The popular totally enclosed fan cooled motors less than 100kW in capacity rating have a structure where a radial fan is mounted as an external fan and air flow blew by the fan cools the outer surfaces of bearing brackets, and a stator frame. Increase in the amount of wind generated by the fan is essential to better cooling performance of this type motors. This paper proposes a new design index to increase the cooling wind without growth of fan-related noise on the basis of theoretical prediction and experimental verification. The resultant optimum external fan provides appreciable increase in the wind amount while suppressing its total pressure. An experiment on a four-pole motor running at 60Hz has demonstrated about 20% increase in the wind amount, also about 20% decrease in fan-driving power, and about 2dB decrease in fan-related noise.
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