Constant Switching Frequency Control Research Articles

Contribution to the Improvement of Sensorless DTC-SVM for Three-Level NPC Inverter-fed Induction Motor Drive

This paper deals with the high performance of multi-level direct torque control (DTC) for induction machines without speed and stator flux sensors. The estimation is performed using a sliding mode observer, which is known for its robustness in high and low-speed operations, the control is based on a backstopping speed controller. This control technique was introduced years ago to circumvent the problems of sensitivity to parametric variations, it presents a high dynamic but their major problems are the variable switching frequency, the size and complexity of the switching tables, and undulations of the torque. The proposed approach is to replace switching tables with constant switching frequency control using three-level spatial vector modulation (SVM). Theoretical elements and simulation results are presented and discussed. As a result, the flux and torque ripple of three-level DTC-SVM control is greatly reduced compared to the flux and torque ripple of DTC-classic control. The advantages of the training system have been validated by the simulation results.

Improved Variable Switching Frequency Control for Capacitor Voltage Ripple Regulation in Multilevel Flying Capacitor Converter

For multi-level flying capacitor (FC) converter in dc-ac applications, variable switching frequency (VSF) control is an attractive solution to regulate the FC voltage ripple compared with the constant switching frequency (CSF) control. However, it is shown that the existing VSF control presents a higher switching loss in the three-phase operation when the identical switching frequency is applied to three phase-legs to reduce the complexity of VSF control implementation. To reduce the device switching losses in three-phase operation, this letter proposes a new strategy for the switching frequency selection, which allows each phase-leg to select its switching frequency individually. The control complexity of the proposed method is also reduced and is very similar to CSF control. The experiment result is also shown to verify the proposed variable switching frequency control strategy.

A novel FPGA‐Based Bi input‐reduced order extended Kalman filter for speed‐sensorless direct torque control of induction motor with constant switching frequency controller

This study proposes an FPGA-based hardware in the loop (HIL) emulator for speed-sensorless of induction motor (IM) constant switching frequency controller-based direct torque control (CSFC-DTC) with a novel bi input-reduced order extended Kalman filter (BI-ROEKF). The full precision single floating point numbers in the IEEE 754 standard are used during the implementation of the HIL emulator which contains closed-loop speed-sensorless drive system of IM on the Xilinx Virtex XC5VLX-110T ML506 FPGA board. In this HIL emulator of speed-sensorless IM drive system, stator stationary axis components of stator flux, rotor mechanical angular speed, load torque, stator and rotor resistances are estimated with the novel BI-ROEKF which is proposed for the first time in the literature. The proposed BI-ROEKF is created by applying two different non-linear and linear system input functions obtained from two different IM models to the single reduced order extended Kalman filter (ROEKF) algorithm. Thus, the order and the computational burden of the EKF are reduced. The HIL emulator of the speed-sensorless drive system of IM is implemented on FPGA using the advantage of hand-written VHDL on getting an optimal logical design to reduce the sampling time which directly effects the estimation performance of the model-based estimator like the novel BI-ROEKF and hence the control performance of drive system. The estimation performance of the novel BI-ROEKF is tested with speed-sensorless CSFC-DTC IM drive system under different challenging scenarios in HIL emulator. Thus, the control and the implementation performances of digitalised emulator are tested. Finally, the estimation and control performance results and the execution time of the each part of the proposed HIL emulator of the speed-sensorless BI-ROEKF-based CSFC-DTC of the IM are presented.

An Efficient of Direct Torque Control of Indirect Three Level Matrix Converter Fed Dual Stator Induction Motor Based on Synergetic Controller

As know, Direct Torque Control (DTC) suffers from the high ripples of the torque and flux, caused by the variable switching frequency. Torque ripple in DTC drives can be reduced if an Indirect Three-Level Matrix Converter (ITLMC) is employed instead of a conventional Two-Level Indirect Matrix Converter. Nevertheless, this problem can be still large if low switching frequencies are used. In order to overcome this problem, a Constant Switching Frequency controller (CSFC) is proposed to replace the five-level hysteresis torque comparator for reducing the ripples and improving the flux regulation in low and zero speed operation. Furthermore, the control scheme performance is enhanced by inserting a robust Synergetic Controller (SC) in the outer loop for speed regulation. Simulation results with a comparative analysis have indicated the special merit of the proposed scheme, which makes it competitive with the existing control strategies.

