Switching-table-based Direct Torque Control Research Articles

Torque Ripple Reduction for Direct Torque Control of Dual Three-Phase PMSM Based on Multiple Virtual Voltage Vectors

The switching-table-based direct torque control (ST-DTC) has the merits of simple structure, easy implementation, and fast response, but suffers from large torque ripples. Previous literature, i.e., the classical DTC, can achieve torque ripple reduction in the high-speed region but has a limited capability for torque ripple reduction in the low-speed region. In this paper, the issue of torque ripple reduction in the full-speed region, i.e., both the low-speed and high-speed regions, is addressed. The effects of each group in all possible six groups of virtual voltage vectors (VVs) on torque variation are firstly investigated. Then, a multiple VV DTC based on only three groups of VVs is proposed and the corresponding optimal selections of three VV groups and level of torque regulator are presented. The proposed DTC not only can effectively reduce torque ripples in the full speed region, but also is simple and easy to implement. The superiority of the proposed DTC with optimal selections of VVs and level of torque regulator are verified by experimental results.

Fuzzy logic-based direct torque control for improvement of the fault-tolerant drive of a five-phase induction motor

Torque ripple reduction is one of the major challenges in switching table-based direct torque control (DTC) while operating for open phase faults of an induction motor, as the switching vectors are unevenly distributed. This can be minimized by increasing the level of the inverter and with the use of multi-phase motors. Fuzzy logic-based DTC is another solution to the above problem. In this paper, a comparative analysis is done between switching table-based DTC (ST-DTC) and fuzzy logic-based DTC for increasing the performance during open phase faults of a five-phase induction motor. The result shows that in fuzzy logic-based DTC with a two-level inverter, the torque ripple is reduced by 5.164% as compared with ST-DTC with a three-level inverter. The fuzzy logic-based DTC with the three-level inverter also gives better performance as compared with fuzzy logic-based DTC with the two-level inverter. The current ripple also reduced by 9.605% with respect to ST-DTC. Thus, fuzzy logic-based DTC is more suitable and cost effective for open phase fault-tolerant drives.

Switching-Table-Based Direct Torque Control of Dual Three-Phase PMSMs With Closed-Loop Current Harmonics Compensation

In this article, the new closed-loop current harmonics compensation strategy is introduced in the switching-table-based direct torque control (ST-DTC) of dual three-phase permanent magnet synchronous machines. The harmonic current distortion is the major problem of ST-DTC of multiphase machines and is typically caused by the lack of control in the auxiliary x-y subspace under the vector space decomposition model. The synthetic vectors to obtain average zero voltage in the x-y subspace can be a solution. However, this cannot compensate for current harmonics; for example, due to dead-time and back-electromotive force distortion, which is also mapped into the x-y subspace. Therefore, in this article, the x-y voltage references are obtained by the closed control loop that controls x-y currents to zero. To modulate the nonzero x-y voltage reference, the new modulation approach employing voltage vector groups in the x-y subspace together with the new switching table is developed and incorporated into ST-DTC. Meanwhile, the analysis of the two available sets of 12 voltage vector groups illustrates a tradeoff between linear range in the x-y subspace and the dc-link voltage utilization rate. Finally, the effectiveness of the proposed ST-DTC strategy is verified through experiments and simulation.

Virtual Subspace-Based DTC Strategy for Torque Ripple Minimization in Six-Phase Induction Motors

Classical switching-table-based direct torque control (ST-DTC) of six-phase induction machine (6PIM) drives is seriously penalized by current harmonics as well as torque ripples because of uncontrolled voltage vectors in the low-impedance non-energy subspace and hysteresis torque regulator, respectively. Hence, a virtual subspace-based DTC strategy is proposed to alleviate the impact of low-order current harmonics and torque ripples. The proposed virtual subspace includes 48 virtual voltage vectors (VVs) in the $\alpha -\beta $ subspace with average volt-seconds of zero in the $z_{1}-z_{2}$ subspace, which offers full extent of freedom degrees for 6PIM fed by a two-level voltage source inverter (VSI). The increased number of VVs allows the possibility to design up to nine-level hysteresis torque regulator to minimize the torque ripples. Nevertheless, in this paper, it is found that torque ripple, current harmonics, average switching frequency, and computational burden come to a compromise with a seven-level hysteresis torque regulator. The impact of these VVs on the performance of ST-DTC is experimentally studied and some details of hardware implementation are presented. The performance of the proposed scheme is verified by simulations as well as laboratory experiments, where a complete comparison with recent schemes is made to reveal the superiority of the proposed scheme.

