A model to predict the motor temperature of an electrical submersible pump (ESP), under variable conditions of flow rate and loading, has been developed. This model takes into account the coupled behavior between motor, pump and production system. Thus, given a defined frequency in the variable speed drive, the motor temperature was determined as a result of the equilibrium between the heat generation, calculated from the power that the pump demands from the motor, and the heat extraction resistance, calculated from the production flow rate around the motor. Furthermore, in real field operations, the measurement of the motor temperature is made at the lower end of the stator winding, which is not exactly the maximum temperature point. In order to predict the maximum motor temperature value, the model developed in this work determines the motor temperature distribution. A convective heat transfer study has also been made comparing models based on fully developed temperature profile to models that consider the development of the thermal boundary layer. A case study has been made with several oil viscosities and water cut. The results showed a fact commonly observed in ESP field operations, that is, the motor temperature rises when the motor speed is continuously increased. It was also shown that neglecting the effect of the thermal boundary layer development may result in an overheated motor prediction where actually, the motor maximum temperature is much lower than its upper limit. Thus, it was observed that fully developed temperature profile models suffer from inaccuracy when used in viscous oil applications, because of their great thermal entry length. (C) 2015 Elsevier B.V. All rights reserved.