented.
Like a every control method has some advantages and disadvantages, DTC method has too. Some of the advantages are lower parameters dependency, making the system more robust and easier implements and the disadvantages are difficult to control flux and torque at low speed, current and torque distortion during the change of the sector in d-q plane, variable switching frequency, a high sampling frequency needed for digital implementation of hysteresis controllers, high torque ripple. The torque ripple generates noise and vibrations, causes errors in sensor less motor drives, and associated current ripples are in turn responsible for the EMI. The reason of the high current and torque ripple in DTC is the presence of hysteresis comparators together the limited number of available voltage vectors . To reduce the torque ripple, the stator flux vector change, which is demanded to compensate the torque and flux errors, determination should be done and also any voltage vector should be produced by the control mechanism. If a higher number of voltage vectors than those used in conventional DTC is used, the favourable motorcontrol can be obtained. Because of complexity of power and control circuit, this approach is not satisfactory for low or medium power applications. One of the methods to increase the number of available vectors is an on-line modulation between active and null vectors .
Cruz et al.  compare FOC, DTC and input-output linearization based on steady-state torque ripple, current peak, and switching frequency. They conclude that FOC and DTC are “good” in dynamic response, and that the parameter sensitivities are “low” and “medium” in DTC and FOC, respectively. Wolbank et al.  compare low and zero-speed applications of DTC and sensorless FOC. They study steadystate stability and speed overshoot, where FOC shows slower dynamics but better steady-state tracking compared to DTC. As both FOC and DTC have drawbacks, an interesting combination of DTC and FOC is presented in . The resulting directtorque and stator flux control method (DTFC) uses no voltage modulation, current regulation loops, coordinate transformations, or voltage decoupling. Casadei et al.  evaluate standard DTC and DFOC and present a unique scheme, discrete space vector modulation (DSVM), which is a variation of the standard SVM. Performance criteria are steady-state current and torqueripples, and dynamic response due to a torque step.
ABSTRACT: This paper, presents speed control and torquecontrol method for Inductionmotor, by using DTC based fuzzy logic, it is applied in switching select voltage vector .The comparison with conventional directtorquecontrol (DTC), show that the use of the DTC_FL , reduced the torque, stator flux, and current ripples. The validity of the proposed methods is confirmed by the simulative results
In Directtorquecontrol of three-phase inductionmotor, the motor is fed by three-phase voltage source inverter. There are 2 3 = 8 switching combinations available in two-level three- phase inverter. These consist of 6 non-zero (active) voltage vectors and two zero voltage vectors having the switching state of either all of the upper switches “on” or all of bottom switches “on”. These voltage vectors are composed of three different sets of vectors having different amplitude. These voltage vectors divide the space plane into six sectors. The more number of vectors increase the choices of optimum voltage vector selection. The different amplitudes of the space voltage vectors increase the flexibility in minimizing the ripple of the stator flux and torque. If the switching controller is fast enough to allow the vector on for a sufficiently short period, then it may be plausible to select the reverse voltage vectors. Vector U5 in particular, can cause a very rapid torque decrease, and most selections of this vector result in an appreciable torque undershoot, especially at higher operation speeds.
ABSTRACT: This paper presents an improved DirectTorqueControl (DTC) of inductionmotor. DTC drive gives the high torque ripple. In DTC inductionmotor drive there are torque and flux ripples because none of the VSI states is able to generate the exact voltage value required to make zero both the torque electromagnetic error and the stator flux error. To overcome this problem a torque hysteresis band with variable amplitude is proposed based on fuzzy logic. The fuzzy logic controller is used to reducing the torque and flux ripples and it improve performance DTC especially at low speed.
