Designing a Sensorless Torque Estimator for Direct Torque Control of an Induction Motor

FPGAs were used to verify the accuracy of the torque estimator.

Induction motors often are the preferred choice among industrial motors due to the modern power electronics that improve their speed control. Vitally important for the speed control of a motor is the accurate estimation of the magnetic flux and the electromagnetic torque. Knowing the electromagnetic torque of a motor, one is able to control it and thus monitor the speed faster and more stably.

In this schematic of the Electromagnetic Torque Estimator test experiment, the induction motor is connected through the proper power electronics with the FPGA.
An electromagnetic torque estimator was designed using a three-phase induction motor, without the use of any sensor. The mathematical model of the induction motor was achieved, as well as deriving the electromagnetic torque equation by means of the proper transformations and mathematical tools. The analysis of the designed estimator is done through Simulink/MATLAB, where the transient and steady-state torque of the motor was determined. Furthermore, the experimental test of the estimator, using a real motor, power electronics, and a field-programmable gate array (FPGA), proved its efficiency.

The quality and reliability of any design are proven by how well it works in a real application. The transition from an ideal case, such as computer simulation, to a real application includes factors that may have undesirable effects. The experiment of an electromagnetic torque estimator using a real induction motor is a real application that involves undesirable external noise, constant offsets at the waveforms, as well as variation in the temperature of the windings and thus in the stator resistance. Nevertheless, the analysis of the results shows that the torque estimator works very well for an induction motor at the frequency of 60 Hz. The calculated steady-state torque is very close to the electrodynamometer settings. In conclusion, at the speed of 60 Hz, the variation of the stator resistance is not dominant and does not affect the torque calculation. Finally, the numerical integration works successfully.

The designed torque estimator can be used in any application that requires the calculation of the torque of an induction motor that works at a high frequency. In fact, the calculation of the electromagnetic torque is an important step before the speed control of the motor.

This work was done by Athanasios Tsoutsas of the Naval Postgraduate School. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Mechanics/Machinery category. NRL-0038



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Designing a Sensorless Torque Estimator for Direct Torque Control of an Induction Motor

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This article first appeared in the June, 2010 issue of Defense Tech Briefs Magazine (Vol. 4 No. 3).

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Overview

The document is a thesis authored by Athanasios Tsoutsas at the Naval Postgraduate School in September 2009, focusing on the development of a sensorless torque estimator for direct torque control of an induction motor. The research addresses the pressing issue of air pollution caused by transportation, proposing electric propulsion as a viable alternative to internal combustion engines.

The thesis emphasizes the need for accurate electromagnetic torque estimation and efficient control systems to enhance the performance of electric vehicles. It outlines the design and implementation of an electromagnetic torque estimator using the Simulink/Matlab environment, integrated with XILINX block sets. This approach allows for the simulation and analysis of the torque estimator's performance in a controlled environment.

A significant aspect of the research is the verification of the torque estimator's accuracy through the use of Field Programmable Gate Arrays (FPGA). The thesis details the methodology employed to ensure that the torque estimation is reliable and can be effectively utilized in real-world applications. By leveraging advanced hardware and software tools, the study draws realistic conclusions about the performance of the electromagnetic torque estimator, which is critical for the control of electric vehicles.

The document also includes a distribution statement indicating that it is approved for public release and has unlimited distribution, making it accessible for further research and development in the field of electric propulsion and motor control systems.

In summary, this thesis contributes to the growing body of knowledge on electric vehicle technology, particularly in the area of induction motor control. It highlights the importance of developing accurate and efficient systems to facilitate the transition from traditional combustion engines to electric propulsion, ultimately aiming to reduce environmental impact and improve air quality. The findings and methodologies presented in this research are relevant for engineers and researchers working on electric vehicle systems, motor control, and related technologies.