Torque Control Of Induction Motor Research Articles

A simple hardware realization of switching table based direct torque control of induction motor

A simple hardware realization of direct torque control (DTC) scheme for fast control of induction motor torque has been presented. The proposed hardware scheme mimics the conventional switching table based DTC scheme and does not require use of any online fast computing device like microprocessor, PC or DSP. The controller uses only commonly used discrete hardware components. The experimental results of the proposed hardware implementation have been presented and supported with simulation results. Another alternate hardware scheme without using the switching table has also been proposed and the experimental results of both the schemes have been compared and found to be in agreement with each other.

Development of torque and flux ripple minimization algorithm for direct torque control of induction motor drive

Direct torque control (DTC) is known to produce quick and robust response in AC drives. However, during steady state, notable torque and flux ripple occur. They are reflected in speed estimation, speed response and also in increased noise. This paper proposes a new control algorithm, which provides decoupled control of the torque, and flux with constant inverter switching frequency and a minimum torque and flux ripple. Compared to the other DTC methods, this algorithm is much simpler and has less mathematical operations, and can be implemented on most existing digital drive controllers. Algorithm is based on imposing the flux vector spatial orientation and rotation speed, which defines the unique solution for reference stator voltage. This paper contributes (a) Calculation of stator flux vector, torque and flux increments (b) The position of new stator flux vector determination (c) Calculation of the stator reference voltage (d) comparison of errors of different control strategies. In this paper, computer simulations and experimental results have been discussed for the proposed algorithm.

Improved direct torque control for induction motor drives with rapid prototyping system

Direct torque control is an AC drives control method especially designed to provide fast and robust responses, but they usually behave like hysteresis controllers. Consequently, there are similar problems like non-constant switching frequencies and high torque ripple, especially when operation conditions result in low switching frequencies. In this paper, the basic concept of DTC will be improved by modifying the classical method. Problems that arise when using standard DTC methods are explained, and improvements are derived to overcome these problems. Two theoretical approaches are verified and compared with standard DTC by measurements and experiments. The proposed methods provide a constant switching frequency and solve the starting problem without dither signal. Further results are a reduction of torque pulsations as well as a new method for stator flux estimation. Experimental results of the new DTC method obtained by an experimental rapid prototype system for drives are presented.

A novel sliding-mode control of induction motor using space vector modulation technique

This paper presents a novel sliding-mode control method for torque control of induction motors. The control principle is based on sliding-mode control combined with space vector modulation technique. The sliding-mode control contributes to the robustness of induction motor drives, and the space vector modulation improves the torque, flux, and current steady-state performance by reducing the ripple. The Lyapunov direct method is used to ensure the reaching and sustaining of sliding mode and stability of the control system. The performance of the proposed system is compared with those of conventional sliding-mode controller and classical PI controller. Finally, computer simulation results show that the proposed control scheme provides robust dynamic characteristics with low torque ripple.

LOW SPEED SENSORLESS VARIABLE STRUCTURE CONTROL OF INDUCTION MOTOR

Torque and speed sensorless induction motor control is presented in this paper. The idea is realized using a sliding mode closed loop rotor flux observer for estimation of electromotive force of machine. This signal is then used in nonlinear stator flux and torque control of induction motor and in rotor flux observer for speed and flux estimation. The analysis of the proposed method is included. Proposed control scheme was implemented on DSP system extended with FPGA where PWM procedure with dead time compensation was realized. Experimental results demonstrated high efficiency of the proposed estimation and control method.

Flux Compensated Direct Torque Control of Induction Motor Drives for Low Speed Operation

Speed control of direct torque controlled (DTC) induction motor drives depends on effectively establishing the stator flux. However, it becomes difficult when the motor operates in the low speed region, because the voltage drop on the stator resistance is comparable with the input stator voltage. Therefore, this study proposes an easy but effective way to compensate the voltage drop on the stator resistance so that the stator flux can be constructed without identifying the stator resistance as done by most authors. As a result, motor torque is constructed due to the effective stator flux compensation, which makes the DTC applicable to induction motor drives in the low speed region. Moreover, a fixed-point digital signal processor (DSP)-based hardware experimental system is built to demonstrate the effectiveness of the proposed control method.

