The proposed system and a hysteresis-based system of DTC-MLI for induction machine have been simulated using MATLAB/Simulink. The parameters of PI controller that obtained in the previous section were used in the simulation as well as in the experiment. To show the feasibility of the proposed system, an experiment was also carried out. Figure 10 shows the block diagram of the experimental set-up. A dSPACE DS1104 controller card based on a TMS320F240 DSP, ALTERA DE2 FPGA board, IGBT-based 5-level CHMI, a 1.5kW squirrel-cage induction machine coupled to a DC machine was used to execute the experiment. The DS1104 was used to implement the hysteresis and the proposed controller also to estimate the torque and stator flux at a sampling period 75µs. The FPGA was used to implement the voltage vector selection table as well as to generate the blanking time for the IGBT.
ABSTRACT: This paper presents plugging operation of induction motor drives for stopping and speed reversal. Classical direct torque control (CDTC) is known to produce quick and robust response in induction motor drives. However, during steady state, notable torque, flux, and current pulsations occur. They are reflected in torque estimation, speed response, and also in increased acoustical noise. A new direct torque and flux control based on space- vector modulation (DTC-SVM) is proposed for induction motor drives, in comparison with the classical DTC method the inverter switching frequency is constant and is dramatically increased, requiring neither any increase of the sampling frequency, nor any high frequency dither signal. The constant switching frequency approach to direct torque controlled induction motor drive results the reduced torque, current and flux ripple.
The new control strategies of direct torque control (DTC) are presented by I.Takashashi and T.Noguchi in 1986 . This both persons were propose circle flux trajectory and hexagon flux trajectory. In addition, a de-coupled control of stator flux and torque is providing fast dynamic response. Basically, the DTC consist a pair of hysteresis comparator, torque and flux estimator, look-up table and 3-phase voltage source inverter (VSI). The torque and stator flux is controlled by 3-level and 2-level hysteresis comparators, respectively. In order to satisfy the demand from both controllers, the appropriate voltage vectors from the look-up table are selected either to increase or decrease torque and flux . The high performance of DTC can be achieve by having accurate estimation of flux and torque. Other else, we also should know that the switching frequency of VSI is contributed by hysteresis comparator. It already been highlighted in  that the operating condition such as rotor speed, stator and rotor fluxes and DC link voltage change also will varies the switching frequency.
A nine level inverter consists of a series H-bridge inverter units connected to three phase induction motor. The general function of this multilevelinverter is to synthesize a desired voltage from several dc sources. The AC terminal voltages of each bridge are connected in series. Unlike the diode clamp or flying capacitors inverter, the cascaded inverter does not require any voltage clamping diodes or voltage balancing capacitors .This configuration is useful for constantfrequency applications such as active front-end rectifiers, active power filters, and reactive power compensation. In this case, the power supply could also be the voltage regulated dc capacitor. One important characteristic of multilevel converters using voltage escalation is that electric power distribution and switching frequency present advantages for the implementation of these topologies. The Fig.1 shows the multilevelinverter topology. This paper makes an overview to find the various induction motor drive configurations used in industry . The various control strategies used to improve drive efficiency and various inverter used to control the motor speed, reduce torque ripple, current ripple and reduce harmonics . Also different topologies and control strategies are useful for different situations. One of the very efficiently used control strategies is the sinusoidal PWM control which can be implemented.
In the conventional DTC, hysteresis controllers are a two value bang bang controllers, which has the same outputs for both small and big torque errors. Therefore torque ripples are produced. The torque ripples can be minimized by dividing the torque errors into several intervals on which control action taken. In this paper fuzzy logic based direct torque control is proposed. Here two fuzzy logic controllers for both flux and torque are proposed along with space vector modulation. The fuzzy controllers allow faster response and SVM technique provide a constantinverter switching frequency so small torque ripples and current distortion.
DTC technique that proposed using conventional inverter has several disadvantages such as less selection of effective voltage vector hence lead to high switching frequency. The limited voltage vectors selection are as shown in Figure 1.1 which comprise of six voltage amplitude. Less effective voltage vector happen because the fixed of two level in the conventional inverter has cause inappropriate selection of switching occur especially when changes of the speed happen. The high rate change of voltage (dv/dt) has cause increase in torque ripple . By this improper selection of switch also lead to the increase in switching frequency. High switching frequency has cause losses to the operation of induction machine and increase possibility of overshoot to happen .
