Auzani Jidin

An Improved FPGA Implementation of Direct Torque Control for Induction Machines

This paper presents a novel direct torque control (DTC) approach for induction machines, based on an improved torque and stator flux estimator and its implementation using field-programmable gate arrays (FPGA). The DTC performance is significantly improved by the use of FPGA, which can execute the DTC algorithm at higher sampling frequency. This leads to the reduction of the torque ripple and improved flux and torque estimations. The main achievements are: 1) calculating a discrete integration operation of stator flux using backward Euler approach; 2) modifying a so called nonrestoring method in calculating the complicated square root operation in stator flux estimator; 3) introducing a new flux sector determination method; 4) increasing the sampling frequency to 200 kHz such that the digital computation will perform similar to that of the analog operation; and 5) using twos complement fixed-point format approach to minimize calculation errors and the hardware resource usage in all operations. The design was achieved in VHDL, based on a Matlab/Simulink simulation model. The Hardware-in-the-Loop method is used to verify the functionality of the FPGA estimator. The simulation results are validated experimentally. Thus, it is demonstrated that FPGA implementation of DTC drives can achieve excellent performance at high sampling frequency.

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.

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 Novel Overmodulation and Field Weakening Strategy for Direct Torque Control of Induction Machines

This paper presents a new simple overmodulation strategy in direct torque control (DTC) of induction machines. The strategy increases the inverter voltage limit and therefore extends the constant torque region. The minor modification on the conventional flux weakening method is also introduced to improve the torque capability in the field weakening region and hence improving the operation over a wide speed range. These strategies are implemented with a DTC-constant switching frequency (CSF) based system. To perform the DTC-CSF in overmodulation mode, the flux error status is modified before it is being fed to the voltage vector selection table. With this proposed strategy, a single-mode overmodulation based on stator flux vector control with smooth PWM to six-step transition is developed. This strategy also proposes a dynamic overmodulation to obtain fast torque transient with six-step operation including the field weakening region. The proposed strategy has been verified through computer simulations and shown to improve the torque capability of DTC induction motor drives with a fast dynamic torque response.

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 produces 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 flux hysteresis controller, before it is being fed to the look-up table. The main benefit in the proposed strategy is its simplicity and it can be implemented in many electric drive applications to obtain maximum torque capability for a wider speed range operation.

A Simple Approach of Space-vector Pulse Width Modulation Realization Based on Field Programmable Gate Array

Abstract Employing a field programmable gate array to realize space-vector pulse width modulation is a solution to boost system performance. Although there is much literature in the application of three-phase space-vector pulse width modulation based on field programmable gate arrays, most is on conventional space-vector pulse width modulation with designs that are complicated. This article will present a simple approach to realize five-segment discontinuous space-vector pulse width modulation based on a field programmable gate array, in which the judging of sectors and the calculation of the firing time are simpler with fewer switching losses. The proposed space-vector pulse width modulation has been successfully designed and implemented to drive on a three-phase inverter system that is loaded by an induction machine of 1.5 kW using the APEX20KE Altera field programmable gate array (Altera Corporation, San Jose, California, USA).

Sliding mode variable structure control design principles and application to DC drives

In this paper, a control system of DC motor for speed and torque control based on variable structure systems with sliding mode control approach is presented. The comparisons between classical and sliding mode techniques are investigated and their advantages and disadvantages are identified. The current limiter mechanism to limit the current during start-up, acceleration and deceleration for speed control loop is proposed and presented. The simulation results of the proposed controller show that the variable structure system with sliding mode control approach is superior in rejecting disturbance, has faster response and is less sensitive to parameter variations compared to the conventional PI controller.

Current control of BLDC drives for EV application

This paper presents a current blocking strategy of brushless DC (BLDC) motor drive to prolong the capacity voltage of batteries per charge in electric vehicle applications. The BLDC motor employs a simple torque hysteresis control (THC) that can offer a robust control and quick torque dynamic performance. At first, a mathematical modeling of BLDC motor and principle of torque hysteresis control will be described, so that the benefit offered by the proposed current blocking strategy can be highlighted. It can be shown that the current control method naturally provides current limitation, in which the current error (or ripple) is restricted within the pre-defined band-gap furthermore provide current protection. The benefit of proposed current blocking strategy will be highlighted such that it can prevent the current drained from the batteries when the torque demand is released to set to 0 Nm. The control scheme is validated and verified by the simulation and experimental results.