I need to quiet "The Beast" AC fan control circuit options? - Page 1
Forum section: electronics / projects

author: almstsobur
author_rank: Contributor
timestamp: June 16, 2016, 10:33:13 am
content: "The Beast" is 1979 EUL-600 Fujitsu-Denso programable load rated for up to 120 amps and weighs in at about 60Lbs. It has 2 large AC 120v fans that draw air across the 6 full length heatsinks. My problem is the noise of those fans, they are full speed full time. The dogs start howling, neighbors come knocking and the woman demotes me to the sofa when I need to use "The Beast" . 98% of the time this thing is overkill for my needs, it's rare I need to test more than about a 6amp load. So I would love to be able to regulate the fan speeds to something reasonable under my typically use scenario. Ideally I would like to screw a thermal probe into one of the heatsinks and have the fan speeds fluctuate by temperature. I've scoured the web and literally found hundreds of write-ups, designs and circuits for DC fans, but these are 120v AC fans and I am not having any luck finding anything that will work for this scenario. Any input or a point in the right direction is appreciated.Thank You,-RP

author: Messtechniker
author_rank: Frequent Contributor
timestamp: June 16, 2016, 11:30:43 am
content: How about first replacing the fans with two 12 V DC types of the same size (and air flow rating).And then get an off-the-shelf fan controller board. Should not be too expensive.Yours - Messtechniker

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 16, 2016, 12:10:29 pm
content: Get good quality PC fans and a small 12V supply, Your would not be able to even hear the fans from over 1 meter.I would recommend "Arctic" fans, they are cheap and very quiet.

author: David Hess
author_rank: Super Contributor
timestamp: June 16, 2016, 02:43:17 pm
content: Speed control of AC fans can be done by switching in different values of capacitance in series; film capacitors up to about 10 microfarads would be typical for a standard size tubeaxial fan. We used to do this where commutation noise from a DC brushless fan or electronic speed control was unacceptable. A variac can also be used with an AC fan but that is probably not what you want.Maybe make a thermal circuit which switches a relay to select whether the capacitor is in series with the fan or not. It would only be two speeds but still a major improvement if the load is idle or at low power most of the time.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 16, 2016, 03:00:34 pm
content: Or since there are two fans, use a relay to select between series/parallel.

author: JWoodrell
author_rank: Contributor
timestamp: June 16, 2016, 03:36:59 pm
content: here is a simple 3 component isolated ac switching circuit. drive it with pwm and run the fans as fast or slow as you want.the VOM1271 MOSFET Driver is super easy to solder and provides isolation between mains and your microcontroller of choice, or even a 555 circuit.pick the mosfets to have low gate capacitance, and a voltage rating to support mains.

author: almstsobur
author_rank: Contributor
timestamp: June 16, 2016, 04:43:54 pm
content: JWoodrell, thank You. That certainly looks like it is worth a try.I was considering the DC fan option, computer case style, ect. But you need to see the quality of these fans, it seemed like such a downgrade. They are pure aluminum, with aluminum housing and appear to be original. 37 years later they still run like a beast with no abnormal noise, vibration or other issues. You just don't see this kind of quality in a plastic DC fan. Also, you can see from the attached picture, they seem to be somewhat structural in nature. The fans are Prince Fan made by Tobishi.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 16, 2016, 05:01:46 pm
content: For a quick-n-dirty hack requiring a minimum of effort you can use a fan speed control "dimmer" (intended for ceiling fans), usually located in the same area as incandescent lamp dimmers at home improvement stores. For example:http://www.homedepot.com/p/Cooper-Wiring-Devices-5-Amp-Single-Pole-Variable-Fan-Speed-Control-White-TFS5-W-K/205618204Don't use a regular incandescent dimmer because the significant inductance of a motor acts to prevent the triac inside the dimmer from ever turning off.

author: almstsobur
author_rank: Contributor
timestamp: June 16, 2016, 05:32:04 pm
content: I ordered one of these off Ebay a couple of days ago. For $9 i figured I couldn't go too wronghttp://www.ebay.com/itm/141409048579It seems more or less like the dimmer principal, it hasn't arrived yet to test it. I know it does not act on a thermal response; which would be my ideal scenario if possible. But it might might keep me from sleeping on the sofa until I hopefully figure out a possible thermal controlled solution, maybe I will need to involve a Audrino, or something similar to do the thermal control aspect. I am completely new to this type of thing, my background is in repair as a hobby, not design.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: June 16, 2016, 06:09:04 pm
content: that thing looks like you can put some decent fans in it, not some cheap bullshit off ebay. Have a look at dB ratings.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 16, 2016, 06:14:04 pm
content: Most likely a phase angle controller would make it noisier.

