1083-4435 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMECH.2019.2912641, IEEE/ASME Transactions on Mechatronics IEEE TRANSACTIONS ON MECHATRONICS Abstract—The direct torque and flux control abandons the current control approach but instead regulates the stator flux and torque directly and independently. Consequently, excellent dynamic performance can be expected from the motor drive, making it attractive for low and medium voltage applications. This paper presents a direct torque and flux controlled interior permanent magnet synchronous motor driven from a three level simplified neutral point clamped inverter. This inverter features fewer switching devices and less DC-link capacitor voltage imbalance problems due to the absence of the medium voltage vectors. A novel space vector modulation algorithm is proposed to generate the reference voltage vector for control purposes while balancing the neutral point voltage. The experimental results included verify the effectiveness of the proposed approach. Index Terms—direct torque and flux control; neutral point voltage balancing; space vector modulation; three-level inverter. I. INTRODUCTION irect torque control (DTC), as opposed to the conventional current control method, regulates the stator flux and electromagnetic torque of the electrical machine directly and independently. As such, the DTC is capable of delivering excellent dynamic performance and needless to say, finds its adoption in a plethora of industrial applications [1]. In its conventional form, the DTC has the major drawback of producing very large torque and flux ripples, resulting in poor power quality. This unwanted feature can be attributed to the fact that the DTC applies a single voltage vector for the complete control cycle. Unless the sampling frequency is very high, the ripples more often than not are unacceptable. Many innovative research proposals have been put forth to alleviate this issue, with varying degrees of success. Notably, the duty ratio control (D-DTC) [2-3], predictive torque control (P-DTC) [4-5], constant switching frequency (CSF-DTC) [6-7] and the space vector modulation (SVM-DTC) [8-11] based control method have been amongst the most popular methods. This work is in part funded by the AUT School of Engineering Research Career Development Grant and in part funded by the Singapore Ministry of Education under Academic Tier1 Grant 2017-T1-002-112. Gilbert Foo and Tung Ngo are with Department of Electrical and Electronic Engineering, Auckland University of Technology, Auckland, 1010, New Zealand. (e-mail: gfoo@aut.ac.nz; tung.ngo@aut.ac.nz). Xinan Zhang (Corresponding author) is with School of EEE, Nanyang Technological University, 639798, Singapore (phone: 65-6790-4003; e-mail: zhangxn@ntu.edu.sg). Muhammed Faz Rahman is with School of EET, University of New South Wales, NSW, 2052, Australia (e-mail: f.rahman@unsw.edu.au) While these methods are all effective, the latter undoubtedly achieves the superior performance in terms of torque and flux ripple suppression as well as total harmonic distortion. This is because the SVM drastically increases both the resolution of the synthesisable reference voltage vector and the effective switching frequency. Despite their success, the aforementioned methods have been mainly limited to low voltage two-level inverter fed DTC drives. Nonetheless, their extension to higher power multilevel inverter fed drives should not be inhibited. Frequently, multilevel inverter drives also demand high dynamic and steady-state performances. The main benefit of the multilevel inverter is its increased number of available basic voltage vectors. This results in lower torque and flux ripples as well as voltage and current waveforms with a much lower total harmonic distortion; attributes that the traditional DTC lacks. The multilevel achieves this ability through an increased number of semiconductor switches which in turn necessitates a corresponding more complex modulation strategy. For example, the most commonly used three-level inverters in industry today are the neutral point clamped (NPC) [12] and the T-type [13] inverters illustrated in figures 1 (a) and (b) respectively. The former requires at least 12 actives switches ( 1−4 , 1−4 , 1−4 ) and 6 diodes ( 1,2 , 1,2 , 1,2 ). On the other hand, while no diodes are needed, the latter needs 12 active switches of which half are bidirectional. Needless to say, the T-type inverter possesses a higher efficiency due to its lower switch count. Nevertheless, because the switches must block the full DC-link voltage during, it is limited to applications operating at the upper end of the low voltage spectrum. In any case, both the NPC and T-type inverters are capable of significantly improving the performance of the DTC but their respective topologies dictate that their two DC-link capacitor voltages inevitably fluctuate and if left unmitigated, will jeopardise the performance of the inverter. Thus, from the drive’s perspective, it is imperative that the control algorithm takes on the task of balancing the capacitor voltages in addition to maintaining accurate torque and flux control [14-16]. A three-level simplified neutral point clamped (3L-SNPC) inverter was proposed in [17] and originally used with gate turn-off (GTO) thyristors. The operation of this inverter is enhanced when equipped with faster switching devices such as IGBTs. In essence, the 3L-SNPC inverter is a combination of a front-end dual Buck converter and a conventional two-level inverter. Its arrangement requires only ten active switches; SVM Direct Torque and Flux Control of Three- level Simplified Neutral Point Clamped Inverter Fed Interior PM Synchronous Motor Drives D Gilbert Foo, Member, IEEE, Tung Ngo, Student Member, IEEE, Xinan Zhang, Member, IEEE, Muhammed Faz Rahman, Fellow, IEEE