Three-phase Permanent Magnet Synchronous Machine Research Articles

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 be concluded that the rotor position estimation accuracy is less affected by the back-EMF harmonics when the simplified EKF method is utilized since it is less sensitive to such noises. When the influence of non-sinusoidal stator currents together with back-EMF harmonics is investigated for the conventional and modified ST-DTC of DTP-PMSM, it is indicated that the simplified EKF exhibits better position and speed estimation accuracy in both the conventional and modified ST-DTC strategies. In addition, its steady-state performance shows a slight superiority over that based on FO, in terms of flux and torque ripples, and THD of phase currents. For the dynamic performance, the estimated speed of simplified EKF shows less phase lag and fluctuations compared to that of FO. Originality/value – This paper introduces the influence of back-EMF and current harmonics on sensorless control performance for single and DTP PMSMs. Detailed experimental results show that the simplified EKF exhibits better rotor position and speed estimation accuracy compared to that of FO due to its higher noise-rejection ability.

Dual-Stator Two-Phase Permanent Magnet Machines With Phase-Group Concentrated-Coil Windings for Torque Enhancement

This paper presents two types of novel dual-stator two-phase permanent magnet synchronous machines (PMSMs) equipped with an advanced phase-group concentrated-coil winding and a spoke-type PM array for direct-drive applications. The key advantage of the proposed two-phase PMSMs is its superior flux-focusing effects, which greatly enhance the torque, benefiting from the whole machine configuration. To highlight the advantages of the proposed two-phase PMSMs, one three-phase PMSM and one two-phase PMSM with the conventional stator and winding configurations are adopted for comparison. All relevant machine characteristics, including the air-gap flux density, back electromotive force, and electromagnetic torque, are predicted by a 2-D finite-element method. Finally, one of the proposed dual-stator two-phase PMSMs is optimized to minimize cogging torque and torque ripples using the Kriging method and a genetic algorithm.

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.

Decoupled Vector Control Scheme for Dual Three-Phase Permanent Magnet Synchronous Machines

Dual three-phase electric machines have many advantages compared with conventional three-phase machines. However, the properties of such machines present challenges for current control that make it difficult to produce sufficient performance for the electric drive. This paper introduces an improved vector control scheme for dual three-phase permanent magnet synchronous machines. The study includes detailed solutions for key parts of the control such as reference frame transformations, decoupling of the current control loops, and modulation. The performance of the suggested control scheme is evaluated using finite-element analyses and experimental results. The results show that the scheme can produce desired dynamics for the current control and guarantees balanced current sharing between the winding sets. In addition, the proposed solution is capable of reducing current harmonics produced by the internal structure of the machine. This problem is, however, only partly solved since complete elimination of harmonic components is not achieved. Nevertheless, the suggested control scheme overcomes many of the disadvantages found with other control solutions. Improved control performance allows the full benefits of dual three-phase drives to be utilized even in demanding applications.

A Fault-Tolerant Permanent-Magnet Traction Module for Subway Applications

A fault-tolerant permanent-magnet traction module is proposed for subway application, which has three different operation modes: normal, isolation, and fault-tolerant mode. In normal mode, 2 three-phase permanent-magnet synchronous machines (PMSMs) are controlled by 2 three-leg voltage-source inverters (VSIs), respectively. The proposed module can output maximum torque and reach the maximum velocity. When one leg fails, the corresponding PMSM is isolated from the fault VSI and the fault subway train operates in isolation mode, in which the traction effort is reduced but the maximum velocity can be maintained under light load. However, the fault subway train will stop if the load is heavy. To avoid the stop, the fault subway train is switched into fault-tolerant mode, in which the healthy VSI and fault VSI are reconfigured to 1 five-leg VSI by a fault-tolerant bridge. Due to lower phase-phase voltage apportionment ratio, the fault-tolerant mode is not used until the velocity is decreased to half of the maximum velocity. In fault-tolerant mode, the nonadjacent selection principle is proposed to prevent the common leg from overcurrent damage. The subway train equipped with the proposed module can maintain basic operation performances if any leg fails. The performances are analyzed and the effectiveness is verified by experimental results.

Improved torque regulator to reduce steady‐state error of torque response for direct torque control of permanent magnet synchronous machine drives

This study presents an improved switching-table-based direct torque control method for three-phase permanent magnet synchronous machine drives. The instantaneous variation rates of stator flux and torque of each converter output voltage vector are analysed. It is found that large steady-state error of torque response exists because of the difference in the decreasing and increasing variation rates. A novel and simple on-line band-shifted torque regulator is proposed to reduce this steady-state error of torque response. The experimental results verify that the proposed method can significantly reduce the steady-state error of torque response with the extra bonus of slight reduction of both the torque ripple and average commutation frequency, while preserve the merits of the conventional direct torque control, that is, simple structure, good robustness and excellent transient response.

