Mastering the thermal behavior of an electric machine is a primordial step in its pre-sizing. Indeed, the optimization of the geometry of this one requires limiting the temperature in the active parts of the machine. This is why this article provides thermal modeling of the active part of the permanent magnet synchronous machine (PMSM). This machine is used for rim driving the turbine of an under-development pump. The machine is supposed to be cooled by water that flows through the submerged airgap, while natural air circulates on the external side. The geometry optimization algorithm considered the temperature in slots (which are the hottest area) as a constraint. A slot temperature of less than 155 °C was chosen, and the resulting geometry from this study perfectly fulfilled the specifications. The transient thermal behavior was studied and simulated for an optimized machine to highlight the sensitivity of the temperature of active components to different heat sources within the motor. It is shown that a.