However, the hysteresis controllers used in the conventional DTC gives two major drawbacks which are variable switchingfrequency and high torqueripple. Theoretically, a reduction of torqueripple can be accomplished by reducing the bandwidth of torque hysteresis comparator such that the torqueripple is restricted to be regulated within its band. In such a way, a high speed processor must be utilized in which normally requires an expensive controller board (i.e. a powerful controller board DSPACE1104) and high-cost design in developing Verilog or VHDL code (i.e. in the case of using FPGA). Although, the use of high speed processor in implementing discrete hysteresis controllers still does not guarantee to perform the DTC at a constant switchingfrequency. This is because the slope of torque may vary on operating conditions (i.e. speed, load torque, flux linkages and etc.), which in turns causes the switching in the torque hysteresis controller that is directly affected the inverter switchingfrequency is unpredictable.
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 torqueripple and variable inverter switchingfrequency. This paper presents an improved switching strategy for reducing flux and torque ripples in DTC of PMSMdrives; wherein the torque hysteresis controller and the look-up table used in the conventional DTC are replaced with a constant frequencytorque controller (CFTC) and an optimized look-up table, respectively. It can be shown that a constant switchingfrequency 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.
Different STs are proposed in References [17–21] which vary according to the number of levels of the torque hysteresis controller, the inclusion of the zero voltage vectors (ZVVs), and sector boundaries. In 1997, the application of DTC in PMSM was firstly reported in Reference  using a two-level (2L) hysteresis torque controller and an ST that employs only the six active voltage vectors (AVVs) of the two-level voltage source inverter (2L-VSI). In Reference , the role of ZVVs in DTC systems of PMSM was investigated, and a DTC strategy was proposed using a three-level hysteresis torque comparator and an ST similar to the basic one which was proposed in Reference  for the induction motors (IMs). It was revealed in Reference  that the torqueripple of the DTC system can be reduced using ZVVs, because they cause torque reduction with a lower rate than that of the AVVs. A modified version of the basic ST is presented in Reference  by redefining the boundaries of the sectors where the stator flux vector is located. The main control objective of this modification was the torqueripple reduction; however, it caused a higher stator fluxripple. The effect of using ZVVs at only one state of the ST was studied in Reference , and the simulation results show that it can be beneficial for the steady-state performance and the average switchingfrequency. However, the dynamic response of the torque was not considered. In addition, according to the analysis of the ZVVs’ effect on the DTC of the PMSM presented in Reference , it is supposed to deteriorate when a torque reduction is commanded. Moreover, from a practical point of view, the control algorithm has to be insensitive to the direction of rotation, but the proposed ST in Reference  will not allow the motor speed reversal, as it employs ZVVs to reduce the torque and the stator flux.
Giovanni Serra, Domenico Casadei, Angelo Tani, Luca Zarri and Francesco Profumo introduced the “Performance Analysis of a Speed-Sensorless Induction Motor Drive Based on a Constant-Switching-Frequency DTC Scheme” and said that in steady-state conditions, the current waveforms are sinusoidal for any value of the speed, and the torque is free of ripple matching the reference value. Some current distortion has been observed in the locked-rotor test with a lowtorque command, which is a critical situation for sensorless drives. The dynamic behaviour has been investigated during starting and speed reversal transients.
Jose Rodriguez gives a new method of DTC which is based on load angle control. To obtain the control algorithm, simple equations are used. This makes it easy for implement and understands. By using SVM, lowtorque ripples are obtained and switchingfrequency is maintained constant. This SVM strategy overcomes the drawbacks of classical DTC . Zhifeng Zhang explains a DTC-SVM scheme in favor of an adjustable speed sensor-less induction motor drive which is supplied by a 2-level SVPWM inverter. By using an input-output feedback linearization control, the inverter reference voltage is obtained. Also a full-order adaptive stator flux observer is designed and a new speed adaptive law is given. Thus the stability of the observer system is ensured . S. A. Zaid  suggested a decoupled control of amplitude and stator flux angle to generate the pulses of voltage source inverter. MATLAB/SIMULINK software simulates the suggested and conventional DTC. The use of SVM enables fast speed and torque responses. Variations of motor parameter do not affect the optimization in the new method. M. satheesh Kumar presents the comparative evaluation of the two popular control strategies for induction motor drive. These strategies are classical DTC and DTC-SVM. The Simulink model of both classical and SVPWM directtorque control drives are simulated in all the four quadrant of operation) and the results are analyzed .
