Research article Full-order observer for direct torque control of induction motor based on constant V/F control technique Narongrit Pimkumwong, Ming-Shyan Wang * Department of Electrical Engineering, SouthernTaiwan University of Science and Technology, No.1, Nan-Tai St., Yung Kang District, Tainan City 710, Taiwan article info Article history: Received 24 April 2017 Received in revised form 20 November 2017 Accepted 14 December 2017 Available online xxx Keywords: Direct torque control Full-order observer Stator flux estimation Feedback gain design Induction motor drives abstract This paper presents another control method for the three-phase induction motor that is direct torque control based on constant voltage per frequency control technique. This method uses the magnitude of stator flux and torque errors to generate the stator voltage and phase angle references for controlling the induction motor by using constant voltage per frequency control method. Instead of hysteresis com- parators and optimum switching table, the PI controllers and space vector modulation technique are used to reduce torque and stator-flux ripples and achieve constant switching frequency. Moreover, the coordinate transformations are not required. To implement this control method, a full-order observer is used to estimate stator flux and overcome the problems from drift and saturation in using pure inte- grator. The feedback gains are designed by simple manner to improve the convergence of stator flux estimation, especially in low speed range. Furthermore, the necessary conditions to maintain the stability for feedback gain design are introduced. The simulation and experimental results show accurate and stable operation of the introduced estimator and good dynamic response of the proposed control method. © 2017 ISA. Published by Elsevier Ltd. All rights reserved. 1. Introduction Direct torque control (DTC) [1e4] is a popular and powerful control technique for three-phase induction motor in present. It has very quick dynamic response, robust control, simple structure and easy to implement because coordinate transformations are not required and the torque and stator flux can be controlled directly [1e 7]. The appropriate stator voltage to control motor can be selected from an optimum switching table in accordance with the sector that the stator flux is located, and the signs of torque and magnitude of stator flux errors that are the output of hysteresis comparators [7]. The use of the hysteresis comparators results in high torque ripple, stator flux ripple and variable switching fre- quency, which produce noises, vibrations and increased losses that are the main drawback of DTC [5,8e10]. Some methods have been introduced in the past decades to mitigate these problems such as DTC based on space vector mod- ulation (DTC-SVM) [5,6, 11e 14], duty ratio control method [9, 15, 16] and model predictive torque control (MPTC) [7 ,8, 17e20]. DTC-SVM still maintains DTC transient merits while the steady-state perfor- mances are improved in the wide speed range. Two PI controllers and SVM technique are used instead of hysteresis comparators and optimum switching table in order to compensate the torque and magnitude of stator flux errors and generate the optimal stator voltage reference vector for motor, respectively. As a result, the torque and stator flux ripples are reduced and the switching fre- quency is kept constant. To improve DTC-SVM, the over modulation technique [13] and DTC-SVM incorporate with sliding mode control [21e23] have been presented. However, DTC-SVM operates in rotating reference frame that requires more complicated compu- tation than DTC. In duty ratio control, a selected active voltage vector is applied for a part of the sampling period and a zero voltage vector is applied for the rest in one switching cycle. This technique can significantly mitigate the torque and stator flux fluctuations but it increases the complexity of the DTC algorithm and stimulates some problems such as the demagnetization phenomenon of the stator flux, especially in the low speed region [16]. In MPTC, the optimal switching table in DTC is replaced by online optimization process. The best stator voltage vector is selected as the stator voltage vector that minimizes the cost function, which defines relating to torque and stator flux errors. The torque and stator flux are kept more accurately within their boundaries than DTC as * Corresponding author. E-mail addresses: p.narongrit@gmail.com (N. Pimkumwong), mswang@stust. edu.tw (M.-S. Wang). Contents lists available at ScienceDirect ISA Transactions journal homepage: www.elsevier.com/locate/isatrans https://doi.org/10.1016/j.isatra.2017.12.014 0019-0578/© 2017 ISA. Published by Elsevier Ltd. All rights reserved. ISA Transactions xxx (2017) 1e12 Please cite this article in press as: Pimkumwong N, Wang M-S, Full-order observer for direct torque control of induction motor based on constant V/F control technique, ISA Transactions (2017), https://doi.org/10.1016/j.isatra.2017.12.014