Design and implementation of sensorless vector and direct torque control induction motor drives for low speed operation

• Main Author: Rashed, Mohamed El-Sayed
• Published: University of Aberdeen 2002
• Subjects: 621.31
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.635966

The invention of the Rotor Field Oriented Control (RFOC) has made the induction machine (IM) dominate the high performance drives market. Fifteen years later, the Direct Torque Control (DTC) strategy has been introduced to give fast torque and flux control with reduced software and hardware requirements. However, the use of flux, speed, current and three-phase stator voltage sensors to monitor the drive states increase the drive cost and reduce the overall system reliability. The number of sensors can be minimised using estimation. However, estimation at low stator frequency operation suffers from pure integration, stability problems, observablity problems, parameter detuning and inverter non-linearity. Thus, it has been suggested to develop and implement stable sensorless RFOC and DTC drives with stator and rotor resistance estimators that can work at low stator frequency with minimal number of sensors. Thus, two types of model reference adaptive system (MRAS) estimators that do not include pure integration have been adopted. A novel design procedure based on linear control theory has been stated to design stable speed, stator resistance and rotor resistance estimators in various operating modes for individual and simultaneous use. The observablity problem at zero stator frequency operation has been eliminated using high frequency injection. The inverter non-linearity has been modeled and thus the model has been used to compensate the inverter voltage losses. The designed estimators have been employed to realise and implement stable sensorless RFOC and DTC IM drives that work at low stator frequency. Furthermore, the classic DTC with fixed switching frequency suffers from torque ripples, flux ripples, torque control offset and flux collapse at low speed and light load. A novel torque ripple minimisation (TRM) DTC scheme has been proposed to predictively eliminate the torque ripple and reduce the flux ripple and also eliminate the torque offset and the flux collapse. In addition, a novel Space Vector Modulation (SVM) DTC has been proposed as an alternative to the TRM DTC scheme and has shown better performance and less computational time. To achieve successful development of sensorless RFOC and DTC drives which can work at low speeds, well-designed stable estimators are the key factor. This research work has given important contribution towards clear vision to analyse and design stable parameter estimators for RFOC and DTC IM drive. It follows, stable speed and stator and rotor resistance MRAS based on back-EMF estimators have been designed for individual and simultaneous use, following the proposed design procedure. These estimators have been employed to implement a stable sensorless RFOC IM drive for low speed operation that insensitive to parameters variation. The performance of the drive has been investigated extensively using simulations and experimental work. In addition, stable speed and stator and rotor resistance MRAS based on current estimation estimators have been designed for individual and simultaneous use. These estimators have been also used to realise a stable sensorless RFOC IM drive for low speed operation. Simulations and experimental work have been carried out to also investigate the performance of the drive. These stable estimators have been also employed to develop two stable sensorless SVM and TRM DTC IM drives for low speed operation and insensitive to stator resistance. The performance of the drives has been investigated for low speed operation using simulations and experimental work. They have shown excellent performance.