Simulation of the proposed DTC controlled induction motor drive is done in MATLAB/SIMULINK environment. Simulation is done using fixed step solver with a step size of 10e -6 . The starting torque is limited to 15.8 N-m. Simulation is done on a 3- induction motor rated at 1.5KW, 1440rpm, four pole, having the following parameters: Rs=7.83 , Rr=7.55 , Ls=0.4751H, Lr=0.4751H, Lm= 0.45351H, J=0.06Kg.m 2 . From the simulation results shown it can be observed that compared with CSVPWM and ADPWM methods using clamping sequences the proposed ADPWM methods results in least harmonic distortion when the drive is operating at near rated speeds. Reduction in stator current ripple can be achieved with the proposed split clamping PWM technique and analytically it is clear that with 30 o , split clamping gives minimum distortion and hence this method is considered as an optimal ADPWM method for the drives operating at near rated speeds. Fig.9- Fig.10 shows the no-load starting and steady state transients in three phase stator currents, torque, speed and stator flux of the CDTC and CSVPWM based DTC induction motor drive. Measured no-load steady state current waveform and its harmonic spectra are also presented for comparison. Fig.11 and Fig.12 shows the simulation results of the split clamping ADPWM based DTC drive using DPWM sequences 012 and 721(with 30 o ) and double switching clamping sequences 0121 and 7212 (with 30 o ) respectively. Elaborated simulation results for the above said ADPWM based DTC IM drive showing different conditions like starting transients, steady state transients, and transients during step change in load, during speed reversal are presented. It is observed that with the proposed method %THD in line current is reduced significantly. Also, observations from the harmonic spectra reveal that with the CDTC method lower order harmonics dominate whereas with the proposed SVPWM method the dominant harmonic components are around the multiples of the switching frequencies. Also with the proposed methods because of usage of DPWM sequences reduction in inverter switching losses can be achieved (though not discussed in this paper). Fig.12f shows the locus of stator flux with the proposed method at high speeds.
electromagnetic torque errors are made within the respective flux and torque hysteresis bands, in order to obtain fast torque response, low inverter switchingfrequency and low harmonic losses. However, DTC drives utilizing hysteresis comparators suffer from high torqueripple and variable switchingfrequency. As in duty ratio control technique, instead of applying a voltage vector for the entire switching period, it is applied for a portion of the switching period and the zero switching state is applied for the rest of the period the ripples is considerably reduced. In this paper the simulation of different DTC schemes (Conventional DTC and Duty Ratio Control DTC) has been carried out using MATLAB/SIMULINK and the results are compared.
The control scheme for PMSM includes director control and vector control out of which directtorque control is popular. DirectTorque Control (DTC) method has been first proposed and applied for induction machines in the mid- 1980’s as reported in . This concept can also be applied to synchronous drives . Indeed, in the late 1990s, DTC techniques for the interior permanent magnet synchronous machine appeared, as reported in .Permanent magnet (PM) synchronous motors are widely used in high-performance drives such as industrial robots and machine tools to their advantages as: high efficiency, high power density, high torque/inertia ratio, and free maintenance. In recent years, the magnetic and thermal capabilities of the PM have been considerably increased by employing the high coercive PM material . For some applications, the DTC becomes unusable, despite it significantly improves the dynamic performance of the drive compared to the vector control due to torque and flux ripples. Indeed, hysteresis controllers used in the conventional structure of the DTC generates a variable switchingfrequency, causing electromagnetic torque oscillations , this frequency is also varying with speed, load torque and hysteresis bands selected . In addition, a high sampling frequency needed for digital implementation of hysteresis comparators and a current and torque distortion caused by sectors changes . Several contributions have been proposed to overcome these problems, by using a multilevel inverter: more voltage space vectors available to control the flux and torque. However, more power switches are needed to achieve a lower ripple and almost fixedswitchingfrequency, which increases the system cost and complexity -. In  and , two structures of modified DTC have been proposed to improve classical DTC performances by replacing the hysteresis controllers and the commutation table by a PI regulator, predictive controller and Space Vector Modulation (SVM). In this paper, a modified DTC algorithm with fixedswitchingfrequency for PMSM is proposed to reduce the flux and torque ripples. It is an extension of the modified DTC scheme for the PMSM proposed by the authors in . The performance of the basic DTC and the proposed DTC scheme is analyzed by modeling and simulation using MATLAB.
