Torque trajectory controller for induction motor

This paper presents a new control scheme of the torque trajectory-tracking controller for induction motors. The proposed approach addresses the problem of torque control of speed sensorless drives at both low and high speed, where rapid speed changes can occur. The scheme allows the precise and quick control of the induction motor flux and torque. A closed-loop rotor flux observer using sliding mode controller and a nonlinear stator frequency dependent gain is presented. The effect of stator resistance estimation quality is discussed. Sliding mode controllers are used as the stator flux controller and in the rotor flux observer. Nonlinear stator frequency dependent gain used in the rotor flux observer enables the operation at low and high speed. Thus at low speed observer turns into the closed-loop observer using stator current, stator voltage and desired rotor flux values as inputs. At high speed the information from desired rotor flux is disconnected and classical voltage model is used. The nonlinear gain is used as the transfer function in order to enable smooth transfer between the schemes. The conventional model of induction motor using rotor and stator flux in stator coordinates is used for the design of the control scheme. Stability and dynamics are described with the use of transfer functions. Also the possibility of a certain rate of decoupling between the stator and rotor fluxes is shown.