author: Delta
author_rank: Super Contributor
timestamp: June 16, 2016, 06:16:15 pm
content: If they are "proper AC" motors (ie induction or synchronous motors) then you can't do any speed control without a Variable Frequency Drive...

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 16, 2016, 06:24:32 pm
content: Quote from: almstsobur on June 16, 2016, 05:32:04 pmThe main difference between one of these fan controls and a regular light dimmer is the addition of an RC snubber across the semiconductor switch (triac or anti-parallel SCRs) that temporarily diverts the lagging inductive current so said switch can turn off.The ebay item you linked to is for 220V, and so the triggering voltage for the SCRs may be too high to give a useful adjustment range on 120V.Quote from: Delta on June 16, 2016, 06:16:15 pmSure you can, under-volting an AC motor without changing the frequency essentially causes the flux to decrease and if that is insufficient flux to supply the torque demanded by the load then speed will decrease (ie - slip will increase). It's not terribly efficient or elegant, but it does work over a modest speed range with loads that have square or cubic law torque curves (ie - not constant torque or traction applications).

author: almstsobur
author_rank: Contributor
timestamp: June 16, 2016, 06:30:02 pm
content: Quote from: MagicSmoker on June 16, 2016, 06:24:32 pmThe Ebay controller says 220v in the title, but in the description it says it supports 120V?? roll of the dice I guess, i't only $9. So are you saying a light dimmer, or the Ebay gizmo is the better solution?(assuming it works at 120v)Also, I happen to work at a place that has/uses a lot of VFD's. I might be able to repair or find a working one that's slated for scrap. But I know nothing about VFD's I think all of ours would be rather large, they control 480V motors, etc.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 16, 2016, 06:50:30 pm
content: I wouldn't use a standard phase-angle triac dimmer. When "dimmed", the overtones of the 50 or 60 Hz buzz are very audible in fan motors because of the sharp edge every mains period. The exhaust fan at my home has such a device and IMO it's less distracting at 100% than when slowed down.A simple multitapped transformer (or autotransformer) is a rather good option if you have one.I'd still consider replacing the fans because modern BLDC fans can be substantially quieter, even the fan blade design has improved in the recent decades.

author: almstsobur
author_rank: Contributor
timestamp: June 16, 2016, 07:19:06 pm
content: I have a Variac that I use for testing. Could it be as simple as ....Use the Variac directly on the fans, get 2 or 3 voltage points, say Low, Medium, High that are acceptable in sound and CFM, then purchase a multitap transformer that is roughly in those 3 selected voltages ranges and wire up a low, Med, High switch for the different taps? That's easy enough if that will work correctly. This thing is mounted in the bottom of my rack and it works flawlessly, it's not going anywhere, so I don't mind hardwiring a permanent solution. I can mount the 3 switches on the side of the rack in a neat little project box beside "The Beast"

author: Gyro
author_rank: Super Contributor
timestamp: June 16, 2016, 07:29:14 pm
content: I saw an project / application somewhere (might have been an EDN circuit idea) that achieved speed control by dropping one complete mains cycle in every so many (I think something like 1 in 3-5) to achieve speed control.It was intended for desk / bedroom fans but might possibly be applicable to cabinet fans.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 16, 2016, 07:34:43 pm
content: What about some kind of triac or SSR phase control solution?

