one that fits on shroud over the motor
Visually like this:
http://www.wistro.com/index.php?co=enHere is some commercial unit data:
www.kmmp-usa.com/products/FBI/Massblaetter/kMMp_0M_technische_Daten_2014.pdfWhat is OD of your motor? Something like 63/71 motor size?
PDF, 2/3 down "Forced ventilation unit" 20W seems to get you up to 71 frme size
http://www.rotor.co.uk/forcevent.htmlWhat sort of air duct fans there are in britain for buildings, like toilet extractor fans, or inline air duct fans? Here stadard cheap bathroom fans are not that expensive.
Found this sort of:
http://www.screwfix.eu/manrose-xf100s-20w-axial-bathroom-fan.htmlThere are few things to consider:
* flow must be good enough. Compuer fans are generally too small. Air is not that great conductor, therefore you need a good flow of it. Bit like a blow on angle grinder. Few wats will not do, 20W a starting point, idustrial ones seems to be 20-40w smallest sizes.
* Check that you will be not fighting agains internal fan on greater rpm.
* Air duct reducer might provide smooth transition and correct size for the shroud.
I don't know how this motor looks, i'm assuming it has internal fan and cooling air entry is at brush end, exhaust at power take out end.
Pekka