# What properties vary with the number of blades on a BLDC fan?

In general, what properties vary with the number of blades on a axial BLDC fan when physical and electrical dimensions are the same (e.g.: rated at approximately 12V and 0.15A and 60mm Ø)?

So far I've seen fans with 7, 9 and 13 blades and unfortunately not all fans come with a datasheet.

Things that I can think of myself include, but don't know how they would be influenced by blade count:

• Fan speed (rpm)
• Volume (${\text{m}^\text{3}}\cdot{\text{min}}^\text{-1}$)
• Application (mounted directly on a heatsink or mounted in equipment housing)
• Artistic mood of the designer

• I know I should only use A-brand parts that come with a datasheet, but I don't always have a choice. Jan 27 '13 at 9:48
• I've been told that the number of pulses produced by the tachometer output may vary with the number of blades, but I have no hard source for that information. Jan 27 '13 at 11:33
• BLDC = Brushless DC (I didn't know this abbreviation) Jan 27 '13 at 21:53

I am not sure whether this applies equally to fans, but for wind turbines (the things that doe the opposite, turn moving air into a rotation) a higher number of blades causes the optimum working point to be at a lower RPM. Hence the USA-type (water pumping?) windmills with lots of blades are slow-runners, contrast them to modern electricity-generating windturbines that have 2 or 3 blades.

Applying this to fans means that a higher number of blades means that it can rotate more slowly.

More blades allow a higher delta-P (pressure) to be developed across the fan. Thus if your application created higher than average backpressure, a fan with more blades might be better. There are design tradeoffs in everything, and I'd suspect that flow is compromised in order to achieve higher pressure. Just anecdotally, fans with more blades tend to be quieter, but this is highly subjective and really depends more on the shape of the blades and RPM than just on the blade cardinality.

Compare the flow curve of a fan with many blades to a similar-sized, similar RPM fan with fewer blades.

• You will want to look at the data presented in my answer. Notice that for the same size family of 60 mm fans that the units with lesser number of blades produce higher maximum pressure than ones with more blades. After looking at that you may agree that the data shown would indicate that you may want to edit your answer. Maybe show some data that bears out your conclusion. Jan 27 '13 at 17:17

Fan designs can come in a huge number of variations. Various manufactures will offer a large range of sizes of fans with varying numbers of blades and blade designs. If you get into the mode of system design where you need to select a fan to meet a particular cooling requirement then you really need to look at some manufacturer catalogs. It is rather surprising at the total number of models of fans made. I did a quick survey of just one product family of fans from manufacturer in a specific size and found a large range available for selection.

The parameters you select on do not necessarily focus in number of fan blades. Instead you would be focusing on size, voltage, rated current, speed, max air flow, max air pressure and noise level.

An example of the fans from Delta in their AFB series in the 60mm x 60mm size yield tremendous variety of selection.

The above set of fan models can be seen in the Delta Fan Catalogue at these links:

Just this single AFB series of fans from delta come in all these size selections:

And to put even more emphasis on the total number Delta is offering all these series of DC powered fans and blowers:

Note that I have no affiliation with Delta other than I've used their fans in some product designs.