The simulation of induction motors is based on static and dynamic models that allow the study of their mechanical performances, as well as their power consumption and losses. The purpose of this paper is to present an improved dynamic model for squirrel-cage induction motors (IM) which includes the core losses in addition to the copper and mechanical losses that are already taken into consideration in the commonly used models. The goal of this model is to compute accurate efficiency values in order to build a control system of the IM, aiming to improve the energy efficiency. Therefore, core losses are computed using the Bertotti model, which is a function of the magnetic field and the frequency. These losses are then included in the induction motor dynamic equations through a variable resistor that depends mainly on the magnetic field and frequency. The proposed model is simulated using Matlab/Simulink and the obtained results are experimentally validated. Then, energy efficiency equations are obtained using an improved model, taking into consideration the main losses in the machine. Energy efficiency contours are drawn, in order to track the best efficiency zone according to flux, speed and load torque.