DTC block is the main element of the simulink has flux and torque hysteresis block, toque and flux calculator block, switching control block and switching table block. For the estimation of the motor flux d-q components and electromagnetic torque, the torque and flux calculator blocks are used. The flux and torque hysteresis block has and three-level hysteresis comparator for the toque control and two-level hysteresis comparator for flux control. For the proper selection of voltage vector the switching table has look-up tables in it working according to the output of the flux and torque hysteresis comparison. Switching control block is used to limit the inverter commutation frequency.
The Direct Torque Control (DTC) applied to the asynchronous machines appeared in the middle of 1980s. DTC is recognized today as a high-performance control strategy for AC drives [3-5]. The main objective of DTC is controlled the stator flux and electromagnetic torque by hysteresis regulators. This is to maintain torque and stator flux within the hysteresis band. DTC technique can give satisfactory results when multilevel inverters are used. This inverter provides nearly sinusoidal voltages with very low distortion, using less switching devices. Due to the small dv/dt’s, torque ripple is greatly reduced. However the simulation cost will increase if we consider the instantaneous model of multilevelinverter and induction machine.
The DTC strategy was implemented using a Texas Instruments DSP TMS320F2812 platform. The sys- tem consists of a three-phase voltage source inverter with insulated-gate bipolar transistors (IGBTs) and the three-phase induction motor shown in the appendix. The stator voltage commands are modulated by us- ing symmetrical space vector PWM, with switching fre- quency equal to 2.5 kHz. The DC bus voltage of the inverter is 226 V. The stator voltages and currents are sampled in the frequency of 2.5 kHz. A conventional PI controller generates a torque reference by using the speed error. The flux and torque estimation, and the flux-torque complex regulator and speed controller have the same sampling frequency of 2.5 kHz. The encoder resolution is 1500 pulses per revolution.
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 .
ABSTRACT: Direct torque control is used over Field oriented control (FOC) because of its simple control structure and in steady state, transient state operating conditions shows better torque control. Direct torque control has the advantages like robust and fast torque responsive. But in low speed operation stator flux estimation raises difficulty due to improper working of an open loop voltage model observer and existence of an open loop integrator. Hence an adaptive flux observer is implemented which eliminates open loop integration, therefore improves the machine performance by minimizing stator current distortions, fast response of rotor speed, stator flux electro-magnetic torque without ripple, constant switching frequency. Voltage distortions are modelled using a non-linear inverter. In this paper a novel approach is seen where fuzzy logic controller is adapted which overcomes high torque ripples and improves the system performance. Simulation results are carried out for the proposed system.
Torque pulsations in BLDC motors brought about by the deviation from ideal conditions are either related to the design factors of the motor or to the power inverter supply, thereby resulting in non-ideal current waveforms. Undesirable torque pulsation in the BLDC motor drivecauses speed oscillations and excitation of resonances in mechanical portions of the drive, leads the audible noise and visible vibration patterns in high precision machines. In this paper, a five level inverter with PI controller is presented for BLDC.This paper has proposed harmonics and torque ripples have been reduced using multilevelinverter with the current controlled technique. The harmonics content of the voltage and Current for a BLDC motor is analyzed and the amount of torque ripple and the THD also calculated. The main advantage of this method is it uses one current controller for the three phases. Finally a generalized expression for highest order harmonic based on switching frequency and number of levels is derived. Matlab/Simulink models are developed.
meet the load angle reference, and the desired torque while keeping the flux amplitude constant. A space vector modulation algorithm is used to apply the required stator voltage vector. It is expected that torque ripple is almost eliminated. By additional of proportional-integral (PI) regulator, a simple flux calculation block is applied and at the same time departs from the rotating coordinate transformation. This strategy is straightforward by using of equation (7) . The block diagram of DTC-SVM based is illustrated in ‘figure 5’. Even this technique has managed to overcome the two main drawback of conventional DTC, it is leads to increase the complexity of DTC configuration and needs for fast processor to be implemented in digital 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 DTCcontroller 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 induction motor using proper electrical transducer. Then flux and torque errors are found out by equation (3) and (4) .
