Abdul Halim Mohamed Yatim

Direct torque control of induction machines with constant switching frequency and reduced torque ripple

Direct torque control (DTC) of induction machines is known to have a simple control structure with comparable performance to that of the field-oriented control technique. Two major problems that are usually associated with DTC drives are: switching frequency that varies with operating conditions and high torque ripple. To solve these problems, and at the same time retain the simple control structure of DTC, a constant switching frequency torque controller is proposed to replace the conventional hysteresis-based controller. In this paper, the modeling, averaging, and linearization of the torque loop containing the proposed controller followed by simulation and experimental results are presented. The proposed controller is shown to be capable of reducing the torque ripple and maintaining a constant switching frequency.

Design and Implementation of a New Multilevel Inverter Topology

Multilevel inverters have been widely accepted for high-power high-voltage applications. Their performance is highly superior to that of conventional two-level inverters due to reduced harmonic distortion, lower electromagnetic interference, and higher dc link voltages. However, it has some disadvantages such as increased number of components, complex pulsewidth modulation control method, and voltage-balancing problem. In this paper, a new topology with a reversing-voltage component is proposed to improve the multilevel performance by compensating the disadvantages mentioned. This topology requires fewer components compared to existing inverters (particularly in higher levels) and requires fewer carrier signals and gate drives. Therefore, the overall cost and complexity are greatly reduced particularly for higher output voltage levels. Finally, a prototype of the seven-level proposed topology is built and tested to show the performance of the inverter by experimental results.

An improved stator flux estimation in steady state operation for direct torque control of induction machines

The paper presents an improved stator flux estimation technique based on a voltage model with some form of low pass filtering. In voltage model-based stator flux estimation, a low-pass (LP) filter is normally used instead of a pure integrator to avoid the integration drift problem due to DC offset, noise or measurement error present in the back electromotive force (EMF). In steady state condition, the LP filter estimator will degrade the performance and efficiency of the direct torque control (DTC) drive system since it introduces magnitude and phase errors, thus resulting in an incorrect voltage vector selection. The stator flux steady state error between the LP filter and a pure integrator estimator techniques is derived and its effect on the steady state DTC drive performance is analyzed. A simple method is proposed to compensate for this error which results in a significant improvement in the steady state drive performances. Simulation based on this technique is given and it is verified by experimental results.

Implementation of Dynamic Evolution Control of Bidirectional DC–DC Converter for Interfacing Ultracapacitor Energy Storage to Fuel-Cell System

An electric vehicle powered by fuel cells (FCs) gives far more promising performance. An FC is a clean energy source and has a high energy-storage capability. However, an FC has a slow dynamic response. A secondary power source is needed during start-up and transient conditions. An ultracapacitor can be used as a secondary power source to improve the performance and efficiency of the overall system. Several methods have been devised to connect an energy-storage device to an FC. This paper presents a converter system for connecting an ultracapacitor as secondary energy storage to an FC electric-vehicle system. A bidirectional dc-dc converter is used for interfacing ultracapacitor energy storage to an FC system. The controller of the converter system was designed and implemented based on dynamic evolution control. The performance of the proposed dynamic evolution control is tested through simulation and experiment. Simulation and experimental results show that the proposed techniques are suitable for controlling bidirectional dc-dc converters.

An Optimized Switching Strategy for Quick Dynamic Torque Control in DTC-Hysteresis-Based Induction Machines

A dynamic overmodulation strategy for fast dynamic torque control in direct torque control (DTC)-hysteresis-based induction machine is proposed. The fastest dynamic torque response with a six-step mode can be achieved in the proposed method by switching only the most optimized voltage vector that produces the largest tangential component to the circular flux locus. This paper also discusses the performance of dynamic torque control in basic DTC in order to justify on how the proposed selected voltage vector results in excellent dynamic torque performance. The main benefit of the proposed method is its simplicity, since it only requires a minor modification to the conventional DTC-hysteresis-based structure and does not require a space vector modulator. To verify the feasibility of the proposed dynamic overmodulation strategy, simulation and experimentation, as well as comparison with the conventional DTC scheme, are carried out. Results showed a significant improvement in the dynamic torque response when compared to the conventional DTC-hysteresis-based method.