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 16, 2016, 08:27:52 pm
content: Quote from: almstsobur on June 16, 2016, 06:30:02 pmI can't comment on the ebay gizmo as I have no experience with it, and I specifically said to NOT use a light dimmer, pointing out that a fan "dimmer" has a few extra components in it to help deal with inductive (motor) loads.Quote from: Siwastaja on June 16, 2016, 06:50:30 pmIt depends on the fan and how it is mounted, but, yes, phase angle control does tend to create some harsh sounds from most motors. I suggested this mainly because it is cheap and easy to try. Your suggestion to use an variac to find a voltage that gives a good compromise of airflow vs. noise will likely result in a superior solution, but is likely to be more difficult to implement (though, see below).Quote from: almstsobur on June 16, 2016, 07:19:06 pmYes, it is indeed as simple as that, but the chance of you finding a multitap transformer that fits the bill are somewhere between slim and none.However, if you find that you only need to lower the line voltage somewhere between 6 and 36V you can wire up a standard voltage transformer to buck the line voltage down by the amount of its secondary (e.g. - a 24V transformer would subtract 24V from ~120V, giving you 96V). You do this by winding the secondary in opposition to the primary. Scroll to the bottom of this page for a good explanation:http://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/winding-configurations/

author: almstsobur
author_rank: Contributor
timestamp: June 16, 2016, 09:28:59 pm
content: Sounds good! Thank you everyone for the input. I think I will start with the Variac and see where I need to be voltage wise, figure out if a transformer is feasible. The load is weak, the fans are only 15 watts a piece. That may open up some more options for getting the correct one. That's where I'll start anyway.

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2016, 01:14:50 am
content: Quote from: Delta on June 16, 2016, 06:16:15 pmThey are not; they use shaded pole AC motors and changing the input voltage without changing the frequency is completely acceptable to control their speed.https://en.wikipedia.org/wiki/Shaded-pole_motorThey may have multiple taps near one electrical end of the winding, which provides variable speed and power via selection of one tap at a time, as in ceiling fans. Moreover, they are compatible with TRIAC-based variable-speed controls, which often are used with fans.We did not use a TRIAC based speed control because of the increased electrical noise. The fans were part of an environmental chamber adjacent to a pair of 8.5 digit voltmeters.

author: almstsobur
author_rank: Contributor
timestamp: June 17, 2016, 03:04:43 am
content: Tested the fans out on the Variac alone after I got home from work. Seemed perfect, no odd noises, nothing unusual. I had full stop to max control with about 65v being a perfect "low" setting. I wonder if this means that the ebay gizmo might work out just fine, or if I should be hunting for a 60, 90, 120 transformer.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 17, 2016, 03:23:23 am
content: A relay to select between series/parallel should work quite nicely, then.

author: Whales
author_rank: Super Contributor
timestamp: June 17, 2016, 05:16:26 am
content: Quote from: NiHaoMike on June 17, 2016, 03:23:23 amHow would the load created by the fans react to being in series? It's definitely worth trying, but make sure you check how they behave if you gently apply friction to one of the fans.I know that for brushed motors you generally can't put them in series (unless mechanically linked). If you physically stall one then it stays stalled after you let go, with most of the voltage drop appearing across the other instead. Atleast IME with small ones.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 17, 2016, 07:01:26 am
content: AC induction motor variant typically used in most fans have slip characteristics without the horrible kind of torque breakdown point like "proper" AC induction motors.These always run at quite a lot of slip and poor efficiency, but are designed to do so. Hence, dropping the voltage works just fine; the efficiency will get even worse, but the motor efficiency is not usually important in small fans, because the delivered power (power to move the air) drops exponentially, too, when slowed down.Any method of dropping the voltage is equally good in principle, as long as you are aware of the audible noise caused by triac phase control. Skipping full mains periods would work quite well most likely.But if you mostly need it for low-dissipation tasks, and full power every now and then, do you really need a multi-tapped transformer? How about a simple 1:2 transformer that can be bypassed? That would give you two fan settings - quiet and loud - and it's easy to source.

author: Berni
author_rank: Super Contributor
timestamp: June 17, 2016, 12:28:13 pm
content: I had a similar problem with a giant rack mount PSU. I simply put resistors in series with the fan to slow it down. Not the proper way to do it but it works pretty well.https://www.eevblog.com/forum/testgear/agilent-6032a-(1000w-60v-50a-psu)-insides/msg941478/#msg94147

author: almstsobur
author_rank: Contributor
timestamp: June 17, 2016, 06:38:18 pm
content: Quote from: Siwastaja on June 17, 2016, 07:01:26 amI think that might be the right track, this started out seeming rather complicated, I wanted automatic thermostatic control of the fans, then an adjustable "trim", then 3 speeds, now it really does seem that KISS applies here. 2 speeds should suffice. Nothing I currently have makes this unit even blow warm-ish air. I mean, 120Amp, I would need to pull a car in the spare bedroom and load test an alternator or something. A normal, low setting with optional high (when the woman makes me mad) should do just fine.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 17, 2016, 08:03:36 pm
content: A resistor works... to get to about 65V across the fan the theoretical value of series resistance needed is 820 ohms (10W), though note that the effective impedance of the fan may change at lower current.Or you can implement the suggestion by David earlier in the thread:Quote from: David Hess on June 16, 2016, 02:43:17 pmThough in this case you'll want to use a 3.3uF non-polarized film capacitor to start.