As shown in Figures. 11(b1) and 13(b1), during the commutation from sector IV to sector V, the un- balancing between the current rising time and the cur- rent falling time affected the electromagnetic torque waveform. As a result of this, the DTC-2 strategy is penalized by a high torque dips. The comparison of Figures.10(c1) and 12(c1) confirms that commutation torque dips is more serious and more strong at high speed.
To improve motor efficiency, the flux must be reduced with an expert control algorithm. That tries to obtain a balance between copper and iron loss when torque is constant . Vector control (FOC) and Direct torque control (sensorless vector control) are the two most popular techniques for induction motor torque control . Unlike Vector control, direct torque control (DTC) does not require coordinate transformation and any current regulator and encoder. It controls flux and torque directly based on their instantaneous errors  . In spite of its implicitly, direct torque control is capable of generating fast torque response. In addition, direct torque control minimizes the use of machine parameters; hence it is much less sensible to parameter variation . For such reasons, DTC has become one of the most popular methods for induction motor drive system control. However, there are many disadvantages with this control method. The most significant problem with DTC is that the nominal value of flux is optimized for nominal motor operating point. But at light load, using the same flux value decreases the power factor and efficiency of the drive .
Sangita Das Biswas completed M.tech in Electrical Engineering in 2003 from University of Calcutta, Kolkata, West Bengal, India. Now she is serving as Assistant Professor in Tripura University, Suryamaninagar, Agartala, Tripura, India from 2005 onwards. Research interests include Multilevel Inverters, Electrical Power Converters, FACTS devices.
Electrical Energy already constitutes quite half-hour of all energy usage on Earth. And this is often set to rise within the returning years. Its huge quality has been caused by its potency of use, simple transportation, simple generation, and environment-friendliness. A part of the full current production is sued to provide heat, light, in electrolysis, arc- furnaces, domestic heating etc. Another massive a part of the current production is employed to be reborn into energy via completely different styles of electrical motors- DC Motors, Synchronous Motors and Induction Motors. Induction Motors square measure typically termed the “Workhorse of the Industry”. This is often as a result of it's one among the foremost wide used motors within the world. It’s employed in transportation and industries, and conjointly in family appliances, and laboratories. The main reasons behind the recognition of the Induction Motors are:
apparatus with require higher power and also some medium voltage motor drives and utility applications need medium voltage and megawatt power levels. It is vexatious to connect only one power semiconductor switch directly for a medium voltage grid. Respectively, a multilevel power converter structure has been introduced for high power and medium voltage situation as an alternative. Multilevel Inverters usually achieve high power ratings, and also enable the use of renewable energy sources such as photovoltaic, wind, and fuel cells. Multilevel Inverter's achieves higher power by using power semiconductor switches with several dc voltage sources. MLI's perform the power conversion by synthesizing a desired staircase voltage waveform. The commutation of these power switches aggregates these multiple DC sources for achieving high voltage at the output. Also the rated voltage of the power semiconductor switches depends only on the rating of the dc voltage sources to which they are connected. In this paper a survey on various multilevelinverter topologies is studied along with its various modulation techniques. Simulation is carried out using MATLAB / SIMULINK. The simulation is carried out for NPCMLI with ILBC for an unmodulated system, driving a motor drive as load. The results show that the DC Link unbalance in NPCMLI can be avoided using ILBC and required motor speed can be maintained for an unmodulated system.
The conventional PWM inverters are able to generate mostly two level output waveforms. Many improvements have been proposed to classic configuration as regards the circuit structures, the control schemes, but the inverter performance was limited by the available number of levels to build the output voltage. To overcome these difficulties several topologies of multilevelinverter have been recently designed and they are now capturing the attention of many industries, specially for high power converters on account of certain advantages that these inverters have as compared to conventional PWM inverters like less switching losses, reduced harmonic losses, etc. In spite of these advantages some problem are also associated with these inverters. Out of these, capacitor voltage unbalance is prominent one. In this paper the SVM technique has been applied to formulate the switching pattern for three level inverter that minimizes the harmonic distortion at the inverter output.