Figs. 7*9 show the experimental results of the classical and proposed ST*DTC strategies for dual*three phase PMSMdrives. The rotor speed reference and stator flux reference are set to 300rpm, 0.075Wb, respectively, while the load is about 2.5Nm (partially loaded). It can be seen from Fig. 7 that the torque, stator flux can be controlled well both in the classical and proposed DTC strategies. The torqueripple is slightly reduced due to the decrease of the average amplitude of the applied voltage in one sampling period. The steady*state error of torque has been significantly reduced by using the band*shifted torque regulator . However, it is obvious that the phase currents of the classical DTC strategy are seriously non*sinusoidal, Fig. 8 (a). This is because the components in the z 1 z 2 subspace are not controlled in the classical ST*DTC strategy, hence, the stator flux in the z 1 z 2 subspace
Torque ripples are produced because of use of hysteresis band controller for torque and flux. It also results into variable switchingfrequency. 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 low speeds 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 low speeds 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 torqueripple there is need to improved the Conventional DTC scheme.
These drawbacks affect the result in increased sub-harmonic currents, current ripple and variable switching losses in the inverter . It is show that, the switchingfrequency is mainly affected by the torque hysteresis band and increases with the width of the band. For a fixed band controller, it is therefore necessary to set the band to the maximum (or worst case) condition so that the switchingfrequency is guaranteed not to exceed its limit which is determined by the thermal restriction of the power devices . An adaptive hysteresis band control strategy has been proposed where the band is controlled in real time by variation of the applied voltage vector in order to keep the switchingfrequency constant at any operation condition. This method reduces the torqueripple while maintaining a constant torqueswitchingfrequency.
Increasing The Induction Motor (IM) drivescontrolled with the vector control method has mostly accepted in the industry. However, this control technique requires complex coordinate transformation, inner current control loop and accurate system parameters. On the other hand, the DirectTorque Control (DTC) method provides robust and fast torque response without such coordinate transformation, PWM pulse generation and current regulators. Moreover, DTC minimizes the use of motor parameters technique suffers from a major disadvantage of steady state ripple in torque and flux, because none of the inverter-switching vector is able to generate the exact stator voltage at proper instants as well as in space. These torque and flux ripples affect the accuracy of speed estimation; result in high acoustic noise and harmonic losses. There are many methods to reduce this torque and fluxripple: (a) the alternative inverter topologies, multilevel inverters and matrix converters which increase the number of switches, and thus cost and complexity; (b) the higher switching frequencies reduce the harmonic content of stator current and thus torque and fluxripple. However, such higher switching frequencies lead to increased switching losses and stress on semiconductor switches of the inverter (c) yet, another method of reducing torque and flux ripples is fuzzy logic controller gives constant switchingfrequency. Moreover this method requires complex control schemes than classical DTC and is machine parameter dependent
Abstract— Directtorque control is a control technique used in AC drive system to obtain high performance torque control. The conventional DTC drive contains a pair of hysteresis comparators, a flux and torque estimator and voltage look-up table. The torque and flux are controlled simultaneously by applying suitable voltage vectors, and limiting these quantities within their hytaresis band. At the same time, de-coupled control of torque and flux were achieved. However, the DTC drives utilizing hysteresis comparators suffer from high torqueripple and variable switchingfrequency. The most common solution to this problem is to use the space vector based which depends on the reference torque and flux. The reference voltage vector is then realized using a voltage space vector modulator (SVM). Nevertheless, by applying this method, the basic structure of DTC is lost and needs for high performance of processor. On the other hand, constant frequencytorque controller also has managed to solve the problem. The proposed controller has retained the basic structure of DTC drive as in hysteresis-based. This paper present the comparison and evaluation of the performances of those techniques of DTC drive that applied to induction machine through simulation using MATLAB/SIMULINK. The evaluation was made based on the drive performances, which include dynamic torque, feasibility, and the complexity of the system. The results obtained showed that the constant frequencytorque controller- based gives better performance in terms of dynamic torque and at the same time retain the simple structure of DTC drive system.
