SPEED ESTIMATION TECHNIQUES FOR SENSORLESS VECTOR CONTROL OF INDUCTION MOTOR

Abstract

The most frequently used electrical machine in various modern high-performance drive applications is the induction motor (IM), especially its squirrel cage type rotor counterpart. For the induction motor controlling to achieve variable speed, Vector Control (VC) or Field Oriented Control (FOC) have become the industrial standard. The vector control technique decouples the two component of stator current: one is responsible for controlling torque and other is controlling flux independently as in the case of separately excited fully compensated DC motor. The closed loop control of Vector control of IM drives required accurate information of speed or rotor position. This information is provided by Tacho -generator or encoder. However the use of speed sensors has many drawbacks such as, higher cost, lower reliability, increase in weight and size and difficulty to use in harsh environment. These drawbacks of speed sensors can be eliminated by using sensorless speed estimation algorithms for induction motor drive. Various speed sensorless techniques are presented, among which MRAS based speed observers offer simple implementation and require less computational effort compared to other methods. Various MRAS based observers have been introduced based on speed tuning signal such as the rotor flux, back e.m.f and reactive power. Steady state and dynamic response of indirect vector control under various operating condition such as starting, reference speed change, load application and load removal for both with and without speed sensor is simulated and examined in MATLAB environment using Simulink and power system toolboxes. The performance of the different speed estimation techniques on the induction motor drive is compared and also their sensitivity towards stator resistance variations is tested. The analysis has been carried out on the basis of result obtained by numerical simulations. The simulation and evaluation of both control technique are performed using voltage source inverter fed three phase squirrel cage induction motor of 2HP rating.