Indirectly air-cooled machine and directly cooled machine end-region fields, losses and temperatures have been compared in detail. The losses in indirectly cooled machines are very substantially reduced due to a number of factors, the most dominant of which is the smaller air-gap length and lower electric loading. Other typical features, such as lower magnetic loadings and increased stator slot number also contribute to this reduction. For the two machines considered the eddy current losses in the stator core end on load are only 10% of that in the directly cooled machine. Three dimensional temperature calculations indicate that despite a poorer cooling circuit in an air-cooled machine the reduction in end-region losses is so great that only very moderate core end temperature rises occur as a result. The stator conductor bar operates at a considerably higher temperature than the core even in the end-region, and it is this feature which imposes the design limit. End-region losses are small enough to require only minimal additional design features in order to reduce them. The electric loading of air-cooled machines can be substantially increased before end-region losses become a major problem.
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