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 use of a switching table 1 for voltage vector selection provides fast torque response, low inverter switchingfrequency and low harmonic losses without the complex field orientation by restricting the flux and torque errors within respective flux and torque hysteresis bands with the optimum selection being made. 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.
an appropriately choosing than FOC method because of its high performance applications due to the advantages of reduction in computations  Since the torque and flux estimators in DTC needs and depends on the parameters identification and accuracy of the estimations, the estimation of the electromagnetic torque is essential for the whole system performance. In PWM and flux vector controlleddrives, voltage and frequency were used as control variables and that are modulated and then applied to the motor. This modulator layer needs an additional signal processing time and restricts the torque and speed response. The idea behind DTC is to directly mix the stator flux vector by applying the appropriate voltage vector to the stator windings. This is done by using a pre-designed switching table to directly update the inverter’s discrete switch positions whenever the variables to be controlled, the electromagnetic torque and the stator flux, exceed the hysteresis bounds around their references.
An advanced directtorque control (DTC) technique using Model predictive control (MPC) is proposed for matrix converter (MC)-based permanent-magnet synchronous motor (PMSM) drive system, which reduces the torque ripples, does not need the duty cycle calculation, and ensures the fixedswitchingfrequency. Analytical expressions of change rates of torque and flux of PMSM as a function of MC – dqo components are derived. The predictive model of PMSM and MC is realized by means of State model. Then, the advanced MC-fed DTC algorithm is implemented based on Cost function evaluation. The simulation results exhibit remarkable torqueripple reduction with the help of MPC. As a result, the proposed strategy is proved to be effective in minimizing the torque ripples for MC-based PMSMdrives.
the difference of injected frequency and fundamental frequency and amplitude fairly direct proportional injection amplitude. Two fundamental currents are considered to compare the, effect of torqueripple at fixed signal injection frequency. Fig.9 and Fig.10 compare the torqueripplefrequency at 20Hz and 200Hz fundamental frequency with injection frequency set to 1000Hz. From Fig.9 and Fig.10 it is clear that when the fundamental frequency comes in the order of injection frequency, phase currents distortion is much higher. Higher torqueripple increase the acoustic noise and vibration, so signal injection techniques should be used only for the low speeds.
Above figures 18 & 19 shows Torque and speed wave forms, these are in positive and negative. It can be observed as the speed and torque decreases (during motor braking) the frequency of the current waveforms decreases and as the speed and torque increase (during motor acceleration) the frequency of the current waveforms increases. By the operation of a BLDC motor it can however work as generator. The polarity of Torque can be reversed by simply reversing the polarity of phase current waveforms w.r.t back e.m.f. waveforms. In regenerative breaking it is used an advantage, in vehicle population. For example special arrangements are needed in the power converter to allow the energy returned by the machine, as conventional diode bridge rectifiers are unable to feed back to the A.C. supply. In automobile applications this situation is considerably simplified by using the battery as a source.
AC motors have always been an area of interest in the field of electrical drives. With improvements in technology there is always a need of effective utilization of electrical power as well as the available resources. Nowadays the focus is given mainly on the efficiency of these drives with improvement in the performance of the motors used in the drives. Permanent magnet motors are classified as BLDC and PMSM among which Brushless DC Motor is one of the highly preferred AC motors used in various applications due to various advantages offered such as high efficiency, better speed versus torque characteristics. Although BLDC drives has several advantages it generates torque ripples which is a major concern in high precision applications especially in spacecrafts. Even though the torque generated is less when compared to BLDC motors, PMSM generates less torque ripples. Field oriented control of PMSMdrives are becoming more popular especially in high precision applications. The main objective of this thesis is to make a comparison study between the FOC control of BLDC motor and PMSM in reducing torque ripples in reaction wheels used in satellites. FOC(Field Oriented Control) is used to achieve high performance control characteristics. FOC controlled AC motor drives provide better dynamic response and lesser torque ripples. The closed loop control of the BLDC Motor drive and PMSM drive using FOC is simulated in MATLAB SIMULINK.
As a result of the technological advances in power electronics and semiconductors fields, Adjustable Speed Drives (ASDs), a.k.a. Variable FrequencyDrives (VFDs), are among the most efficient and reliable drive systems of IMs. ASDs have been known with attractive features such as a reliable transient response, control of a continuous range of speed and considerable energy saving . Furthermore, the torque control performance of the ASDs are much superior to that of DC machines drives. This is because of the unprecedented technology evolution in digital microprocessors and DSPs that effectively help to handle complex control problems in the electrical drives schemes.