Atikah Razi

A simple potential balancing strategy for neutral-point-clamped inverter fed direct torque control induction machines

This paper presents a simple and robust potential balancing strategy for Direct Torque Control (DTC) of induction machines. In the proposed method, a two-level hysteresis comparator is used to restrict the error voltage, i.e. difference of upper and lower capacitors voltage, within the hysteresis band. The comparator will produce suitable digitized output status to be indexed into a look-up table for selecting appropriate switching states to compensate the mismatch voltages. In doing so, the variation of two capacitors voltages can be controlled, consistently, as compared to that of using the common SVM approaches where the variation (or error of ripple) is not strictly maintained. Some experimental results are presented to highlight the improvements of DTC performances as well as the robustness of control balancing strategy for steady state and dynamic conditions.

Reduced torque ripple and switching frequency using optimal DTC switching strategy for open-end winding of induction machines

This paper presents an optimal switching strategy to minimize torque ripple and switching frequency for Direct Torque Control of induction machines. In this proposed method, two inverters are connected to the terminals of open-end windings of an induction machine. By doing so, number of voltage vectors is greater same as available in three-level multilevel inverters. The most optimal voltage vectors will be used to improve DTC performances, i.e. reduced torque ripple and switching frequency. The identification of the vectors are based on operating conditions, specifically by examining the behaviors of torque error and switching frequency of error status produced from the hysteresis controllers. It can be shown that, the proposed optimal switching vectors according to the operating conditions can reduce the rate of change of torque and hence minimize torque ripple and switching frequency. The improvements offered in the proposed method were verified through experimental results.

Constant switching frequency torque controller for DTC of induction motor drives with three-level NPC inverter

This paper presents Direct Torque Control (DTC) of induction motor drives using three-level Neutral Point Clamped (NPC) inverter. The usage of hysteresis controller that utilized in conventional DTC lead to varying switching operation, therefore a Constant Switching Frequency (CSF) torque controller is proposed to replace the torque hysteresis controller. The proposed CSF controller consists of Proportional-Integrator (PI) controller and triangular carrier waveforms are compared with the output of PI controller to produce a torque error status to implement DTC control. The proposed controller is able to improve the DTC drives performance in terms of torque ripple reduction and constant switching frequency operation. The improvement of proposed method is verified using experimental works.

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.

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.

Improved torque control performances using Torque Error Modulator for Multilevel Inverter based DTC induction motors

This paper presents the advantage of Torque Error Modulator (TEM) implementation in conventional Direct Torque Control (DTC) of 3-phase induction motor with 3-level Cascade H-bridge Multilevel Inverter (CHMI). 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 ripple. The major factor contributes to these problems are the usage of hysteresis based comparators in controlling the output torque. The implementation of TEM in DTC provides solution to these problems while retaining the simple control structure of conventional DTC. The combination usage of 3-level CHMI with TEM can further minimize the output torque ripple of 3-phase DTC system by providing greater number of vectors. The 3-level CHMI of 3-phase system provides 18 active voltage vectors where each vector may give different effect on controlling performance of 3-phase induction machine in various torque demands. In order to verify the feasibility of the proposed method experimentation, the proposed method compared with convention DTC system via simulation and experiment results.

A novel Hexagonal Flux control method to improve constant switching performance of multilevel 3-phase DTC

This paper presents the advantage of implementing a novel Hexagonal Flux Control Method to improve constant switching performance of multilevel 3-phase DTC scheme. The Direct Torque Control (DTC) scheme is well known to provide rapid decoupled control of torque and flux in motor control drive via a simple control structure. The utilization of 3-level CHMI in this DTC can minimize the output torque ripple by providing larger number of voltage vectors. However, DTC Scheme is known to have two major shortcomings, which are the irregular switching frequency of power switches and high torque output ripple. The usage of torque hysteresis controller plays a major role in cause of this problem. The implementation of PI based constant switching controller to replace the hysteresis controller able to solve these problems while remaining the simple DTC control structure. Conversely, there are presents of minor oscillation in the torque regulation of constant switching method in which contributed by the flux regulation factor. This paper presents about the Hexagonal Flux method implementation in-order to mitigate the flux regulation problem. The detail explanation and calculation of optimal PI parameter tuning strategy with the combination of Hexagonal Flux method have been discussed. In order to validate the feasibility, the proposed method has been compared with convention DTC system via simulation and experiment results.

Constant switching frequency using proposed controller with optimal DTC switching strategy for dual-inverter supplied drive

This paper presents a simple solution to the variable switching frequency and high torque ripple which encountered by hysteresis-based Direct Torque Control (DTC) drive. To solve this two major problems, a new carried-based torque controller is introduced to replace the previous hysteresis torque controller which operate by comparing torque error with the triangular waveform. Since the dual-inverter supplied drive is implemented, an optimal switching strategy scheme is adopted for ensuring the most appropriate amplitude of voltage vectors is selected according to machine operation condition. As a result, the torque error become easier to be control inside triangular waveform since the rate change of torque or torque slope become lesser then the slope of triangular waveform. By combining this simple carrier-based torque controller with optimal switching strategy scheme, the DTC performances able to be improved further by providing constant switching frequency as well as torque ripple reduction. Then, all this improvement offered by proposed schemes are validated through the experimental results.

Improved wave shape-pattern performance using Phase Opposite Disposition (POD) method for cascaded multilevel inverter

Nowadays, power electronics inverters are widely used due to the increasing of the power demand. Thus the device such as multilevel inverter was used to perform the required energy conversion. However, multilevel inverter is operated without harmonic-less. This total harmonic distortion (THD) producing from multilevel inverter is generated from the sinusoidal output waveform. This paper presents level-shifted SPWM technique of single-phase 5-level Cascaded H-Bridge (CHB) inverter. This research aims of assessing the importance of accuracy of the pattern of the wave shape and harmonic distortions of the output voltage by controlling the modulation indices. The proposed method uses Phase Opposite Disposition (POD) modulation strategy to evaluate the waveform and harmonic performances. The simulation results were verified through experimental study.