EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: aeberbach on August 21, 2022, 07:13:50 am
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I picked up a 100mm fan unit - I think this kind is intended for ventilating the greenhouse, to grow "herbs" indoors. I'm interested in making a filtration unit for soldering and for 3D printing using a combination of activated charcoal filters and HEPA filters using the Miele vacuum pattern. These things come with a housing that is intended to interface to 100mm tube, and they connect to the housing with a pair of circular clips, so it is easy to 3D print a mating surface for the clip and to loft that into a mount for the filters and intake pipe. Plenty of air movement, filters are easily replaced and affordable. Total cost well under $100.
(https://i.imgur.com/FMwybl9.jpg)
Specification sticker.
(https://i.imgur.com/EuDpuCO.jpg)
The plan.
What I want to know is what kind of electric motor is this and can it be speed controlled? It might be good to reduce the fan speed to the point where it catches all the smoke while being as quiet as possible. In 3D printing the plastics that print at a higher temperature (ASA, ABS etc) are the more worrisome for particulates release and smell, but they also tend to like a hot environment more and an enclosure helps keep them warm while they print and reduces warping. Being able to adjust speed so that solder smoke for example is just sucked in to the enclosure would be good - keep all the air exiting the enclosure going through the filters while not cooling the interior any more than necessary.
I have opened up this thing and it appears to be a brushless motor with three wires connecting to the coils. Three wires are connected from mains to the stator, one through what must be a capacitor. By looking at this can you say what kind of motor this is, whether it can be speed controlled and if so what kind? I'm happy just to buy one but I know some kinds of AC motors can't be controlled.
(https://i.imgur.com/svSjgvp.jpg)
Wiring housing showing mains connection with capacitor(?)
(https://i.imgur.com/cz9rlLf.jpg)
Three wires enter motor housing.
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It is an asynchronos induction motor with a capacitor to make it run on 1 phase. So it it not well suited for speed control. It is possible but would need a relatively expensive variable frequency drive. Even than the frequency / speed range is limited because of the capacitor.
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Maybe it would be better to leave the fan speed as is and reduce the amount of air drawn through the intake by a variable gate selecting between the intake pipe and an alternative. Same amount of air exits the filter, proportion drawn from the flexible tube can be varied.
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For DIY projects where you have to improvise anyway, just use bog standard BLDC fans (computer fans) and any 12V wallwart to power them.
Those AC induction fans are hugely inefficient so mostly produce heat, and when you try to slow them down (not all are designed to be slowed at all), they get even more inefficient, to the point the motor might overheat due to reduced cooling.
Typical BLDC fan can be PWM'd with a simple ground side NMOSFET, and the efficiency stays good while doing that.
If you still want to do that using the AC fan, typical triac based dimmer circuit (light bulb dimmer, basically) can be used at the expense of audible noise. A transformer can be used to lower voltage, this has the advantage of no audible buzz. Note these solutions cannot reduce the mains frequency, so the induction motor will be running at larger slip, further from the optimum efficiency operating point. Some motors are specifically designed to run this way, basically they suck even more over the whole operating area. Better efficiency motors are more picky and can't be run at large slip at all, or they just stall and overheat.
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By the time the intake air has passed through two filters and a length of flexible ducting, I really doubt that you'll need to reduce the fan speed. Those small inline bathroom fans are normally placed as close as possible to the (un-filtered) air intake, they blow a lot better than they suck, static pressure wise. I would actually consider putting the HEPA filter on the fan exhaust in order to get up to usable airflow - that's what vacuum cleaners do.
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The pressure from the fan is rather small, and thus not much difference from blowing to sucking. Even a good vaccum cleaner is more like 25% vaccum at most, the small fan here is way less pressure and thus hardly any difference.
Higher grade shop vaccums with a proper Hepa filter rating usually have the filter before the turbine: this way any leakage in the filter housing will cause air to come in and not dust to be pushed out. In addition this keeps the turbine clean of dust. In the EU this is the required configuration for wood dust (and likely some other dust) in a professional environment. A common problem with broken filter / bag is to have dust build up in the turbine, than falling off at one side and causing imbalance in the turbine to cause vibrations and possibly damage the bearings.
Ideally one would have some filtering on both side of the fan to keep the noise down and insects and similar out in the time the system is not used.
A Hepa / charcoal filter may need quite some pressure differential. The static pressure of this fan of 150 Pa = 1.5 mbar looks rather low to me. A normal vaccum cleaner gets more like 100-250 mbar. A pressure loss at the Hepa filter may well be in the 50 mbar range for a reasonable flow rate.
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Many of those little induction motors work fine on a cheap triac based speed control, they're used in ceiling fans and older forced air furnace blowers. The ones you can't easily control are those that have a centrifugal switch and start winding.
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The problem with this fan is not getting lower power, but more like getting more pressure, which would need more speed.
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To get more pressure centrifugal blowers are a good option, I have no data on hand but they can typically make a lot more pressure than an axial fan all else being equal. Indeed vacuum cleaners use a form of centrifugal blower.
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Larger filters can be used to ease the pressure requirement, if you have space, that is.
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That's a very good idea. I think I can stack two HEPA filters (95x195) in a 3D printed housing. I guess they are like resistors in how they will work in parallel.
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Many of those little induction motors work fine on a cheap triac based speed control, they're used in ceiling fans and older forced air furnace blowers. The ones you can't easily control are those that have a centrifugal switch and start winding.
I have a triac speed controller for each of my two ceiling-attic ventilators. They work surprisingly well.
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Static pressure 150 Pa which is 15mm (0.6") H2O, will be OK if you have a HEPA filter with plenty of surface area.
I've used 120mm fans with the same sort of pressure, and a 30x30x4 vornado (https://www.amazon.ca/Vornado-AQS-Replacement-HEPA-Filter/dp/B000063D2S) filter. Sometimes they are on sale for <$20. Third party ones are really cheap (for vornado/levoit/xiaomi) but unsure how trustworthy they are.
Miele filter might be a bit more restrictive though, just due to the small size and thickness.
You could want to look around for room fan or furnace filter based stuff if any are on sale. But its harder to design around a big square filter.
I like the design, will be a good system in the end, 31dB is very quiet.