Recent years, there is a great increase in application of Field Oriented Control(FOC) for speed and torque of industrial induction motor drives. This trend can be seen in various industries like Steel, paper and also various machinery tools etc. Among the various method of drive controls, Direct Torque Control (DTC) and Vector Control are considered most high performance induction motor drives.
An induction motor based on DTC is being increasingly used in various industrial applications. DTC offers faster torque response, better speed control and lesser hardware costs in compared with vector controlled drives. However, the conventional DTC suffers from high torque ripple, current harmonics and low performance during torque transients.
In this paper, an improved direct torque control technique of induction motor is presented. In comparison with the conventional DTC technique, a well-developed Space Vector Modulation(SVM) technique is applied for more accurate current control. The torque and flux magnitude are under control using each PI controller, thereby generating the voltage command for inverter control.
To control the speed and torque for induction motor, the system usually necessitates an information of speed or flux of the motor. However, there are many problems in case of using speed or flux sensors. A sensor requires a mounting space on the motor, reduces the reliability, and increases cost of the drive system. Therefore the various sensorless control algorithms have been proposed for the elimination of speed or flux sensors.
For most of those sensorless methods, the control performance in high speed range is good, but it is difficult to obtain satisfactory result in low speed region.
This paper presents sensorless speed control system for induction motor drive with an improved DTC method. The system consists of closed loop stator flux and torque controller, speed and torque estimators, PI controllers, PWM(Pulse width modulation) technique, IGBT(Insulated gate bipolar transistor) voltage source inverter, and micro-processor. The motor speed is estimated by using differential operator based on flux calculator.
The simulation and experimental results based on an improved DTC technique indicated good speed and load response from the low speed range to the high speed range. The torque and speed ripple could be reduced in comparison with the conventional switching table DTC method.