Direct torque control scheme of IM drive with 12-sided polygonal voltage space vectors

The application like electric vehicles requires frequent torque control to control speed of IM Drives. The direct torque control is very good control scheme for fast torque control and it is easy to implement and decoupling of motor variables along rotor flux axis is not required. In this paper, direct torque control scheme is proposed using 12-sided polygonal space vector based inverter fed IM drive. The proposed DTC scheme uses the fundamental stator voltage vector for identification of sector for selection switching of vector to control stator flux and the torque. Because the DTC scheme requires the information of sector of stator voltage vector position only. The exact position of stator voltage is not required for control like rotor oriented vector control scheme of IM drives. Extensive simulation and experimental results are shown to verify the proposed concept. The control scheme is extended from 1 Hz to rated 50 Hz operation of the inverter. The advantages achieved due to 12-sided polygonal space vector are mainly fast transient of torque, less switching frequency variation, reduced torque error and high resolution of sector identification with accuracy of ±15°.

Introduction

Nowadays, there is an increasing demand for inverter-fed Induction Motor (1M) drive with good performance and simple hardware. 1M drive is used in various applications such as electric vehicles, ventilation systems for large buildings and for industrial drives with variable speed requirements. They are also used for pump, elevator, conveyor and machine tool drives. These applications require frequent torque control to control speed of vehicle. This has resulted into the need of control scheme with high performance, fast transient and accurate control of torque for induction motor drive. There are mainly two schemes used for these applications. One is vector control and the other is the Direct Torque Control (DTC) [1], [2]. The DTC gives fast control of torque compared to vector control [2], [3], [4]. And it is easy to implement also. The DTC has been implemented using either variable switching frequency or constant switching frequency techniques. In conventional DTC [2], variable switching frequency hysteresis controller is applied for inverter control. It is simple and gives large benefits; the main drawback of this method is the wide band of the switching frequency of the inverter even when the flux and torque references are kept constant. Constant switching frequency space vector modulation based DTC is proposed in [5]. This method has the advantage of reduced line current harmonics but it requires intensive computations and does not operate well at low speeds.