A comparison of stator flux linkage estimators for a direct torque controlled PMSM drive

In an increasing number of applications highly dynamic electrical drives, characterized by high quality torque control, are demanded. Direct torque control (DTC) for AC machines, permanent magnet synchronous motors (PMSM) or induction machines, can provide this accurate and fast torque control. When applying DTC the change of the stator flux linkage vector is controlled, based on torque and flux errors. As such the estimation of the stator flux linkage is essential for a DTC drive. Furthermore the quality of the estimation directly determines the capability of the drive. In the literature several possible solutions for the estimation of the stator flux linkage are proposed. However, a comprehensive comparison between these solutions is not present. This paper gives an overview of several techniques for the estimation of the stator flux linkage for DTC in PMSMs. The theoretical advantages and disadvantages of the methods are outlined. After a short discussion on the effects of erroneous estimations the results from simulations for the different methods are reviewed. It is shown that, despite their simplicity stabilized voltage model methods can offer good performance. Still they can not reach the performance of an extended Kalman filter implementation of a current model. Aspects of the practical implementation on FPGA are discussed

An improved direct torque controlled interior permanent magnet synchronous machine drive without a speed sensor

Some essential and important improvements of the direct torque controlled interior permanent magnet (IPM) synchronous machine drive are presented in this thesis. These studies, including analysis, modeling and experimental implementations confirm the possibility of a high performance direct torque controlled IPM synchronous motor drive without any continuous rotor position and speed sensor and without any current controller. The direct torque control technique, the comparison between DTC and FOC, and compensation methods for the problems/limitations associated with DTC have been investigated in this thesis. A number of important problems that affect the accuracy of the estimated machine flux linkage on which the DTC technique is built are thoroughly examined. Estimation of stator resistance variation, analysis and compensation of the non-linear effects of the inverter such as forward voltage drop and dead-time, speed sensorless control, and torque and flux ripple minimization for a direct torque controlled IPM motor drive are of major concern in this thesis. A Proportional-Integral stator resistance estimator based on stator current has been investigated for the compensation of any variation in stator resistance. It is shown that the estimator can track the variation of the stator resistance adequately. The scheme utilizes the error between the actual current and the reference current and requires no position signal. Modeling and experimental results will be shown. The non-linear effects of the inverter affect flux estimation greatly, especially at low speed. The effects such as forward voltage drop, dead-time and switching delay is analyzed, they degrade the system performance by introducing error between the estimated values and the actual values. The effects of the forward voltage drop and deadtime can be compensated by using a look-up table. The performance improvement of the drive has been shown in experiments. A speed estimation scheme based on stator flux linkage estimation is adopted and investigated experimentally. Furthermore, the possibility of fielding-weakening operation of the speed sensorless control is also investigated by modeling. The torque and flux ripples are significant problems of the DTC, and are mentioned widely. In order to solve this problem, the changes of torque and flux linkage over a sampling period are derived. Based on the analysis, a modified DTC is proposed to overcome these significant problems. Modeling and experimental results confirm the effectiveness of the proposed scheme. The field weakening control and speed sensorless control scheme is also combined with the proposed scheme. The experimental results show the new DTC scheme can achieve wider range operation and speed sensorless control successfully. The torque and flux ripples are reduced greatly under the new scheme in all experimental results. These abovementioned studies have clearly established that the DTC technique for the IPM machine is now much closer to being a viable and cost-effective candidate for a sensorless PM synchronous motor drive

Improved direct torque control of permanent magnet synchronous electrical vehicle motor with proportional-integral resistance estimator

Abstract -Electric vehicles (EVs) require fast torque response and high drive efficiency. This paper describes a control scheme of fuzzy direct torque control of permanent magnet synchronous motor for EVs. This control strategy is extensively used in EV application. With direct torque control (DTC), the electromagnetic torque and stator flux can be estimated using the measured stator voltages and currents. The estimation depends on motor parameters, except for the stator resistance. The variation of stator resistance due to changes in temperature or frequency downgrades the performance of DTC, which is controlled by introducing errors in the estimated flux linkage vector and the electromagnetic torque. Thus, compensation for the effect of stator resistance variation becomes necessary. This work proposes the estimation of the stator resistance and its compensation using a proportional-integral estimation method. An electronic differential has been also used, which has the advantage of replacing loose, heavy, and inefficient mechanical transmission and mechanical differential with a more efficient, light, and small electric motors that are directly coupled to the wheels through a single gear or an in-wheel motor

Impact of the motor magnetic model on direct flux vector control of interior PM motors

The stator-field-oriented, direct-flux vector control has been proven to be effective in terms of linear torque control and model independent performance at limited voltage and current (i.e. in flux weakening) for AC drives of various types. The performance of the direct-flux vector control relies on the accuracy of the flux estimation, as for any field oriented control. The knowledge of the motor magnetic model is critical for flux estimation when the operating at low speed. This paper addresses the effects of a limited knowledge of the motor model on the performance of the control at low speed, for an Interior Permanent Magnet motor drive. Experimental results are give

Direct torque control of brushless DC drives with reduced torque ripple

The application of direct torque control (DTC) to brushless ac drives has been investigated extensively. This paper describes its application to brushless dc drives, and highlights the essential differences in its implementation, as regards torque estimation and the representation of the inverter voltage space vectors. Simulated and experimental results are presented, and it is shown that, compared with conventional current control, DTC results in reduced torque ripple and a faster dynamic response

On the stator flux linkage estimation of an PMSM with extended Kalman filters

The demand for drives with high quality torque control has grown tremendously in a wide variety of applications. Direct torque control (DTC) for permanent magnet synchronous motors can provide this accurate and fast torque control. When applying DTC the change of the stator flux linkage vector is controlled. As such the estimation of the stator flux linkage is essential. In this paper the performance of the Extended Kalman Filter (EKF) for stator flux linkage estimation is studied. Starting from a formulation of the EKF for isotropic motors, the influence of rotor anisotropy and saturation is evaluated. Subsequently it is expanded to highly isotropic motors as well. In both cases the possibilities to add parameter estimations are evaluated

On extended Kalman filters with augmented state vectors for the stator flux estimation in SPMSMs

The demand for highly dynamic electrical drives, characterized by high quality torque control, in a wide variety of applications has grown tremendously during the past decades. Direct torque control (DTC) for permanent magnet synchronous motors (PMSM) can provide this accurate and fast torque control. When applying DTC the change of the stator flux linkage vector is controlled, based on torque and flux errors. As such the estimation of the stator flux linkage is essential. In the literature several possible solutions for the estimation of the stator flux linkage are proposed. In order to overcome problems associated with the integration of the back-emf, the use of state observers has been advocated in the literature. Several types of state observers have been conceived and implemented for PMSMs, especially the Extended Kalman Filter (EKF) has received much attention. In most reported applications however the EKF is only used to estimate the speed and rotor position of the PMSM in order to realize field oriented current control in a rotor reference frame. Far fewer publications mention the use of an EKF to estimate the stator flux linkage vector in order to apply DTC. Still the performance of the EKF in the estimation of the stator flux linkage vector has not yet been thoroughly investigated. In this paper the performance of the EKF for stator flux linkage is studied and simulated. The possibilities to improve the estimation by augmenting the state vector and the consequences of these alterations are explored. Important practical aspects for FPGA implementation are discussed