What sort of brushless motor to make an extractor fan? - Page 1
Forum section: electronics / general

author: e100
author_rank: Frequent Contributor
timestamp: April 24, 2022, 08:52:28 am
content: I'm looking to replace a conventional single speed mains powered extractor fan for a bathroom with something that can be speed controlled via a low voltage ESC and micro. The operating voltage has to be under 48v due to the onerous wiring regulations in my country. Looking at the specs of new fans it looks like I'll need approximately 10-20 watts of power.Would looking at the motors used in quadcopters and RC planes be a good place to start?There is a bewildering array of options and I have no idea how to even begin to figure out what sort of motor would be a suitable drop in replacement. I'm guessing that I would need to use a sensored model to avoid cogging, and therefore noise, at low rpms.

author: David Hess
author_rank: Super Contributor
timestamp: April 24, 2022, 06:17:01 pm
content: Given your requirements I would consider using one or more standard 12 volt 4-wire PWM 120mm computer fans.

author: Benta
author_rank: Super Contributor
timestamp: April 24, 2022, 06:56:19 pm
content: I'm not sure how long a computer fan would withstand the moist environment of a bathroom.

author: wraper
author_rank: Unknown
timestamp: April 24, 2022, 07:04:29 pm
content: Computer case fans usually are not optimized for high static pressure which is needed to push the air thru a pipe. So unless there is no pipe or a very short pipe, it probably won't work well. CPU fans which are more likely to be optimized for higher static pressure may work better.

author: Benta
author_rank: Super Contributor
timestamp: April 24, 2022, 07:15:33 pm
content: That apart, sensorless BLDC drive is a good candidate for a fan application, as it starts under no-load conditions.However, a minimum RPM is necessary for the back EMF for the controller.RC boat motor and controller is a possibility, but they probably run too fast.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 24, 2022, 11:50:37 pm
content: Quote from: Benta on April 24, 2022, 06:56:19 pmThere are moisture resistant versions for outdoor equipment.

author: Benta
author_rank: Super Contributor
timestamp: April 25, 2022, 04:19:57 pm
content: Quote from: NiHaoMike on April 24, 2022, 11:50:37 pmOutdoor computers?

author: thm_w
author_rank: Super Contributor
timestamp: April 25, 2022, 09:05:34 pm
content: What is the intent of the speed control? Is this something that can already be commercially done with a timer, or humidity sensor?As wraper said, computer case fan is not suited to the task.One option is 12/24V marine vent fans:https://www.mrboats.com.au/4-hose-boat-bilge-air-in-line-blower-uv-stabilised(can be loud)I'm sure you can find RV type 12/24V vent fans as well.Another might be replacing the existing fan in the housing, if you can find one that matches:https://www.alibaba.com/product-detail/24v-Exhaust-Fan-Industrial-Ventilation-Backward_1600221394017.html

author: Simon
author_rank: Unknown
timestamp: May 13, 2022, 06:06:21 pm
content: There is an abundance of decent pressure fans 120mm frame and speed controllable. I've used an ebmpapst one that pulls 120W and does 11'000 rpm for a very good suck.

author: james_s
author_rank: Super Contributor
timestamp: May 13, 2022, 06:25:08 pm
content: Quote from: Benta on April 25, 2022, 04:19:57 pmSure, things like traffic light controllers and stuff like that, typically mounted in a cabinet outdoors that is not climate controlled. There is lots of equipment using that style of fans besides desktop PCs.

author: Simon
author_rank: Unknown
timestamp: May 13, 2022, 06:40:22 pm
content: You can get IP65 or more fans, used them no end, the 120W ones were not that high but their 65W versions were, just ordered one at work, 92mm fan that takes 40W and really does blow. The market is full of rugged small fans. What do you think cools all those servers, 12V, 24V, 48V take your pick.

author: thm_w
author_rank: Super Contributor
timestamp: May 13, 2022, 09:27:36 pm
content: I would not want to use a 11krpm Delta screamer anywhere near where I live or work.It looks like there are some IP68 120mm fans on Digikey for not too unreasonable prices, as Simon indicated:https://www.digikey.ca/en/products/detail/mechatronics-fan-group/MS1238E12B-FHR-2EM/99629171.2" H2O static pressure, 270 CFM.I got this 140mm one to try for a HEPA filter (no water involved), 2" H2O, 330 CFM:https://www.aliexpress.com/item/1005003213526421.html

author: james_s
author_rank: Super Contributor
timestamp: May 13, 2022, 10:51:05 pm
content: Quote from: thm_w on May 13, 2022, 09:27:36 pm"violent large air volume cooling fan"I don't think I have ever seen a cooling fan described as "violent".

author: Simon
author_rank: Unknown
timestamp: May 14, 2022, 06:48:36 am
content: I'm not really sure why the IP rating. I doubt the existing fan is IP68 rated. Plenty of air conditioning fans are not that highly IP rated. The air around us is always full of moisture. Just turned my dehumidifier on and it wanted to run even set at 90% but it's not like the room windows were all steamed up.

author: thm_w
author_rank: Super Contributor
timestamp: May 16, 2022, 10:27:44 pm
content: Quote from: james_s on May 13, 2022, 10:51:05 pmOnce you have these type of fans bare in front of you for testing, its not a bad descriptor. They can easily chop your fingertips off.I like how they word the listings too, you can tell they don't want to deal with naive buyers:"This is a super high speed fan with a monster performance. Because the speed is too fast, there will be some noise, please choose carefully!"Hooked up to a low cost USA HEPA filter and still sucks like a hoover through some 4" duct, total overkill. But thats where PWM input will come in handy.

author: Whales
author_rank: Super Contributor
timestamp: May 16, 2022, 11:10:48 pm
content: Quote from: e100 on April 24, 2022, 08:52:28 amN.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.

author: Whales
author_rank: Super Contributor
timestamp: May 16, 2022, 11:22:00 pm
content: 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 hoursNormally 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?