Direct drive generators have become advantageous for wind turbines, especially offshore wind, due to their increased reliability and efficiency. Any type of generator must be adequately cooled in order to meet operating conditions preventing the degradation of copper winding insulation and thermal deflections. The amount of cooling is related to the current density of the coils at maximum loading conditions. This condition is set by the cooling technique as a more effective cooling technique allows a greater stator current density while achieving the desired temperature. In this work, four different cooling techniques for a 10 MW direct drive generator were assessed for maximum current density allowable within a class F insulation rating. These techniques included natural convection, forced convection, direct liquid cooling, and two-phase cooling. Machine parameters were compared between cooling techniques and a large drop in generator size and mass found as the heat transfer coefficient increased. For the same power rating and efficiency, the volume of the two-phase cooled stator was reduced by 40% compared to forced air convection. Discussion on the practical employment of each cooling technique was explored and simulations of temperatures within the stator given for each cooling technique. The current density and cooling technique influence on generator efficiency is discussed as well as implications for high power density offshore wind turbine generators.

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