Vector Shifted Model Predictive Power Control of Three-Level Neutral-Point-Clamped Rectifiers

Model predictive power control (MPPC) has been emerging as one of the most promising control schemes for three-level neutral-point-clamped (NPC) rectifiers. However, conventional MPPC (C-MPPC), which only selects one switching state during the entire sampling period, leads to high active and reactive power ripples. Moreover, the heavy computational burden and variable switching frequency limit the applications of MPPC. In this article, vector shifted MPPC (VS-MPPC) methods are investigated. With the shifted vectors, the constant-switching-frequency MPPC of three-level NPC rectifiers can be simplified as that of a two-level rectifier, and the balanced neutral capacitor voltage can be easily achieved by adjusting the duty cycle of the redundant switches without any weighting factor employed. Only eight voltage vectors are calculated and shifted based on the small hexagon selection. Consequently, the computational burden is significantly reduced, even 35% less than that of C-MPPC. Furthermore, the proposed VS-MPPC presents a constant switching frequency and better steady-state control performance without evaluating all the switching states. Simulation and experimental evaluations of the proposed VS-MPPC methods with C-MPPC have been conducted to validate the superiority of the proposed VS-MPPC methods.

Impact of Observability and Multi-objective Optimization on the Performance of Extended Kalman Filter for DTC of AC Machines

It is well known that the selection of extended Kalman filter (EKF) covariance elements has a considerable bearing on the effectiveness of EKF performance. The observability at very low frequency is also an essential property for the selection of EKF elements. This paper investigates the optimization of the EKF covariance elements when zero frequency is included in the training profile for direct torque control (DTC) of induction motor. In addition, the paper studies the optimization of EKF by speed and torque fitness functions using a non-dominated sorting genetic algorithm-II at zero and high speeds under stable flux regulation. For this purpose, DTC with constant switching frequency controller which has the capability of establishing continuous flux rotation regardless of speed variation is used. The optimized results of EKF for both DTC motor drives and speed and torque cost functions are verified experimentally.

Multivector Model Predictive Power Control of Three-Phase Rectifiers With Reduced Power Ripples Under Nonideal Grid Conditions

Model predictive power control (MPPC) is a promising control scheme for bidirectional ac–dc rectifiers. However, the control performance of conventional MPPC (C-MPPC) deteriorates under nonideal grid conditions. Furthermore, the one-switching-vector-per-control-interval characteristic of C-MPPC leads to high ripples of control variables. To improve the steady-state performance of rectifiers under nonideal grid voltage conditions, a multivector MPPC (MV-MPPC) scheme is proposed. The proposed method presents a constant switching frequency and better steady-state control performance without increasing its sampling frequency. By selecting two active vectors and one zero vector, the range of optimal vector for active and reactive power regulation can be extended from fixed phase and magnitude to arbitrary phase and magnitude. Incorporated with a second-order generalized integrator for obtaining the quadrature component of grid voltage, the proposed method is applicable to the grid conditions where low-order harmonics exist. The preliminary calculation is adopted to avoid repetitive computation of the predicted values in the implementation of MV-MPPC, which reduces the calculation burden of the proposed scheme. A thorough experimental evaluation of the proposed scheme with the C-MPPC and duty-optimal MPPC has been conducted to validate the superiority of the proposed MV-MPPC solution.

0.293-mm2 Fast Transient Response Hysteretic Quasi-<inline-formula> <tex-math notation="LaTeX">$V^{2}$ </tex-math> </inline-formula> DC–DC Converter With Area-Efficient Time-Domain-Based Controller in 0.35-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>m CMOS

A time-domain-control-based quasi- $V^{2}$ buck converter with small area, fast transient response, wide load current range, and constant switching frequency is proposed in this paper. The proposed time-domain-based controller achieves compact core area and simplified constant switching frequency control by replacing the conventional voltage-domain comparator with a phase detector having an adaptive detection window. Moreover, a coupling-based inductor current emulator is also proposed to support the discontinuous conduction mode operation in the light load. The proposed buck converter is implemented using 0.35- $\mu \text{m}$ CMOS process. The core area of the proposed buck converter is only 0.293 mm2. The measured peak efficiency in the continuous conduction mode is 92% and the end-to-end efficiency in the load current range of 25–700 mA is 73%. The undershoot/overshoot and recovery time with the up/down load current steps of 510 mA are 38/20 mV and 2.5/2.6 $\mu \text{s}$ , respectively.

A power quality enhanced grid voltage sensorless predictive direct power control for active front end rectifiers

In this research work, a power quality enhanced grid voltage sensorless deadbeat predictive direct power control (DB-DPC) approach for AC/DC power converters based on latest virtual flux (VF) estimation, DB-DPC and space vector pulse width modulation (SVPWM) is proposed. At first, the grid voltage sensorless technique with switching table based DPC, which is a conventional approach, is discussed and its performance is analyzed under both increase of load as well as decrease of load conditions. Further, the proposed technique performance is also analyzed under the same circumstance and its performance is compared with the traditional method. The proposed technique has numerous advantages over the conventional method such as constant switching frequency, better active and reactive power control, good regulation of dc bus voltage, and enhanced power quality performance. As the proposed approach utilizes VF, DB-PC, and SVPWM techniques, this ensure line voltage sensorless approach, excellent control dynamics and constant switching frequency, respectively. At the end, experimental validation is done to confirm the supremacy of the proposed technique under load varying conditions and has a phenomenal performance under these circumstances.

