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At steady-state operation, power losses cause a heating of rotating electrical machines. In air-cooled machines, these losses are evacuated by a forced cooling airflow through the active parts. When designing and optimizing such a machine, the design engineer must be able to get a full picture of the power losses, the cooling airflow, and the temperatures inside the active parts (e.g., core laminations, windings) and the periphery (e.g., winding overhangs). The aim of the designer is to fulfill the customer's requirements regarding the guaranteed temperatures. This paper presents a computation method, where the power loss, airflow, and temperature calculations for the world's largest air-cooled hydrogenerators are coupled in an iterative process. The new contribution of this paper is a calculation software developed by the authors. It includes a state-of-the-art loss computation, an automated airflow network, and a set of linked thermal networks. These computations result in a complete overview of the temperature gradients and allow fine tuning of the cooling airflow and, consequently, optimization of ventilation losses.