Improved Direct Torque Control Method for Dual-Three-Phase Permanent-Magnet Synchronous Machines With Back EMF Harmonics

In this article, we propose a novel switching-table-based direct torque control (ST-DTC) method to eliminate the current harmonics in dual-three-phase permanent-magnet synchronous machines due to the back electromotive force (back EMF) harmonics and voltage vector selection. By employing a novel space voltage vector selection strategy and the optimal action time for different groups of vectors, a novel ST-DTC method is developed. Furthermore, in order to implement standard pulsewidth modulation (PWM) switching sequence, a centralization PWM method for 36 voltage vectors is introduced. The proposed method can effectively reduce the phase current harmonics caused by both back EMF distortion and voltage vector selection, and still reserve the advantages of the conventional ST-DTC, such as fast torque response and simple structure. Its effectiveness is verified by simulations and experiments.

Improved Duty-Ratio-Based Direct Torque Control for Dual Three-Phase Permanent Magnet Synchronous Machine Drives

This article investigates the duty-ratio-based direct torque control strategies for dual three-phase permanent magnet synchronous machine (PMSM) drives. The classical switching-table-based direct torque control (ST-DTC) for the dual three-phase drives usually suffers from significant harmonic currents and a large torque ripple. To reduce the harmonic currents, 12 combined voltage vectors are generated and employed as active vectors. Then, the duty-ratio-based ST-DTC is introduced to reduce the torque ripple. Considering that the typical duty-ratio-based ST-DTC exhibits the drawbacks of machine parameter dependent and a certain degree of failure rate in the duty ratio regulation, a simple and effective method taking into account the effect of the machine speed is employed to obtain the duty ratio. Afterward, to prevent the issue of the inverter thermal imbalance caused by using only one type of zero vectors, two types of zero vectors are employed in the switching sequences. With the proposed method, not only the harmonic currents have been suppressed, but also the torque ripple can be considerably reduced, while the merits of the classical ST-DTC such as simple structure and excellent dynamic performance are preserved. The experimental results validate the effectiveness of the proposed strategy.

Modified Switching Table-Based Direct Torque Control of Six-Phase Induction Motor Drive

This paper presents “Switching Table based Direct Torque Control” of Six-Phase Induction Motor (SPIM) Drive. The SPIM consists of two sets of stator windings spatially shifted by an angle of 30°. The extension of DTC technique of three-phase induction motor for a six-phase induction motor yields conventional DTC, in which one large voltage vector per sampling period is utilized. This technique suffers from large stator current harmonics of the order of 6k ± 1 (k = 1, 3, 5..). Initially, the conventional DTC technique has been implemented for comparison purpose. Further, its performance is improved by using two voltage vectors in each sampling period. In order to investigate the performance of ST-DTC based SPIM, a simulation study is carried out under different operating conditions. To validate the simulation results, a prototype model of six-phase induction motor rated for 1.5 kW, 200 V, 50 Hz is developed. Results obtained during simulation and experimentation show the effectiveness of modified ST-DTC technique in the reduction of stator current harmonics.

Performance Analysis of Switching Table Based DTC for 5-Phase Induction Motor with 3-Level Inverter

When leg and level of an inverter are increased, the number of switching states increases cumulatively. When the switching table-based DTC with higher phase (leg) and level is implemented for multiphase induction motor, the switching table becomes more accurate for different types of loading. Here, a 5-phase induction motor is simulated in MATLAB/Simulink from its dynamic mathematical equations. The switching table based direct torque control (ST-DTC) has been implemented to the simulated model through a 5-leg 3-level inverter. A performance study for ST-DTC has been carried out among 5-phase induction motor using 2-level (FPIM 2-L) inverter, 5-phase induction motor using 3-level (FPIM 3-L) inverter and 3-phase induction motor with 3-level (TPIM 3-L) inverter, and results are compared. In FPIM 3-L inverter, out of 243 switching states 20 active switching vectors are chosen for preparation of switching table, whereas in TPIM 3-L inverter, out of 27 switching states 12 switching vectors are chosen. In FPIM 2-L inverter, ten are chosen out of 32 switching states for DTC application. As the active states are increased, the numbers of sectors are also increased for FPIM 3-L inverter. With FPIM 3-L inverter, there is an improvement in settling time during step fall. The torque ripple in case of FPIM 3-L inverter is reduced by 4.18% with respect to FPIM 2-L and 2.7% with respect to TPIM 3-L inverter. However, the torque response during start is slightly sluggish in case of FPIM 3-L inverter as compared to TPIM 3-L inverter.

A Novel Vector Control Strategy for a Six-Phase Induction Motor with Low Torque Ripples and Harmonic Currents

In this paper, a new vector control strategy is proposed to reduce torque ripples and harmonic currents represented in switching table-based direct torque control (ST-DTC) of a six-phase induction motor (6PIM). For this purpose, a new set of inputs is provided for the switching table (ST). These inputs are based on the decoupled current components in the synchronous reference frame. Indeed, using both field-oriented control (FOC) and direct torque control (DTC) concepts, precise inputs are applied to the ST in order to achieve better steady-state torque response. By applying the duty cycle control strategy, the loss subspace components are eliminated through a suitable selection of virtual voltage vectors. Each virtual voltage vector is based on a combination of a large and a medium vector to make the average volt-seconds in loss subspace near to zero. Therefore, the proposed strategy not only notably reduces the torque ripples, but also suppresses the low frequency current harmonics, simultaneously. Simulation and experimental results clarify the high performance of the proposed scheme.

