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What sort of brushless motor to make an extractor fan?

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--- Quote from: e100 on April 24, 2022, 08:52:28 am ---I'm looking to replace a conventional single speed mains powered extractor fan for a bathroom with something that can be speed controlled

--- End quote ---

N.B. Conventional mains fans can be speed controlled.  Add a "run capacitor" in series with them (typically a few uF, plastic film type, physically big). 

My bathroom fan has one permanently wired in to slow it down.  Without it the fan is really noisy and so strong that it opens the bathroom door.

These caps are often used inside the switchboxes of (non-bathroom) ceiling fans to let you choose their speed.  The example I picture above has two caps inside it for just this purpose (Low = 1.5u, Medium = 2.5u, High = no cap)

Obligatory: don't wire this without the necessary safety training/equivalent for mains wiring.  Pain & death, hazardous work.  It sounds like you want something lower-voltage so that you're more confident doing it yourself, so my suggestion here might not be any use.

If you go the route of using a hobby brushless motor:

* Make sure it has ball bearings, not sleeve bearings.  Your bathroom fan will do a lot of hours
* Normally there is epoxy on the motor windings.  Make sure it's also covering where the wires connect to the windings (so they don't corrode)
* Get as low of a 'Kv' as possible.  This is the relationship between voltage and speed for a motor (you want low speed).  Most hobby brushless motors are designed for very fast speeds and small diameter props, your bathroom fan is going to have a very large prop that you want to spin dramatically slower than an RC aeroplane.

* Most ESCs (Electronic Speed Controllers, aka brushless motor drivers) will overheat if they don't have lots of airflow on them.  I'm not sure how to fix this problem safely in a bathroom situation, perhaps there are better quality (but more $$$) ESCs out there that are rated for no airflow under a layer of roof dust?


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