A simple method to reduce torque ripple and mechanical vibration in direct torque controlled permanent magnet synchronous motor

The Direct Torque Control (DTC) technique of Permanent Magnet Synchronous Motor (PMSM) receives increasing attention due to its simplicity and robust dynamic response compared with other control techniques. The classical switching table based DTC presents large flux, torque ripples and more mechanical vibrations in the motor. Several studies have been reported in the literature on classical DTC. However, only limited studies that actually discuss or evaluate the classical DTC. This paper proposes a simple DTC method / Switching table for PMSM, to reduce flux and torque ripples as well as mechanical vibrations. In this paper two DTC schemes are proposed. The six sector and twelve sector methodology is considered in DTC scheme I and DTC scheme II, respectively. In both DTC schemes a simple modification is made in the classical DTC structure that is by eliminating two level inverter available in the classical DTC is replaced by three level Neutral Point Clamped (NPC) inverter. To further improve the performance of the proposed DTC scheme I, the available 27 voltage vectors are allowed to form different groups of voltage vectors such as Large - Zero (LZ), Medium - Zero (MZ) and Small - Zero (SZ), where as in DTC scheme II, all the voltage vectors are considered to form a switching table. Based on these groups, new switching table is proposed. The proposed DTC schemes are comparatively investigated with the classical DTC and existing literatures from the aspects of theory analysis and computer simulations. It can be observed that the proposed techniques can significantly reduce the flux, torque ripples, mechanical vibrations and improves the quality of current waveform compared with traditional and existing methods

Analysis of 37-kW Converter-Fed Induction Motor Losses

This paper presents an energy efficiency analysis of a 37-kW standard squirrel-cage induction motor under sinusoidal and nonsinusoidal supply. The motor losses are analyzed using the conventional IEC loss segregation method and also numerically modeled using finite-element simulations. The measured and simulated loss components are compared with three different modulation methods. The overall simulated losses are in good agreement with the measured ones, but there exist differences in the loss components

An Improved Model Predictive Torque Control for PMSM Drives Based on Discrete Space Vector Modulation

In this article, an improved model predictive torque control (MPTC) method based on discrete space vector modulation (DSVM) is proposed for permanent magnet synchronous motor (PMSM) drives. Aiming at solving the two problems of large torque ripples and high computational complexity in conventional MPTC, the proposed method adopts a second optimization and a new simplified search strategy. The key idea of second optimization is to make the output voltage vector closer to the actual optimal solution. In this case, a more suitable voltage vector is applied in each sampling period. The simplified search strategy reduces the calculation time by cutting down the number of candidate voltage vectors without affecting drives performance. Compared to the conventional MPTC without DSVM and with DSVM, the proposed method can produce superior steady-state performance and lower computational complexity. Simulation and experimental results are presented to validate the effectiveness and feasibility of the proposed method

Minimization of torque ripples in direct torque control of induction motor at low speeds

Direct Torque Control (DTC) of induction motor has attracted a considerable attention in the motor drives industry. The key merits of DTC include fast torque dynamic response, simple structure, insensitivity to motor’s parameters. Nevertheless, DTC inherently suffers from two major downsides namely: high torque ripples and variable switching frequency. This thesis presents a new technique to minimize the torque ripples inherited in the digital-based DTC of induction motor. The typical discrete-based DTC imposes a delay time which frequently allows the torque to overshoot beyond hysteresis bands. This triggers the selection of reverse voltage vectors which, in turn, cause large torque decrements. The torque ripples become of great significance at low speeds where torque overshoot is most likely to occur due to steep positive torque slope. A multi-level DC link voltage is proposed to vary the DC voltage of Voltage Source Inverter (VSI) according to motor’s speed. By varying the DC link voltage, the torque slopes can be controlled and, hence, the torque overshoots are mostly avoided. Therefore, the torque ripples are significantly minimized. The viability of proposed technique has been validated using MATLAB/Simulink software. Results show the proposed technique may yield over 50% reduction in the RMS torque ripples while maintaining a low switching frequency. Also, the torque dynamic response is maintained as good as in the conventional DTC schem

DIRECT AND INDIRECT TORQUE CONTROL OF UNBALANCED PERMANENT MAGNET SYNCHRONOUS MACHINES

Electrical machines may exhibit various types of imbalances and undesirable harmonic distortions. These may increase the torque and flux ripples, acoustic noise, unbalanced three-phase currents, while also reducing efficiency. These types of imbalances and undesirable harmonic distortions cannot be controlled by using the conventional indirect torque control (ITC) and direct torque control (DTC) strategies. For some high-performance motion control, such as precision machine tools, robotics, and servo drives, low torque ripples are, however, obligatory. Nowadays, more studies have been conducted on the ITC strategy to control undesired current harmonics, such as double synchronic reference frames (DSRF), resonant controller, second order generalized integration, and reference current generation. Such strategies, however, can rarely be applied to DTC strategy. In this research, the influence of asymmetric winding impedances, unbalanced back-EMF, and inverter nonlinearity in three-phase surface-mounted PMSMs has been systematically investigated by employing space vector modulations (SVM) based ITC and DTC strategies. This thesis firstly presents a modified ITC strategy by extracting the positive and negative sequence components in the stationary abc frame, and then a coordination transformation is used to control the machine in DSRF. This strategy provides faster dynamic response when compared with the conventional DSRF strategy, since the filters and the decoupling network are not required. Due to the lack of research regarding the DTC strategy under unbalanced conditions, this research investigates and proposes modified cascaded and parallel DTC-SVM strategies. The conventional cascaded DTC strategy is investigated under balanced and unbalanced conditions. Then, a modified control strategy is introduced by adding two compensators (the conventional PI-controller with a resonant controller, and the use of the negative- and positive-sequence voltage vectors) to suppress the 2nd harmonic components in the torque and stator flux linkage. Furthermore, for parallel DTC-SVM, the compensation of the 2nd and 6th harmonic components is investigated by means of either a resonant controller or an adaptive filter. In addition to the simplicity of the proposed strategies, these may also be able to significantly reduce the torque and flux ripples, while maintaining the merit of the fast dynamic response of the conventional DTC strategy even under variable fundamental frequency. Moreover, it has been proven that the compensation from using a resonant controller or an adaptive filter is parameter independent. Thus, regardless of unbalanced conditions, an effective torque ripple minimisation can still be achieved by properly selecting the dominant harmonic compensation

The Fuzzy Logic-based Stator-flux-oriented Direct Torque Control For Three-phase Asynchronous Motor

This article proposes a Takagi-Sugeno fuzzy logic-based (TS-FLB) controller applied to the stator-fluxoriented direct torque control (SFO-DTC) scheme. The conventional SFO-DTC scheme has two PI regulators to generate the reference stator voltage vector necessary to maintain the load. The objective is to reduce the quantity of these controllers, hence we propose a single TS-FLB controller to substitute both PI regulators. The rule base for the proposed TS-FLB controller is defined in function of the stator flux error and the electromagnetic torque error, and it utilizes trapezoidal and triangular membership functions (MFs) for the fuzzification of its inputs. Constant switching frequency and low torque ripple are obtained using the space vector modulation technique. The simulation and experimental results show that the TS-FLB controller obtains a decoupling torque and stator flux control and low torque ripple. These results were obtained for various operating conditions such as step change in the motor load, speed reversion with no load and the application of an arbitrary speed profile. 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