This paper presents the results of an investigation into the suitability of a Direct Torque Control method for an electric vehicle application. For this investigation, a 2.2kW specially rewound induction motor driven using a three-level IGBT inverter switching at 10kHz was used.
The control scheme used and the test system that has been developed are explained. Simulated and experimental results are presented. The control scheme is computationally complex, but is shown to have low current distortion, low torque ripple, and a fast torque response.
INTRODUCTION:
The University of Canterbury is currently developing an electric vehicle driven by a single 70kW, 10 000 RPM induction motor. This high-speed induction motor was chosen based on its high power to weight ratio. Such induction motors typically have low inductance and therefore need a controller with a fast current response.
A suitable torque controller is required for this high-speed induction motor. Torque control is preferred for electric vehicle applications instead of precise closed loop speed control because it mimics the operation of an internal combustion engine. It is important to make an electric vehicle drive like a standard vehicle.
Matlab and Simulink were used to perform simulations on a number of control schemes. These schemes were Field Oriented Control, Direct Torque Control (DTC), DTC using Space Vector Modulation, and DTC with Minimal Torque Ripple. DTC using Space Vector Modulation was chosen based on its low current distortion (therefore high system efficiency) and fast torque response. A disadvantage of this control scheme is its high computational complexity.
Initial simulations and testing have been based on a 2.2kW, 50Hz, induction motor that has been rewound to run at up to 150Hz (4500 rpm) from a 270 volt supply. This motor is being driven through a three-level 150 amp (peak) IGBT inverter switching at 10kHz.
A three-level inverter has the advantage of being able to produce three different levels of output phase voltage compared to the standard two levels. With a greater number of available levels for the output voltage the desired sinusoidal voltage can be achieved more accurately without increasing the switching frequency. This results in less current harmonics and therefore more efficient utilisation of the available energy. However, this improvement is at the expense of additional hardware and complexity.
In this paper, the Direct Torque Control method and the three-level Space Vector Modulation technique that were used are explained. Simulation results from Matlab/Simulink are presented. The test platform that has been developed is explained. Initial experimental results from the evaluation of the three-level inverter are presented.
Source: University of Canterbury
Author: J. C. Trounce, S. D. Round, R. M. Duke
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