author: Seekonk
author_rank: Super Contributor
timestamp: June 17, 2016, 08:39:25 pm
content: Funny, I use the same capacitor method to boost voltage to fans. Have used 4uf in series with 240V fans to boost 120V to 185V. As you get near LC resonance the voltage will climb. I've turned some transformers to smoke when I've guessed wrong with capacitive value. Some caution is suggested if you use this method.

author: almstsobur
author_rank: Contributor
timestamp: June 17, 2016, 09:04:30 pm
content: Couple more questions, sorry. I started taking this thing apart and 2 things caught my eye. First the fans say "Impedance Protected" I googled that to get the concept, but it wasn't clear to me if that will effect what I am trying to do? The next thing, is the noise filter, the fans are wired through that, leave it, or remove the fans from that part of the circuit? Also, took a crude dB reading from about a meter away for a before and after compare, 83 dB and it sounded more quiet than normal, I think because the case is off.

author: almstsobur
author_rank: Contributor
timestamp: June 17, 2016, 09:05:53 pm
content: Sorry missed a pic

author: Berni
author_rank: Super Contributor
timestamp: June 17, 2016, 09:44:57 pm
content: Impedance protected means that they don't catch fire if something gets caught in it and stops it from spinning.Usually asynchronous motors are designed to have as little internal leakage inductance as possible to get as much field as possible trough the rotor to get as much power as possible. But due to this they pull a very large current from the mains if the rotor is not spinning resulting in the motor overheating. If there is a lot of leakage inductance in the stator this current is limited to a safe value. This is why its safe to reduce the speed of these fans by adding resistors to reduce the operating voltage, normal async motors don't like this.If you want to keep it simple i would just add a resistor in series with the fans and also add one of those bimetal temperature switches across it. That way when its cold the fan runs with a resistor in series to slow it down (Try something from 100 Ohm to 1K), but when the bimetal switch gets hot it will short out the resistor and give the fans full wack mains voltage.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 17, 2016, 09:46:23 pm
content: Quote from: Seekonk on June 17, 2016, 08:39:25 pmYeah, I thought about the possibility of series resonance, but didn't think it likely because the required inductance would be so large (~2H). That said, probably cheaper and simpler to just go with a wirewound resistor.

author: Delta
author_rank: Super Contributor
timestamp: June 17, 2016, 11:08:43 pm
content: Put an incandescent lamp in series with the fan, with a switch to bypass it.K.I.S.S.

author: almstsobur
author_rank: Contributor
timestamp: June 17, 2016, 11:20:19 pm
content: Well, I just tried the resistor theory, it slows the fans, something in the +/- 200 ohm range seemed about right. But it smoked 2, 1 watt 470ohm parallel in a matter of seconds. Hunting through junk gear for something 5w+

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 18, 2016, 02:11:49 am
content: How well does it work to put the fans in series?

author: almstsobur
author_rank: Contributor
timestamp: June 18, 2016, 02:19:35 am
content: Quote from: NiHaoMike on June 18, 2016, 02:11:49 amIt works well, they are just a little slower than needed I.E. silent, but still pushing decent air, especially at the loads I'm testing. That's the method I'm currently shooting for now. I smoked a 5w resistors as well, so I would need a 15+ on a heatsink or something it looks like. I just don't have anything like that around, and it seems like a waste of power to just turn that into heat. Anyway, the series fan wiring is done and tested, I'm just trying to work out how I can use probably a switch and power relay to put it in parallel for high speed. I need to break 2 connections and make 3 connections, trickier than you would think....