Inductionmotor is widely used in industry because of its simple construction, low cost, low maintenance and good reliability. and also it can be used in any environment because of absence of commutator and related spark problems. IM control methods can be divided into scalar and vector control. The general classification of the variable-frequency methods is presented in Fig. 1. In scalar control, which is based on relationships valid in steady state, only magnitude and frequency (angular speed) of voltage, current, and flux linkage space vectors are controlled. Thus, the scalar control does not act on space vector position during transients. Contrarily, in vector control, which is based on relations valid for dynamic states, not only magnitude and frequency (angular speed) but also instantaneous positions of voltage, current, and flux space vectors are controlled. Thus, the vector control acts on the positions of the space vectors and provides their correct orientation both in steady state and during transients.
could be necessary to prepare a table with several com- plex gains designed for each speed desired or to specif- ically speed range. Thus, the complex vector notation and the complex controller can become an interesting tool for the implementation of three-phase inductionmotordirectcontrol drives. Operation at low speed was explored but it requires a more complete study in the future.
The compensation technique presented in  was implemented both in simulation and in hardware. Experimental results will be discussed in section 6.1. The simulation results of the implementation are presented here. Figure 3-7 shows the Simulink model of the implemented algorithm on an open-loop Volts-Hertz controlled inductionmotor drive. A fixed step size of 100 s was used for simulation. The parameters for the inductionmotor used are the same as the actual motor on which hardware implementation is done. The motor was excited by a voltage with a stator frequency of 5Hz and amplitude corresponding to the rated V/f ratio. The amplitude of the input voltage was compensated for the drop in voltage due to the stator resistance. This effect is more prominent at lowspeeds when the magnitudes of the two voltages are comparable. The machine is operated at no-load conditions. The effects of the nonlinear behavior of the inverter have not been considered.
In industry, more than half of the total electrical energy produced is consumed by electric motors . Among several types of electric motors, three-phase induction machines (IMs) occupy a prominent place. Indeed, at least 80% of industrial control systems use induction motors , which have gradually taken the place of DC machines because of their good performance: reliability, simple construction, low cost and simple maintenance [3, 4]. However, these numerous advantages are not with- out inconvenience, the dynamic behavior of the machine is often very complex [5, 6], since its modeling results in a system of nonlinear equations, strongly coupled and mul- tivariable. In addition, some of its state variables, such as flux, are not measurable . These constraints require more advanced control algorithms to control the torque and flux of these machines in real time . For several
In the 80’s, new IM torquecontrol techniques was developed. Takahasi& Noguchi presented the DirectTorqueControl (DTC). It is most convenient for low and medium power applications. In the DTC scheme, the inverter bridge switch connections are directly selected using a qualitative behavior rule set in order to control the stator flux and the torque. The DTC scheme produces a fast torque response while keeping the IM stator flux and torque decoupled. With this scheme the torque and flux presents a high ripple.
Directtorquecontrol is one of the methods which is used in variable frequency drives for the control of the inductionmotor. Directtorquecontrol has emerged over the last decade to become one possible alternative to the well-known Vector Control of Induction Machines. In DTC, the stator flux and the torque are directly controlled by selecting the appropriate inverter state. The output of the speed regulator (PI controller) results in generation of the reference torque. However the PI controller cannot result in perfect control if its parameters Kp, Ki are not properly chosen. The undesired torque and flux ripple may occur in conventional directtorque controlled inductionmotor drive. DTC can improve the system performance at lowspeeds by continuously tuning the regulator by adjusting the Kp, Ki values. Many artificial intelligence techniques and random search methods have been employed to improve the control parameters.
In discrete duty cycle based control method for DTC of asynchronous motor drives with model predictive solution , they proposed a DTC which provides a global less torque ripple, which satisﬁes the root-mean-square torque ripple criteria. The global less torque ripple DTC is a two-step design. The ﬁrst step derives the torque error to zero at the end of the control period. Then, the next step minimizes the torque bias and rms ripple by changing the voltage vector asymmetry pattern in switching of the ﬁrst step into symmetry ones. The common concept to reducing the torque ripple is the synthesis of a higher amount of voltage space vectors with respect to those used in basic DTC techniques. In DTC of asynchronous motor using SVM , they proposed a new duty cycle based control technique to minimize torque and ﬂux ripples of the conventional DTC.