Measurement of stator heat transfer in air-cooled axial flux permanent magnet machines

Relatively little fundamental research has been undertaken on heat transfer in disc type electrical machines, a common example being the axial flux permanent magnet (AFPM) machine. Power density is usually thermally limited and therefore understanding of stator convective heat transfer is crucial in the design of AFPM machines and electrical machines in general. This paper presents direct measurements of radially resolved stator convective heat transfer in a rotor-stator system, using a thin-film electrical heater array to measure heat transfer. The heater array combines surface temperature sensing with heat flux measurement. It is found for the three gap sizes considered here that average Nusselt number (non-dimensional convective heat transfer coefficient) increases from Nu ¿ 100 at low disc speeds/rotational Reynolds numbers (Re_¿min = 3.7e4) up to Nu ¿ 300-400 at higher speeds (Re_¿max = 5.6e5).