Yuan Ren

Direct Torque Control of Permanent-Magnet Synchronous Machine Drives With a Simple Duty Ratio Regulator

The conventional switching-table-based direct-torque-controlled (DTC) ac machine drive is usually afflicted by large torque ripple, as well as steady-state error of torque. The existing methods, which optimize the duty ratio of the active vector, are usually complicated and parameter dependent. Based on the analysis of instantaneous variation rates of stator flux and torque of each converter output voltage vector, a simple and effective method considering the effect of machine angular velocity is proposed to obtain the duty ratio. The experimental results carried on a dSPACE platform with a laboratory prototype of the permanent-magnet machine verify that the proposed duty-based DTC method can achieve excellent transient response, less torque ripple, and less steady-state error, without resorting to the complicated control method over a wide range of operating regions.

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.

Enhancement of Steady-State Performance in Direct-Torque-Controlled Dual Three-Phase Permanent-Magnet Synchronous Machine Drives With Modified Switching Table

This paper investigates the improved switching-table-based direct torque control strategies (ST-DTC) for dual three-phase permanent magnet synchronous machine drives. The classical ST-DTC scheme for dual three-phase drives is usually seriously penalized by significant current harmonics, which cause large losses and reduce the efficiency of the drive system. Hence, a modified switching table consisting of 12 new synthetic voltage vectors is proposed. With this new switching table, not only the variables relating to the torque production are well controlled, but also the variables contributing to the current harmonics are eliminated successfully within one sampling period. Furthermore, a simple modified torque regulator is proposed to reduce the torque ripple and the steady-state error of torque which is usually observed in the real-time system. The switching sequence which is most suitable for the implementation of the real-time system has been proposed to minimize the computation burden. The proposed strategy only changes the switching table and torque regulator without any major modifications, and hence, the merits of the classical direct torque control, i.e., simple structure and good robustness, are still preserved. The experimental results validate the effectiveness of the proposed strategy.

Torque Improvement of Dual Three-Phase Permanent-Magnet Machine With Third-Harmonic Current Injection

This paper presents dual three-phase permanent magnet (PM) machines utilizing third-harmonic back electromotive force (EMF) and current to improve the output torque, but the torque ripple remains almost similar. The feasibility and practical issues of dual three-phase machines with third-harmonic EMF and current are first discussed, and the output torque is analytically derived. Furthermore, the optimal value of the third harmonic injected into a current waveform for maximum torque improvement is also analytically derived and validated by finite-element analysis (FEA) with the consideration of the constraints of both the peak and root-mean-square currents. For feasible dual three-phase PM machines, torque characteristics, including the average torque and the torque ripple, are compared without and with third-order harmonic injection. It is found that, for a dual three-phase machine without third-harmonic back EMF, the optimal third harmonic into the current is 1/6 of the fundamental current, and the average torque with the current having the optimal third harmonic injected can be improved by 15%. However, for the dual three-phase PM machines with third-harmonic back EMF, the optimal third harmonic into the current is dependent on the ratio of the third-harmonic back EMF to the fundamental back EMF, and the interaction of the third-harmonic current and the third-harmonic EMF results in the further improvement of the output torque. Finally, the experiments on a prototype are given to verify both the analytical and FEA methods.

Analysis of Dual Three-Phase Permanent-Magnet Synchronous Machines With Different Angle Displacements

In this paper, a novel nonconventional angle displacement (i.e., 15°) between two sets of three-phase windings is proposed for dual three-phase permanent-magnet (PM) machines. First, all feasible angle displacements between the two three-phase winding sets for various slot/pole number combinations of dual three-phase machines are discussed in general. Then, an in-depth investigation is carried out on a 24-slot/10-pole dual three-phase PM machine as an example covering various electromagnetic performances. The investigation shows that under healthy and three-phase open-circuit conditions, the proposed 15° configuration has a comparable electromagnetic performance to the 30° configuration, and a better performance than the 0° counterpart. Furthermore, under three-phase short-circuit (SC) condition, the proposed 15° configuration has the lowest SC current, smallest braking torque, and the best PM demagnetization withstand capability. Finally, three prototype 24-slot/10-pole surface-mounted PM machines are built and tested to verify the theoretical analyses.

Influence of back-EMF and current harmonics on sensorless control performance of single and dual three-phase permanent magnet synchronous machines

Purpose – The purpose of this paper is to investigate the influence of back-EMF and current harmonics on position and speed estimation accuracy for single and dual three-phase (DTP) permanent magnet synchronous machines (PMSMs) with two fundamental-model-based sensorless control strategies which are widely utilized for AC machines, i.e. flux-linkage observer (FO) and simplified extended Kalman filter (EKF). Design/methodology/approach – The effect of distorted back-EMF is studied for sensorless vector control of single three-phase PMSM. For the influence of current harmonics, unlike the existing literature where the current harmonics are deliberately injected, in this paper, sensorless switching-table-based direct torque control (ST-DTC) strategies for DTP-PMSM which inherently suffer from non-sinusoidal stator currents in addition to the distorted back-EMF, are investigated experimentally. Findings – By employing the FO and simplified EKF-based sensorless vector control of single three-phase PMSM, it can...

Sensorless switching-table-based direct torque control for dual three-phase PMSM drives

The inherent harmonic currents in conventional switching-table-based DTC (ST-DTC) for dual three-phase PMSM can affect its sensorless control performance, but it can be reduced by utilizing a modified switching-table strategy. In this paper, the influence of non-sinusoidal stator currents on the position and speed estimation accuracy for conventional flux-linkage observer (FO) and simplified Extended Kalman Filter (EKF) is investigated. Experimental results verify that the simplified EKF show better position and speed estimations due to its higher noise-rejection ability.

Cascaded Direct Torque Control of Unbalanced PMSM With Low Torque and Flux Ripples

During operation or manufacturing, the electrical machines are often exposed to parasitic impedance in the phases, causing unbalanced three-phase currents and increased torque and flux ripples. To mitigate the undesired torque and flux ripples, this paper presents a modified cascaded direct torque control strategy for three-phase PMSMs, having asymmetric phase impedances. The proposed method aims to generate the positive and negative reference voltage vectors by extracting the sequential components of the stator flux and current in the stationary αβ frame. Then, these two vectors are combined to produce the modified reference voltage vector for space vector modulation. Moreover, for further torque ripple suppression, a standard proportional-integral regulator has been enhanced by a resonant controller tuning at twice of the fundamental frequency. Various experimental results verify the feasibility of the proposed strategy under dynamic and steady-state conditions.