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.

Speed sensorless direct torque control of IMs with rotor resistance estimation

Direct torque control (DTC) of induction motors (IMs) requires an accurate knowledge on the amplitude and angular position of the controlled flux in addition to the information related to angular velocity for velocity control applications. However, unknown load torque and uncertainties related to stator/rotor resistances due to operating conditions constitute major challenges for the performance of such systems. The determination of stator resistance can be performed by measurements, but methods must be developed for estimation and identification of rotor resistance and load torque. In this study, an EKF based solution is sought for determination of the rotor resistance and load torque as well as the above mentioned states required for DTC. The EKF algorithm used in conjunction with the speed sensorless DTC is tested under eleven scenarios comprised of various changes made in the velocity reference beside the load torque and rotor resistance values assigned in the model. With no a priori information in the estimated states and parameters, it has been demonstrated that the EKF estimation and sensorless DTC perform quite well in spite of the uncertainties and variations imposed on the system.

Variable-structure torque control of induction motors using space vector modulation

Compared with field orient control (FOC), direct torque control (DTC) is known to provide fast and robust response for induction motors. However, while offering high dynamic performance, classical DTC produces notable torque, flux, and current ripples, and operates with a variable inverter switching frequency. In this paper, a novel torque control scheme for induction motors is proposed. The stator flux and the electrical torque are directly controlled by variable-structure controllers, and stator voltage vectors calculated by the variable-structure controllers are applied to the motor by means of space vector modulation. The proposed scheme therefore provides smooth, fast and robust regulations of the electrical torque and the stator flux. Moreover, the implementation of the scheme is simple. Theoretical analysis shows the asymptotical convergence of electrical torque and stator flux tracking. Simulation and experimental results are provided to evaluate the performance of the proposed scheme.

Modeling and simulation of FPGA-based variable-speed drives using Simulink

This paper presents a novel approach for the design and simulation of variable-speed drives when the control algorithm is to be implemented in an Field Programmable Gate Array (FPGA). The specialized Simulink tools used and the design procedure are presented. An application example using a Direct Torque Control (DTC) induction motor drive is presented and discussed.

Sensorless direct torque control of induction motors used in electric vehicle

A three-phase squirrel-cage induction motor is used as the propulsion system of an electric vehicle (EV). The motor is controlled at different operating conditions using a direct torque control (DTC) technique combined with a new switching pattem producing low harmonics. The operating flux of the motor is chosen optimally for both loss minimization and good dynamic response. Since speed estimation is sensitive to rotor resistance variations, the rotor resistance value is calculated and modified in real-time continuously. Simulation and experimental results show that the proposed DTC is able to follow the reference speed (which may be only input reference of the system) with a reasonable dynamic and relatively low error.

Sensorless Direct Torque Control of Induction Motors Used in Electric Vehicle

A three-phase squirrel-cage induction motor is used as the propulsion system of an electric vehicle (EV). The motor is controlled at different operating conditions using a direct torque control (DTC) technique combined with a new switching pattem producing low harmonics. The operating flux of the motor is chosen optimally for both loss minimization and good dynamic response. Since speed estimation is sensitive to rotor resistance variations, the rotor resistance value is calculated and modified in real-time continuously. Simulation and experimental results show that the proposed DTC is able to follow the reference speed (which may be only input reference of the system) with a reasonable dynamic and relatively low error.

Nonlinear adaptive speed and torque control of induction motors with unknown rotor resistance

In this paper, we propose a nonlinear adaptive speed and torque controller of induction motors with unknown rotor resistance. All the system parameters except rotor resistance are assumed to be known, and only the stator currents and rotor speed are assumed to be available. The desired speed and torque should be a smooth bounded function. A complete proof of the global stability without singularity is given, and the output error will converge to zero asymptotically. Finally, the simulation and experimental results are given to demonstrate the effectiveness of the proposed controller.