A Hybrid Direct Torque Control Scheme for Dual Three-Phase PMSM Drives With Improved Operation Performance

The direct torque control (DTC) is widely used in motion control of electric drives, especially for the switching table based direct torque control (ST-DTC) and the space vector modulation (SVM) based DTC (SVM-DTC). The classical ST-DTC schemes can achieve fast dynamic response but may suffer from distinct torque and flux ripples in steady-state operation. On the other hand, the classical SVM-DTC schemes have lower ripples in torque and flux, but the methods slow down dynamic response of DTC. Different from them, a novel hybrid DTC scheme is proposed for dual three-phase permanent-magnet synchronous motor (PMSM) drives in this paper, which can achieve fast dynamic response of classical ST-DTC scheme and good steady-state performance of classical SVM-DTC scheme simultaneously. The key of the proposed scheme is to implement different torque control methods under dynamic operation mode and steady-state operation mode, respectively. A switching method has been designed to guarantee smooth transition between the two operation modes. Besides, the decoupling control of torque and stator flux is maintained with the proposed control scheme, and the stator flux is controlled stably all along. By experiments on a laboratory prototype, it has been verified that the proposed scheme can achieve both good steady-state and transient performance compared to other control methods for dual three-phase PMSM drives.

Reduction of Both Harmonic Current and Torque Ripple for Dual Three-Phase Permanent-Magnet Synchronous Machine Using Modified Switching-Table-Based Direct Torque Control

Due to the fixed and limited sampling period in the real-time system, three-phase permanent-magnet synchronous machine (PMSM) drives using switching-table-based direct torque control (ST-DTC) usually suffer from steady-state error and ripple of torque. For the case of the dual three-phase system that has been widely investigated recently, the harmonic currents inevitably occur, which can be regarded as the third issue. This harmonic currents lead to the increase of losses and the decrease of system efficiency. Previous literature has addressed the aforementioned issues but only deals with either the torque ripple issues or harmonic currents. Therefore, this paper first introduces two types of synthetic vectors, which can reduce the harmonic currents effectively, as well as the most suitable switching sequences. Then, a modified five-level torque regulator has been proposed to improve the torque performance. With the proposed method, not only the harmonic currents have been suppressed, but also the steady-state error and ripple of torque can be considerably reduced, whereas the merits of classical ST-DTC, such as simple structure and excellent dynamic performance, are preserved. The experimental results validate the effectiveness of the proposed strategy.

Direct torque control of induction motor using three-level diode clamped inverter with DC-link capacitors voltage balancing

A simple switching table-based direct torque control of induction motor using three-level diode clamped inverter with DC-link capacitor voltage balancing is proposed in this paper. This scheme effectively utilises redundant states of small voltage vectors for charging and discharging of DC-link capacitors. The capacitor voltage balancing of the inverter is incorporated in the switching table of direct torque control. The switching table employed in this scheme balances the capacitor voltages for all operating conditions and it does not require any additional circuitry. To validate this scheme, extensive simulations are carried out for different load conditions. The steady state and transient operations of drive are investigated. The dynamics of DC-capacitor voltages are also tested. The results show that the proposed scheme provides capacitor voltage balancing and reduces neutral point drift voltage. The proposed scheme can be considered for medium and low voltage industrial drive applications.

Modified switching‐table strategy for reduction of current harmonics in direct torque controlled dual‐three‐phase permanent magnet synchronous machine drives

The switching-table-based direct torque control (ST-DTC) strategies of dual-three-phase permanent magnet synchronous machine (PMSM) drives are investigated in this study. Owing to its inherited advantage of suppression of sixth harmonic torque pulsation, dual-three-phase machine has attracted more and more interests among various sorts of multiphase machines. However, unexpected stator harmonic currents are often observed in the classical ST-DTC strategy for dual-three-phase PMSM drives, which cause large losses and decrease the efficiency of the drive system. An optimised ST-DTC strategy which consists a two-step process to determinate the most appropriate inverter voltage vector is proposed to reduce the stator harmonic currents. The merits of the classical DTC, that is, simple structure and good dynamic performance, are still preserved. The experimental results verify that the proposed method can significantly reduce the current harmonics.

Matrix Converter Induction Motor Drive Employing Direct Torque Control Method

The presented paper deals with the implementation of the Switching Table Based Direct Torque Control (ST-DTC) in the matrix converter-fed induction motor drive. In order to improve the control of the matrix converter input power factor, the use of the simplifled Indirect Space Vector Modulation (ISVM) was proposed. To verify the feasibility of the proposed solution, experimental results were presented.