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.

Control Strategies for Open-End Winding Drives Operating in the Flux-Weakening Region

This paper presents and compares control strategies for three-phase open-end winding drives operating in the flux-weakening region. A six-leg inverter with a single dc link is associated with the machine in order to use a single energy source. With this topology, the zero-sequence circuit has to be considered since the zero-sequence current can circulate in the windings. Therefore, conventional overmodulation strategies are not appropriate when the machine enters in the flux-weakening region. A few solutions dealing with the zero-sequence circuit have been proposed in the literature. They use a modified space vector modulation or a conventional modulation with additional voltage limitations. This paper describes the aforementioned strategies, and then, a new strategy is proposed. This new strategy takes into account the magnitudes and phase angles of the voltage harmonic components. This yields better voltage utilization in the dq frame. Furthermore, inverter saturation is avoided in the zero-sequence frame, and therefore, zero-sequence current control is maintained. Three methods are implemented on a test bed composed of a three-phase permanent-magnet synchronous machine, a six-leg inverter, and a hybrid digital signal processor /field-programmable gate array controller. Experimental results are presented and compared for all strategies. A performance analysis is conducted as regards the region of operation and the machine parameters.

Space-Vector PWM Control Synthesis for an H-Bridge Drive in Electric Vehicles

This paper deals with a synthesis of space-vector pulsewidth modulation (SVPWM) control methods applied for an H-bridge inverter feeding a three-phase permanent-magnet synchronous machine (PMSM) in electric-vehicle (EV) applications. First, a short survey of existing architectures of power converters, particularly those adapted to degraded operating modes, is presented. Standard SVPWM control methods are compared with three innovative methods using EV drive specifications in the normal operating mode. Then, a rigorous analysis of the margins left in the control strategy is presented for a semiconductor switch failure to fulfill degraded operating modes. Finally, both classic and innovative strategies are implemented in numerical simulation; their results are analyzed and discussed.

Fast Computation of Electromagnetic Vibrations in Electrical Machines via Field Reconstruction Method and Knowledge of Mechanical Impulse Response

The main objective of this paper is to provide an efficient computational model for fast and accurate calculation of electromagnetically induced vibrations in electrical machines. Although a three-phase permanent-magnet synchronous machine has been used in this paper, the same methodology can be extended to other types of electric machinery. A brief comparison between the different methods for structural analysis in electrical machines is introduced. The method proposed in this paper uses finite-element analysis in order to extract the impulse responses that are used in calculating the vibration. A field reconstruction method is used for the electromagnetic field analysis.

High-Performance Control of Two Three-Phase Permanent-Magnet Synchronous Machines in an Integrated Drive for Automotive Applications

The closed-loop control of an integrated dual AC drive system is presented to control two three-phase permanent-magnet motors. A five-leg inverter is employed in the drive system; three of the inverter legs are for a main traction motor, but only two are needed for a three-phase auxiliary motor by utilizing the neutral point of the traction motor. An integrated drive with reduced component count is therefore achieved by eliminating one inverter leg and its gate drivers. A modified current control scheme based on the rotor flux orientation principle is presented. Simulation and experimental results are included to verify the independent control capability of the integrated drive.

A Series-Connected Two-Motor Six-Phase Drive With Induction and Permanent Magnet Machines

A novel concept for multimotor drives, based on utilization of multiphase machines, has been proposed recently. Independent vector control of all the machines of the group can be achieved while using a single multiphase inverter supply, provided that the stator windings are connected in series in an appropriate manner. One specific case of such a drive system is a two-motor drive, comprising a symmetrical six-phase machine connected in series with a three-phase machine. The group is supplied from a six-phase current-controlled voltage source inverter (VSI). The available considerations related to this two-motor drive system, consisting of two induction motors, are currently of purely theoretical nature, with the proof-of-concept provided by simulation only. This paper analyzes series-connected two-motor six-phase drive, comprising a six-phase symmetrical induction machine and a three-phase permanent magnet synchronous machine (PMSM) and provides, for the first time, detailed experimental verification of the possibility of independent control of the two motors. The operating principles of the drive system are at first reviewed and a brief description of the experimental system is then given. The emphasis is placed on an extensive presentation of experimental results, collected from a laboratory setup. It is shown that a truly independent and decoupled vector control of the two machines results, although a single supply source is used. The two-motor drive system of the proposed structure is seen as a potentially viable industrial solution for applications requiring one high-power and one low-power machine.