The Constant Switching Frequency And Torque Ripple Reduction Of Direct Torque Controlled Induction Machine With Neutral Point Clamped Inverter

Direct Torque Control (DTC) of induction motor drives are widely accepted in variable speed drive applications because it offers high performance in terms of fast torque response as well as simplicity of control structure. However, conventional OTC drives which utilize two-level inverter and hysteresis controllers inherently suffer from two major drawbacks which are large torque ripple and variable switching frequency particularly at low speed operation. This is because the torque is unable to restricted within the hysteresis band (commonly happened in digital implementation of hysteresis controller) that leads to condition of overshoot and undershoot which result in large torque ripple. Secondly, inappropriate selection of voltage vectors for different speed operation as two-level inverter provides only limited numbers of voltage vectors. This research aims to improve the performance of the OTC drives by utilizing a three-level Neutral Point Clamped (NPC) inverter as appropriate voltage vectors can be utilized for different speed operations due to availability of larger number of voltage vectors . Nevertheless, the utilisation of NPC inverter lead to imbalance of upper and lower capacitor voltage due to application of short amplitude of voltage vectors which have two redundant switching states and each switching state produces different effect towards the capacitor voltage. Therefore, a simple capacitor voltage balancing strategy is also proposed to select the appropriate switching states based on capacitor voltage status. Next, the selection of appropriate voltage vectors according to speed operation are determined to produce reduction of torque ripple and switching frequency. Furthermore, a constant switching frequency operation is obtained by replacing the hysteresis torque controller with constant switching frequency (CSF) torque controller. The proposed improvement method was conducted experimentally using DTC-N PC inverter drives with CSF torque controller (with acronym DTC-N PC-CSF) and was compared with conventional OTC drives as well as OTC drives with N PC inverter and hysteresis torque controller (with acronym DTC-NPC-HTC) for performance analysis. The performance result showed that the torque ripple was minimized and the switching frequency was remained constant for all range of speeds

Simple Switching Strategy for High-Torque Control Performance utilizing Neutral Point Clamped Multilevel Inverter

<p>Three-level Neutral Point Clamped (NPC) inverter allows the configuration of switching devices to operate at high voltage and produce lower current/voltage harmonics. It is known that, DTC of induction machine which employs hysteresis controller has major drawbacks namely larger torque ripple and variable switching frequency. This paper aims to propose a suitable voltage vector selection to provide better torque regulation and lower switching frequency by employing DTC with 3-level NPC multilevel inverter. A simple switching strategy was formulated using 7-level torque hysteresis and 2-level flux hysteresis controllers to give more options in selecting an appropriate voltage vector, inherently, according the motor operation conditions. The improvements offered were verified through simulations.</p

Direct Torque Control Of Induction Machine Using 3-Level Neutral Point Clamped Inverter

This paper presents a Direct Torque Control (DTC) of induction motor drives utilizing 3-level neutral-point clamped (NPC) multilevel inverter. This research aims to provide a simple strategy to address capacitor voltage balancing problem of NPC inverter as well as produce better performance for DTC of induction motor in term of reducing torque ripple and high switching frequency. The proposed method uses 2-level hysteresis comparator to restrict the error of upper and lower capacitor voltages and then generate a signal which determine either to increase or decrease the particular capacitor voltage. Using that information, the appropriate selection of small voltage vectors is tabulated in look-up table (LUT) to stabilize the mismatched of capacitor voltage. Simulation results are presented to show the effectiveness of the proposed capacitor voltage balancing strategy

Minimization Of Torque Ripple And Switching Frequency Utilizing Optimal DTC Switching Strategy For Dual-Inverter Supplied Drive

This paper presents an optimal switching strategy employed in direct torque control (DTC) of induction machine utilizing dual-inverter supplied drive. In the proposed switching strategy, the utilization of appropriate amplitude of voltage vectors based on machine operating condition can reduce the rate change of torque slope while improving the performance of DTC drive in term of torque ripple reduction and also minimization of switching frequency variation. This is achieved by modifying the torque error status according to the capability of torque regulation which is done by evaluating the switching frequencies of torque and stator flux error status from the hysteresis comparators. By doing that the suitable amplitude of voltage vectors is able to be applied based on machine operation. All the improvements offered in the proposed strategy are verified through simulation results

Constant Switching Frequency For Direct Torque Control Of Induction Motor

DTC drives utilizing hysteresis comparator is one of the simplest and most popular technique used in induction drives. However, due to its disadvantage of variable switching frequency causes serious problem in DTC especially for electromagnetic torque ripple. This paper proposed a technique of constant switching that could reduce the ripple on both torque and phase current for DTC by using triangular waveform or carrier frequency. Thus, the conventional and proposed DTC will be simulated and demonstrated by using Altera field-programmable gate array (FPGA) and digital signal processor. Simulation results presented in this paper shows the torque and stator current ripple with the proposed DTC algorithm when varying the speed and the carrier frequency

Improve Torque Control Performance Of 3-Phase DTC Constant Switching Method Using Optimal PI Parameter Tuning Strategy

This paper presents the advantage of using optimal PI parameter tuning strategy of constant switching method in the three phase Direct torque control (DTC) scheme. The DTC system is known to offer fast decoupled control of torque and flux via a simple control structure. Nevertheless, DTC system has two major drawbacks, which are the variable inverter switching frequency and high torque output ripple. The major factor that contributes to these problems the usage of hysteresis based comparators to control the output torque. The implementation of PI based constant switching method in DTC able to solve these problems while retaining the simple control structure of conventional DTC. The combination usage of 3-level CHMI in this system can further minimize the output torque ripple by providing greater number of vectors. This paper presents detail explanation and calculation of optimal PI parameter tuning strategy consecutively to enhance the performance of 3-level DTC system. In order to validate the feasibility, the proposed method compared with convention DTC system via simulation and experiment results

Investigation Of Torque And Flux-Current Producing Components In Indirect Rotor Flux Oriented Control (IRFOC) Of Induction Machines

This paper presents an investigation of torque and flux-current producing components in indirect rotor flux oriented control (IRFOC) of induction machine. Induction machine faced the problems when the motor rotates at the highest speed due to complex mathematical model based on the previous research. Therefore, to prove that there have the coupling effect, the investigation towards the torque and flux current component of IRFOC is done. The method that have been used to detect the problem is by using simulation method on MATLAB software. The algorithm and controller that been used is Field Oriented Control (FOC) because it is the first method in drive system that are been used in order to solve the induction machine problems. Simulation results presented in this paper show the torque, rotor flux, direct and quadrant current component when varying the value of torque and flux