Cascade sliding mode control of a field oriented induction motors with varying parameters

An adaptive nonlinear sliding mode controller combined with a field orientation scheme has been developed for the control of induction motor to achieve rotor angular speed and rotor flux amplitude tracking objectives. The inputs to the controller are the reference speed, the reference flux, the measured stator currents, the measured rotor speed, the estimated rotor flux, and estimates of the varying non measurable parameters. The unknown load torque and the rotor resistance which may vary during operation. The controller outputs are the reference stator voltages. An accurate knowledge of the rotor flux, the load torque and the rotor resistance is the key factor in obtaining a high-performance and high-efficiency induction-motor drive. The rotor flux is estimated using the induction-motor rotor-circuit model. Although the estimated rotor flux is insensitive to the stator-resistance variation, it does depend on the rotor resistance. A stable model reference adaptive system (MRAS) rotor-resistance estimator insensitive to stator-resistance variation as well as a load torque estimator have been designed. The use of cascade sliding mode control structure and continuous adaptive update of the machine parameters ensures accurate flux estimation and high-performance operation. Simulation results are presented to verify the stability of the induction-motor drive in various operating modes.