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 18, 2016, 03:11:19 am
content: You can do it with a single DPDT relay. It's like how the voltage selection switches work with dual primary transformers.https://www.eevblog.com/forum/beginners/dual-primary-transformers-use-switch-to-choose-between-110v-input-and-230v/msg495617/#msg495617

author: almstsobur
author_rank: Contributor
timestamp: June 18, 2016, 04:14:30 am
content: Ah ha! Very nice, thank you sir. I'm going to get crakin' on this right now. I've got a slew of dpdt's on hand.

author: David Hess
author_rank: Super Contributor
timestamp: June 18, 2016, 06:03:24 pm
content: Quote from: Whales on June 17, 2016, 05:16:26 amIt works fine with shaded pole motors but at half the voltage, they run awfully slow.Quote from: MagicSmoker on June 17, 2016, 08:03:36 pmA resistor works fine but wastes energy producing more heat to dissipate so I have always used a high voltage film capacitor instead; at the values needed, they are not expensive.

author: SeanB
author_rank: Super Contributor
timestamp: June 18, 2016, 06:35:39 pm
content: running bthem in series will work fine, as they are 100VAC fans in any case 120VAC is a little high voltage to apply to them, and the lower voltage will improve noise, lower the winding temperature and reduce bearing wear, which in these fans is the thing that kills them.

author: almstsobur
author_rank: Contributor
timestamp: June 18, 2016, 07:26:28 pm
content: Quote from: SeanB on June 18, 2016, 06:35:39 pmYeah, this thing has a bit of a unique history. I was trying to do some research on it before I bought it, but about the only information I could find on this unit was university research papers from the late 70's and 80's where it was used a lot for switch-mode power supply design work. It was a VERY expensive piece of equipment in 1979. No matter how hard I tried I could not even find a user manual, much less a service manual or schematic. Fujitsu-Denso was broken off from Fujitsu many years ago and changed hands as a "mini" Fujitsu subsidiary a few times, Fujitsu Ten, Fujitsu Access and now it seems the old documentation went with the Computer Services division of Fujitsu. I was able to make email contact with a person in that division in Japan with some help from Google Translate and I asked for any documentation they may have. He said it was never intended for mass export and it never even had an English language user manual. He was able to scan me a .PDF of the original Japanese user manual though. I took that .PDF and OCR'ed it, then put it in Google Translate and got myself a pretty readable user manual in English. I "guess" Japan users 100V Mains AC, or at least did? because the primary spec is all 100V AC for this unit, but the manual clearly says everything is 100V-120V 50/60Hz. This info made me confident pulling the trigger and buying it. How this unit got to the US, I may never know, but the serial number is 0002. Seems like maybe the second unit ever made, I bet it has some stories to tell if it could talk.Anyway, should be done today, just wrapping up now. Went with a DPDT Serial/Parallel switch is all. Simple yet effective.

author: SeanB
author_rank: Super Contributor
timestamp: June 18, 2016, 08:07:29 pm
content: Japan has a 100VAC electric grid, so this is what they use for equipment designed for local use only.https://en.wikipedia.org/wiki/Energy_in_Japan#National_gridRunning at 120VAC will increase power losses in transformers and in linear regulators, but as most Japanese made transformers are designed very conservatively ( historically but modern stuff might not be as good) they will be fine on 120VAC.

author: Berni
author_rank: Super Contributor
timestamp: June 18, 2016, 09:28:59 pm
content: The higher voltage would likely only affect the regulators. Motors tend to be perfectly happy with a bit more voltage. Since the speed is determined by the line frequency it should run at roughly the same rpm but the current in the wingdings is lower. Only problem is if the voltage gets high enough to push the iron core in the motor towards saturation, at that point the current trough it would start to rapidly increase and cause extra heating.But that point should be pretty far off as anything running on mains has to assume quite a bit of tolerance in the voltage. Over here the mains should be 230V but its often 240V or even 245V sometimes.

author: almstsobur
author_rank: Contributor
timestamp: June 18, 2016, 11:08:46 pm
content: All finished! I want to thank you all, I really appreciate the assistance with this. It's pretty much perfect now. Almost completely silent with the series switch engaged but still decent flow for my under 10 amp testing. Couple of pictures; one wide shot to show the scale of the beast, then a front shot with it on, and a couple of the back including the on/off DPDT toggle switch I added for the parallel/series control.

