Speed sensorless vector control of induction motors with parameter estimation

The induction motor has a very nonlinear multivariable structure which makes it very difficult to control. By placing a vector controller between the induction motor and the main controller, the motor can be forced to yield a fast torque response similar to that of a separately excited DC motor. However, a prerequisite for the correct operation of the vector controller is an accurate knowledge of the rotor velocity and rotor time constant. Evaluation of the rotor velocity requires extra speed transducers and the value of the rotor time constant is directly affected by changes in the resistance of the rotor bars due to changes in temperature. This paper addresses these problems by introducing a novel, cheap and practical method for continuous on-line evaluation of these variables. The presence of a tachogenerator/optical shaft encoder in vector control of induction motors or closed-loop speed control is very undesirable. A novel method of speed estimation is described which eliminates the need for a tachogenerator. The proposed method uses an approximation to eliminate the derivative terms in the machine model equations in the stationary reference frame. Thus a single equation for the rotor velocity can be derived.