Lookup-Table-Based DTC of Induction Machines With Improved Flux Regulation and Extended Kalman Filter State Estimator at Low-Speed Operation

Direct torque control (DTC) based on lookup table is known to have poor flux regulation at low speed that can cause a significant deterioration in the speed estimation of a sensorless drive. In this paper, stator flux regulation at low speed is improved under no load by replacing the hysteresis torque comparator with constant switching frequency controller, without any changes to the conventional lookup table; hence, the simple structure of DTC is maintained. In order to reduce the sampling time and hence improve the estimation process at very low speed, a simple fifth-order extended Kalman filter (EKF) is used in this paper. The fast execution time helps the linearization process of the EKF; hence, results in an improvement in its performance. Simulations and experimental results of the proposed sensorless drive system with the EKF estimator proves excellent flux regulation, and good speed robustness and stability at very low speed.

Simple Flux Regulation for Improving State Estimation at Very Low and Zero Speed of a Speed Sensorless Direct Torque Control of an Induction Motor

This paper presents a simple flux regulation for a direct torque control (DTC) of an induction motor (IM) to improve speed and torque estimations at low- and zero-speed regions. To accomplish this, a constant switching frequency controller (CSFC) is used to replace the three-level hysteresis torque comparator of a DTC IM. The DTC of IM utilizing CSFC (DTC-CSFC) retains the simple structure of a lookup table-based DTC drive. With DTC-CSFC, constant switching frequency is maintained, and at the same, the flux droop problem that normally occurs in DTC with the hysteresis controller (DTC-HC) at low speed is solved; subsequently, the stator flux and torque estimations at low speed are also improved. In the proposed system, the speed feedback for the closed-loop speed control is estimated using an extended Kalman filter, which requires heavy real-time computation. However, due to the simple structure of DTC-CSFC, small sampling time, hence large control bandwidth is possible. The performances of the speed sensorless DTC-HC and DTC-CSFC are compared experimentally under different operating conditions. With the improved stator flux regulation, experimental results of the DTC-CSFC showed a significant improvement in speed and torque estimations at very low and zero-frequency operations.

Power Quality Analysis of Vienna Rectifier for BLDC Motor Drive Application

<p>This paper presents the power quality improvements for a BLDC driver using Vienna rectifier as front end converter. The major drawbacks in the BLDC motor drive and non linear load applications are the line pollution and depreciation of the power factor. The conventional power factor correction method is not economical and efficient. It requires bulky components as load changes and produce high THD and less Power factor. The front end converter of BLDC bridge inverter drive is proposed by the Vienna rectifier, which can contribute more significantly in improving the power factor and reduce the line pollution. The Vienna rectifier topology is controlled by the constant switching frequency control technique for wide range of load variation and it reduces the THD, improve the power factor and provide a steady DC link voltage to the bridge inverter to drive the BLDC motor. The design calculation and performance characteristics of BLDC motor and Vienna rectifier are verified by using Matlab simulation.</p>

Using Constant Switching Frequency Control in Linear-Assisted DC-DC Switching Converters for Photovoltaic Solar Regulators

This article shows the proposal of a linear–assisted converter or linear–&–switching hybrid converter with a constant switching frequency for photovoltaic solar DC-DC regulators. The control loop of the system is based on the current–mode technique. The main disadvantage of a converter with current–mode control is the inherent instability of the loop when switch duty ratios are greater than 0.5. In order to make stable the proposed linear–assisted converter, the article shows the technique based on a slope compensation.

A Space-Vector-Based Hysteresis Current Controller for a General $n$-Level Inverter-Fed Drive With Nearly Constant Switching Frequency Control

A current-error space-vector-based hysteresis current controller for a general <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$n$</tex></formula> -level voltage-source inverter (VSI)-fed three-phase induction motor (IM) drive is proposed here, with control of the switching frequency variation for the full linear modulation range. The proposed current controller monitors the space-vector-based current error of an <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n$</tex></formula> -level VSI-fed IM to keep the current error within a parabolic boundary, using the information of the current triangular sector in which the tip of the reference vector lies. Information of the reference voltage vector is estimated using the measured current-error space vectors, along the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\alpha$</tex> </formula> - and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\beta$</tex></formula> -axes. Appropriate dimension and orientation of this parabolic boundary ensure a switching frequency spectrum similar to that of a constant-switching-frequency voltage-controlled space vector pulsewidth modulation (PWM) (SVPWM)-based IM drive. Like SVPWM for multilevel inverters, the proposed controller selects inverter switching vectors, forming a triangular sector in which the tip of the reference vector stays, for the hysteresis PWM control. The sector in the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n$</tex></formula> -level inverter space vector diagram, in which the tip of the fundamental stator voltage stays, is precisely detected, using the sampled reference space vector estimated from the instantaneous current-error space vectors. The proposed controller retains all the advantages of a conventional hysteresis controller such as fast current control, with smooth transition to the overmodulation region. The proposed controller is implemented on a five-level VSI-fed 7.5-kW IM drive.

