M. Khairi Rahim

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.

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.

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.

DTC brushless DC motor by using constant switching frequency

This paper propose direct torque control of constant switching frequency for brushless dc motor DTC-CSF BLDC. It is due to overcome the drawback of conventional DTC such as high torque ripples and variable switching frequency. It is applied by using PI controller to regulate the error of torque producing output error of PI controller, and then compare it with carrier or triangular frequency to generate constant switching frequency. The experiment is implemented by using Altera field-programmable gate array (FPGA) and digital signal processor. Results of experiment show the torque ripples waveforms are maintained in each level of speed and constant switching frequency is proved by the existence of fundamental frequency of 3125 Hz where it is clearly appeared in the frequency spectrum in any variation speed level.

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.

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.