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A novel nonlinear affine model for an induction motor with core loss is developed in the well-known (α, β) stationary reference frame, where the core is represented with a resistance in parallel with a magnetization inductance. Then, an optimal rotor flux modulus is calculated such that the power loss due to stator, rotor, and core resistances is minimized, and as a consequence, the motor efficiency is raised; therefore, this flux modulus is forced to be tracked by the induction motor along with a desired rotor velocity by means of a high-order sliding-mode controller, the supertwisting algorithm. Using a novel Lyapunov function, the closed-loop stability of the system is demonstrated. Moreover, a classical sliding-mode observer is designed for the estimation of unmeasurable variables like rotor fluxes and magnetization currents. For the load torque, a Luenberger observer is designed. The performance of the proposed controller is finally studied by simulation and experimental tests. It was observed that the steady-state optimal flux signal corresponds to the load torque profile. This fact suggests that the flux demand is the necessary one to produce the electric torque that can cancel out the load torque.