This paper presents an original use of a lumped thermal network. This model has been developed for a 900 kW hydrid excitation synchronous generator dedicated to a direct drive wind turbine. The analytical model takes into account conduction, convective and mass transfers. The latter is based on a flow distribution inside the generator estimated by numerical approach. An inverse method is applied to the model in order to estimate losses inside the generator using measured temperatures. The generator has been manufactured and tested with sinusoidal power supply. Both direct and inverse use of the model have shown good accuracy with experimental results. The generator is installed on a wind turbine to replace an older machine. The existing inverter has been kept and is connected to the machine. The inverse method applied to on-site thermal steady state temperatures has shown important additional losses due to the inverter.