Basic requirements for low noise cooling fans

Low noise fans for electric motors have been an industrial requirement for many years. The author traces the recent history of what has been designed and tested by looking at the aerodynamic requirements for cooling flow, the limitations placed on design by industrial needs, noise generative processes, some innovative designs and the possible future. The author concentrates on high speed synchronous sealed motors where the cooling must take place first by conduction and then by convection, but the comments are equally applicable to other units and systems. The solution to the basic problem is that of a uni-directional, small diameter fan running at low speed, with aerodynamically designed large chord blades. These give high efficiencies and low noise for example 70 dBA at 1 m on a 250 frame compared to 90 dBA or more from the conventional bi-directional fan. Pressure must be maintained on industry to accept uni-directional units and to make provision for fitting fans appropriate to the shaft rotation in each application.