Speed Sensorless Field Oriented Control of Induction Motor through Speed and Flux Estimation

Abstract

Separately excited dc motors were used in industry for high performance applications and servo applications. This was due to simple control of dc motor contrasted with an ac motor. Now days dc motors have been replaced by the induction motors for high performance applications because induction motors are cheaper, robust and have low moment of inertia and light weight. The entry of vector control strategies the control of an induction motor is changed into that of a separately excited dc motor. In vector control torque and flux can be controlled independently. Accepting that the rotor flux position is known the stator current phases are resolved along and in quadrature to it. The quadrature component of stator current is the torque current, i_ßs and the in phase component of stator current is field current, i_as. The transformation of current requires unit vectors which are derived from the rotor flux signals. When the measured field angle is used to determine unit vector in the vector control scheme is known as direct vector control and that using evaluated field angle is called indirect vector control scheme. In conventional vector control a rotational transducer is used to sense the speed. It increases complexity of the drives and decreases reliability of the system. In some cases it is undesirable such as high speed drives. But all these sensorless indirect vector control have a disadvantage that motor parameters variation with temperature causes an estimation speed error in steady state and transient state. This increases the losses of the motor and reduces efficiency of the drive. However, speed sensorless vector control based on rotor flux observer method has finite parameters variation effect and speed accuracy is improved by observer. Efforts are being made to decrease the sensitivity of the drives system to motor parameters variation. The aim of this project work is to build up a vector controlled induction motor drive working without a speed sensor. The methodology is to detect the motor speed by using rotor flux observer. It estimates the stator currents and rotor flux by measuring terminal currents and voltages, and the speed is then estimated by utilizing the rotor flux and the current errors between the actual stator current and the estimated stator current.