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.

A Novel Dynamic Overmodulation Strategy of Direct Torque Control

The direct torque control-space vector modulation (DTC-SVM) is well-known for achieving excellent dynamic overmodulation. However, the method uses complicated predictive stator flux vector control and requires high precision stator flux vector to define overmodulation mode. This paper proposes a straightforward dynamic overmodulation method and constant switching frequency controller without SVM to obtain a faster dynamic torque response in DTC for induction machines. To perform this method, the flux error status produced from the flux hysteresis comparator during dynamic condition is modified slightly before being fed to the look-up table. The verification of proposed method was performed using MATLAB/Simulink simulation package. The effects of different switching and operating condition on dynamic torque performance are analyzed. The result shows that a faster torque response is achieved in proposed dynamic overmodulation DTC method. The main benefit of the proposed method is its simplicity since only minor modification is made to the basic DTC hysteresis-based structure.

Optimum Design of the Current-Source Flyback Inverter for Decentralized Grid-Connected Photovoltaic Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Two alternative modes of operation for the current-source flyback inverter are investigated in this paper. The discontinuous conduction mode (DCM), where a constant switching frequency (CSF) control method is applied, and the boundary between continuous and DCM (BCM) that is introduced for photovoltaic (PV) applications in this paper (where a variable switching frequency control method is applied). These two control methods are analytically studied and compared in order to establish their advantages as well as their suitability for the development of an inverter for decentralized grid-connected PV applications. An optimum design methodology is developed, aiming for an inverter with the smallest possible volume for the maximum power transfer to the public grid and wide PV energy exploitation. The main advantages of the current-source flyback inverter are very high-power density and high efficiency due to its simple structure, as well as high-power factor regulation. The design and control methodology are validated by personal computer simulation program with integrated circuit emphasis (PSPICE) simulation and experimental results, accomplished on a laboratory prototype. </para>

Constant and High Switching Frequency Torque Controller for DTC Drives

The letter presents a new method of increasing the switching frequency of a direct torque control (DTC) of induction machines. The method simply replaces the hysteresis comparator of the conventional DTC drives with a triangular waveform-based constant switching frequency controller. By synchronizing the digital signal processor (DSP) sampling with a triangular waveform and with an appropriate systematic controller design, a high switching frequency DTC drive is possible without requiring a high-frequency space-vector modulator. The implementation of the controller is simple and operates based on waveform comparisons; in this letter it is implemented using a combination of a DSP and a field programmable gate array device. Simulation and experimental results indicate that the controller both reduces the torque ripple and maintains a constant switching frequency.

Predicted (on-time) equal-charge criterion scheme for constant-frequency control of single-phase boost-type AC-DC converters

A new constant switching frequency control method for single-phase boost-type AC-DC power converters is presented. The on times of the converter switches in each switching period is determined such that the average input current tracks the reference template in every switching cycle. The problems encountered in achieving smooth and stable operation and the modifications made to overcome them are discussed. The simulation studies done on a power converter controlled with this method, which is known as the predicted (on-time) equal-charge criterion (PECC) method, indicate stable operation at different input-current and voltage levels and power factors. The method was implemented on an insulated gate bipolar transistor (IGBT) power converter rated at 1 kVA using a 80 386 processor system for computations. The experimental results are presented and discussed in this paper.

High-performance hysteresis modulation technique for active filters

A new, substantial improvement of the hysteresis current control method for voltage source inverters is presented. A simple and fast prediction of the hysteresis band is added to a linearized version of the phase-locked loop control, thus ensuring constant switching frequency and tight control of the position of modulation pulses. This allows high accuracy in tracking highly distorted current waveforms and minimizes the ripple in multiphase systems. The implementation of this technique is very simple and robust, employing only a small number of conventional inexpensive analog and logic components. It does not require trimmings or tunings, giving the control the capability to adjust itself to the different operating conditions. The proposed method is compared with the most diffused modulation techniques, demonstrating its superior performance in responding to the most demanding conditions met in active filters. The behavior of the method has been fully verified by simulation and by experimental tests.