A Wide-Speed High Torque Capability Utilizing Overmodulation Strategy in DTC of Induction Machines With Constant Switching Frequency Controller

This paper presents a simple overmodulation method employed in direct torque control (DTC) constant switching frequency (CSF) controller of induction machines. The proposed overmodulation method is utilized to extend a constant torque region and hence produce high torque capability in field-weakening region with six-step operation. It will be shown that the overmodulation operation using the DTC-CSF scheme can be established by controlling the stator flux locus from circular to the hexagonal shape. This is achieved by modifying the flux error status produced from the flux hysteresis controller before it is fed to the lookup table. The main benefit of the proposed method is its simplicity since it requires only a minor modification to the conventional DTC hysteresis-based structure and does not require a space-vector modulator.

Extending switching frequency for torque ripple reduction utilizing a constant frequency torque controller in dtc of induction motors

Direct torque control (DTC) of induction machines is known to offer fast instantaneous torque and flux control with a simple control structure. However, this scheme has two major disadvantageous, namely, a variable inverter switching frequency and a high torque ripple. These problems occur due to the use of hysteresis comparators in conventional DTC schemes, particularly in controlling the output torque. This paper reviews the utilization of constant frequency torque controllers (CFTC) in DTC to solve these problems while retaining the simple control structure of DTC. Some extensions of the work in utilizing a CFTC will be carried out in this paper which can further reduce the torque ripple. This is particularly useful for a system which has a limited/low sampling frequency. The feasibility of a CFTC with an extended carrier frequency in minimizing the torque ripple is verified through experimental results.

Dynamic evolution control for synchronous buck DC–DC converter: Theory, model and simulation

Abstract This paper proposes a new control technique for synchronous buck DC–DC converter. Theory, design and implementation of the proposed control technique are provided. A new approach for converter controller synthesis based on dynamic evolution control theory is presented. In order to synthesize the converter controller, this method uses a simple analysis of nonlinear equation models of the converter. The synthesis process is simple and requires a quite low bandwidth for the controller. Therefore, this control method is suitable for digital control implementation. As an illustrative example, the synthesis of synchronous buck DC–DC converter controller is discussed in detail. The model of the synchronous buck DC–DC converter system was implemented using SimPowerSystems toolbox of MATLAB-SIMULINK. Performance of the proposed dynamic evolution control under step load change and step input voltage condition was investigated. Simulation results confirm that the proposed control method is superior to traditional PI based controller because of fast transient response and good disturbance rejection.

Simple Dynamic Overmodulation Strategy for Fast Torque Control in DTC of Induction Machines With Constant-Switching-Frequency Controller

This paper presents a simple dynamic overmodulation method to obtain a fast dynamic torque response in direct torque control (DTC) of induction machines with constant-switching-frequency controller. A fast dynamic torque response can be obtained by switching only the most optimized voltage vector during torque dynamic condition. The optimized voltage vector can be identified by comparing the rate of change of torque produced between applications of two possible active voltage vectors, according to the flux position. The selection of the optimized voltage vector can be simply implemented by modifying the flux error status before it is being fed to the lookup table. It will be shown that the proposed switching strategy facilitates the DTC to perform under six-step mode to achieve the fastest dynamic torque response. The effectiveness of the proposed dynamic overmodulation to obtain the fast torque response is verified with some experimental results.

A new topology -Reversing Voltage (RV) - for multi level inverters

Multi level inverters have been widely accepted for high power high voltage applications. Their performance is highly superior to conventional two level inverters in reduced harmonic distortion, lower EMI (electro magnetic interference) and higher DC link voltages. However it has some disadvantages as increased number of components, complicated PWM control method, and voltage balancing problem at neutral point. In this paper a new topology called reversing voltage (RV) is proposed to improve the multi level performance by compensating the disadvantages already mentioned. This topology requires less components compared to available inverters (especially in higher levels) and requires less carrier signals and does not need balancing of the voltages. The topology and the control method based on PD (phase disposition) SPWM are shown and the required components are also compared to other topologies to show the superiority of the topology. Finally, output voltage and currents are demonstrated by simulation.