This paper deals with a model reference adaptive system (MRAS) estimator based on second-order sliding mode (SOSM) strategy for speed-sensorless direct torque control (DTC) of six-phase induction motor (6PIM). A compensated flux observer is proposed using well-known super-twisting algorithm (STA), which is inherently an SOSM technique. This observer is adopted as the reference model in the MRAS-based speed estimator to overcome the chattering problem of the classical sliding-mode techniques. The estimator is robust against parameter uncertainties and dc-offsets. In addition, a super-twisting algorithm-based second-order sliding-mode (ST-SOSM) controller is designed for speed loop in the DTC strategy to increase its robustness under applying external load disturbances. The speed control loop needs the load torque signal. For this purpose, a ST-SOSM load torque observer is proposed, where it provides a robust performance without the penalty of high chattering. Simulation and experimental results confirm the validity and the effectiveness of the proposed approaches.

Any mismatch between the real and tuned values of the stator resistance (R ₈ ) leads to a degraded performance of the direct torque control (DTC) strategy. The underestimation of R ₈ causes an error in the stator flux and torque estimations, thereby weakening the DTC performance; however, an overestimation of R ₈ can make the DTC unstable. The main objective of this paper is to present a novel estimator based on the model reference adaptive system (MRAS) for online estimation and tracking of R ₈ in the DTC of six-phase induction motors (6PIMs). Voltage and current vectors of 6PIM are mapped into α - β, z ₁ - z ₂ , and o ₁ - o ₂ subspaces, where only the α - β components are related to electromechanical energy conversion. The proposed technique uses additional degrees of freedom, i.e., the z ₁ - z ₂ subspace to estimate R9. The attained merits include simplicity, low computation burden, and low sensitivity to other parameters of the 6PIM. The adaptation law is derived from the Lyapunov stability theory and Popov hyperstability theory. The proposed technique is applied under a duty cycle control based DTC, where z ₁ - z ₂ currents possess reduced amplitudes in comparison with the conventional DTC. The validity of the proposed system is verified by simulation and experimental results.

Accurate rotor time constant (Tr) value is necessary to ensure an acceptable performance of the indirect field oriented control (IFOC) strategy, where a detuned Tr affects rotor flux orientation, in turn, results in poor dynamic and steady-state response of torque. The problems associated with detuned Tr will be exacerbated in sensorless drives, when a model-based speed estimation algorithm is employed. This paper investigates the problem of simultaneous identification of the rotor time constant and rotor speed for sensorless IFOC of six-phase induction motor (6PIM) drives. First, an adaptive observer is proposed for online detection of the low-frequency sinusoidal signal, which is intentionally injected to the rotor flux command. Then, a novel model reference adaptive system (MRAS)-based Tr estimator is proposed using the detected signal. The Lyapunov stability theorem is used to ensure the asymptotic stability of the proposed identification system. The proposed method is based on the induction machine model in primary subspace, i.e., α-β subspace. Hence, it is usable for other multi-phase induction machine (three-phase and higher). Nevertheless, the 6PIM is adopted here as a case study. The simulation and experimental results clarify the effectiveness of the proposed parallel estimation system.

In this paper, a new vector control strategy is proposed to reduce torque ripples and harmonic currents represented in switching table-based direct torque control (ST-DTC) of a six-phase induction motor (6PIM). For this purpose, a new set of inputs is provided for the switching table (ST). These inputs are based on the decoupled current components in the synchronous reference frame. Indeed, using both field-oriented control (FOC) and direct torque control (DTC) concepts, precise inputs are applied to the ST in order to achieve better steady-state torque response. By applying the duty cycle control strategy, the loss subspace components are eliminated through a suitable selection of virtual voltage vectors. Each virtual voltage vector is based on a combination of a large and a medium vector to make the average volt-seconds in loss subspace near to zero. Therefore, the proposed strategy not only notably reduces the torque ripples, but also suppresses the low frequency current harmonics, simultaneously. Simulation and experimental results clarify the high performance of the proposed scheme.