author: almstsobur
author_rank: Contributor
timestamp: June 18, 2016, 11:10:08 pm
content: Last 2, thanks again!

author: Berni
author_rank: Super Contributor
timestamp: June 18, 2016, 11:43:58 pm
content: Nice clean mod there.Oh and not a bad workbench, also a fan of Keithleys i see.

author: almstsobur
author_rank: Contributor
timestamp: June 19, 2016, 01:46:01 am
content: Yeah, I got that first 2015 THD for the audio capabilities, but I liked it a lot, fast, accurate, easy to use, front/rear inputs. So I saw the 2700 in an auction for an absolute steal that needed repair, very simple fix so I added that to the collection. I have no complaints, both great meters in my mind. I use them both pretty much daily.

author: Berni
author_rank: Super Contributor
timestamp: June 19, 2016, 07:54:00 am
content: Yeah i love the speed on them and that VFD display is nicer to read than modern LCDs. I was pondering getting a Keithley 2015 just because its pretty cheep on ebay and you can never have too many multimeters. Don't really need the THD part (Built my own audio tester that goes down to 0.0004% THD) but hey if just in case.Nice rack off to the side too, been considering getting one since some of the old test gear is over 50cm deep and weighs over 15kg a piece (And i just got a deal on an old 50kg HP spectrum analyzer). I designed the equipment shelf on my bench for more modern 40cm deep gear nor did i specifically design it to hold over 100kg.

author: SL4P
author_rank: Super Contributor
timestamp: June 19, 2016, 08:56:01 am
content: How about a suitably rated diode in series?The fans will run quieter, and a tiny bit slower on the half-wave forward cycles.We used to do this with desktop equipment with small high rpm fans that were too noisy - reducing the motors kickback on the negative cycles.Not sure of the long term effect on the fan, but we never lost any.

author: almstsobur
author_rank: Contributor
timestamp: June 19, 2016, 09:23:04 am
content: Quote from: Berni on June 19, 2016, 07:54:00 amYeah, the rack is very handy for the big stuff and less commonly used things.Obviously I'm not sure what your lab/shop configuration looks like, but if you decide to do a rack and your floors will support it; I highly recommend doing a setup that will support casters/wheels. My rack has big casters and the floor is hardwood and even though I have a couple hundred pounds in the rack I can literally move it with 2 fingers. I have a rack mounted 12 plug power strip at the top and everything is wired within the rack, so it's just 1 power cable to the strip outside of the rack, makes it very convenient to move it where you need it to see screens or use a particular piece of gear without long cables all over, or just roll it off into the corner for more room when your not using that gear. Very slick and handy when you can easily move the rack.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2016, 08:41:25 pm
content: Quote from: SL4P on June 19, 2016, 08:56:01 amUsing a series diode or several will work for a low voltage DC brushless motor but not for an AC shaded pole motor. Destruction of the AC motor winding is likely do to heating when the leakage inductance saturates.

author: Berni
author_rank: Super Contributor
timestamp: June 21, 2016, 05:20:43 am
content: Quote from: David Hess on June 20, 2016, 08:41:25 pmYeah this is like putting a diode in series with a transformer primary, The large DC component would likely saturate its core, at that point the only thing holding back the current flow is the winding resistance. So lots of current flows and it overheats.This is a valid way of doing it in brushed AC motors. Most hair driers use a diode to get the slow speed setting on them. As you probably noticed hair driers are really loud as these types of motors tend to not run very quietly, also the brushes wear down but a hair driver is never run for very long. This is why fans tend to all be DC brushless motors or AC asynchronous (Offten the simpler inefficient shaded pole version).

author: SL4P
author_rank: Super Contributor
timestamp: June 21, 2016, 05:53:25 am
content: I take the learned advice with sincerity, and my experience was many years ago (when I was a nipper), but I recall the units/fans we did that to - were brushless AC in desk mounted consoles that ran 24/7 without any issues.I was wondering about magnetic effects, but don't recall any negative symptoms.I can only guess the fans were well over-specced in that application (and may have been 2x110v fans series - in a 220v device..A long time ago, but it certainly worked!I appreciate the technical criticism of the suggestion.

