Given the increasing complexity of rotor designs across applications such as the oil and gas industry, developing mathematical models for these systems is essential. These models support both the identification of operational characteristics during the design phase and real-time monitoring via digital twins. To accurately represent system behavior, models must be created for each component, incorporating complex calculations such as bearing coefficients and gear meshing contacts. Coupling these models enables the analysis of interactions that define the system overall response. A planetary transmission system with two inputs, one from a generator and the other from a hydraulic torque converter powered by the same generator, constitutes a reliable mechanical speed variator for energy applications. This type of system requires a complete mathematical model, as its dynamic response is influenced by multiple interconnected components. Understanding how critical parameters affect its behavior is essential. Therefore, this work investigates the influence of a hydraulic torque converter on the dynamic response of a reduced model of an mechanical speed variator using a time-domain lateral vibration model. The system responses with and without the torque converter are compared to assess its impact.