A new approach to direct torque control of induction motor drives for constant inverter switching frequency and torque ripple reduction

In this paper, a new approach to the direct torque control (DTC) of induction motor drives is presented. In comparison with the conventional DTC methods, 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 well-developed space vector modulation technique is applied to inverter control in the proposed DTC-based induction motor drive system, thereby dramatically reducing the torque ripple and speed ripple. As compared to the existing DTC approach with constant inverter switching frequency, the presented new approach does not invoke any concept of deadbeat control, thereby dramatically reducing the computations. Experimental results are illustrated in this paper confirming that the proposed DTC method has the above-mentioned features even at the low speed range down to /spl plusmn/1 r/min.

Direct torque control of induction motors: stability analysis and performance improvement

Practitioners in the electric drives community have recently introduced a novel torque control strategy for high power induction motor applications, called direct torque control (DTC), which has three distinctive features: 1) it focuses on stator flux regulation; 2) it does not aim at an asymptotic system inversion; and 3) it explicitly takes into account the discrete nature of the control actions. Besides its simplicity, it is claimed that the achieved performance is superior to field oriented strategies because of the digital form of the control structure and the reduced dependence on parameter variations of the stator flux calculations. Our objective in this paper is twofold: 1) to carry out a mathematical analysis of the stabilization mechanism of DTC, which helps us assess the achievable performance of the current scheme and provide guidelines for its tuning; and 2) to propose a modified DTC to improve its dynamical behavior. Experimental results are presented to illustrate the main points of our paper.

Improvement of low-speed operation performance of DTC for three-level inverter-fed induction motors

A direct torque control algorithm for three-level inverter-fed induction motors is presented. Basic voltage selection methods similar to a two-level inverter provoke some problems such as stator-flux drooping phenomenon and undesirable torque control deterioration appeared, especially at low-speed operation. To overcome these problems, an algorithm with the basic switching sectors subdivided and intermediate voltage vectors applied is proposed in this paper. This algorithm basically considers applications in which direct torque-controlled induction motors are fed by three-level inverters with maximum switching frequency lowered around 1 kHz. An adaptive observer is also employed to bring better responses at the low-speed operation, by estimating some state variables and motor parameters which take a deep effect on the performance of the low-speed operation. Simulation and experiment results verify effectiveness of the proposed algorithm.

00/01371 Direct torque control of induction motors for electric propulsion systems

00/01371 Direct torque control of induction motors for electric propulsion systems

Speed-sensorless direct torque control of induction motors using an adaptive flux observer

This paper presents a new sensorless induction motor drive. The drive uses an adaptive flux observer for speed estimation and the discrete-time direct torque control technique for torque and stator flux control. The adaptive flux observer uses a mechanical model to improve the behavior during speed transients. The estimated stator flux of the adaptive observer is used in the discrete-time direct torque control method to provide fast torque response combined with torque-ripple-free operation over the whole speed range. The sensorless drive system is capable of working from very low speed to high speed and exhibits good dynamic and steady-state performance. The steady-state and dynamic performance of the proposed sensorless drive using simulation and experimental results are demonstrated.

New direct torque control of induction motor for minimum torque ripple and constant switching frequency

In this paper, a direct torque control (DTC) method of an induction machine is proposed which enables the minimum torque ripple control, while maintaining constant switching frequency. Some drawbacks of the conventional DTC are the relatively large torque ripple in a low speed range and the variation of switching frequency according to the amplitude of hysteresis bands and the motor operating speed. In the proposed strategy, an RMS torque-ripple equation is derived using instantaneous torque equations and, at each switching cycle, an optimal switching instant which satisfies the minimum torque-ripple condition is determined based on the ripple equation. The proposed strategy improves the performance of the DTC by combining a low-torque-ripple characteristic in steady state with the fast torque dynamics. Experimental results prove the feasibility of the proposed strategy as compared with the conventional method.

Direct torque control of induction motors for electric propulsion systems

This paper describes an induction motor drive for use in electric propulsion. The specifics of the application (electric vehicle) require fast response and high efficiency for the drive. Direct torque control (DTC) is used to control the motor. In this control strategy, efficiency is optimized by adjusting the magnetic flux of the motor. The sensitivity of the DTC to temperature variations, leading to stator resistance changes, is eliminated by online estimation of stator resistance.