author: nikodean1
author_rank: Newbie
timestamp: June 22, 2016, 01:40:21 am
content: Fan technology has improved considerably over the years. Maybe you can find an equivalent pair of new 120 volt fans which are much quieter, and just swap them out without speed control.I'm not sure what the CFM requirements for yours is, but I came across this 110 CFM, 120mm 115 volt model:https://www.amazon.com/AC-Infinity-AXIAL-Muffin-Cooling/dp/B009OWRMZ6/ref=sr_1_1?ie=UTF8&qid=1466559258&sr=8-1&keywords=AC+Infinity+120+mm+fanThe noise output is 47 dBA, which shouldn't be too loud.What size are your fans?

author: Berni
author_rank: Super Contributor
timestamp: June 22, 2016, 06:26:34 am
content: Its possible that some fans are so overbuilt that they don't saturate from DC that easily.Once you get in to these high airflow fans the actual fan is only part of the story of making it quiet. These powerful fans create so much airflow that pockets of turbulence can form along the flow inside the case, causing extra noise. As i mentioned earlier i made the fan in a HP rack mount PSU way quieter by simply flipping it around so that it blows out of the case rather than suck in to it. That was mostly due to a heatsink being pretty close(cm or two) to the fan, this caused the blades to "wosh" past it and make a howling noise in sync with its rpm.In another rack PSU i found a unused header on the CPU board with 12V on it so i simply replaced the 110V fan with a computer fan and plugged it in to there.

author: almstsobur
author_rank: Contributor
timestamp: June 22, 2016, 07:05:54 am
content: So far I'm thrilled with the simple series/parallel switch. I don't think I personally need to do anything else to the beast, though the additional input might help a future search'er. Last night a was able to load test in complete silence about ten 12v wall-worts to find one that truly met 12v at 2amps. 8/10 did not, 2 did and one is being used now to power my new self designed desoldering pump setup. I'm just happy to load test in peace 😉

author: edpalmer42
author_rank: Super Contributor
timestamp: June 22, 2016, 07:21:00 am
content: The only improvement I can think of for your series/parallel arrangement would be to replace the switch with a relay. Power the relay through a normally open thermal switch bolted to one of the big heatsinks. When the heatsink gets hot enough (maybe 60C?) to trip the thermostat, the relay operates and changes the fans from series (slow and quiet) to parallel (fast and noisy - but lots of cooling).

author: almstsobur
author_rank: Contributor
timestamp: June 22, 2016, 07:26:42 am
content: Quote from: edpalmer42 on June 22, 2016, 07:21:00 amDamnit Ed! I like thatI like it a lot. Now your going to get me to crack the beast open again.

author: Berni
author_rank: Super Contributor
timestamp: June 22, 2016, 11:16:40 am
content: Quote from: almstsobur on June 22, 2016, 07:26:42 amWell using a switch does have an advantage in one area. This sort of automatic speed control will in most cases cycle on and off all the time. When the unit gets hot the fan kicks on and cools it down so its not hot anymore so the fans goes back to slow mode again, but then the heatsink slowly becomes hot again so it kicks on...etc This tends to be even more annoying to listen to than having it constantly be loud.I guess you could fix that by adding a relay wired to self latch itself across the thermostat switch (or use a relay with a 3rd contact in it) so that when it kicks on it stays on until the instrument is power cycled when cold.

author: edpalmer42
author_rank: Super Contributor
timestamp: June 22, 2016, 04:31:20 pm
content: Yes, cycling is a possibility.But due to the mass of heatsinks involved and the built-in hysteresis of the thermostat, it might not be a problem. The only way to find out would be to try it. Attach a temperature sensor to one of the heatsinks and manually flip the switch when the heatsink reaches e.g. 60C. How long does it take to drop by 10C? Then look for a thermal switch with 10C hysteresis or more. Extra points if you can log the data and show us a graph.If it looks like it's going to be a problem, you'd have to replace the thermal switch with a smarter circuit - or a switch!

author: edpalmer42
author_rank: Super Contributor
timestamp: June 22, 2016, 04:37:58 pm
content: On second thought, the first test you should do is to measure the heatsink temperature with the fans in series. How much power do you have to dissipate to get them "too hot"? If you'll never reach that point, you could consider the automatic switch from series to parallel as more of a safety feature like a flashing warning light instead of an operational feature.

author: nikodean1
author_rank: Newbie
timestamp: June 22, 2016, 07:11:46 pm
content: Quote from: almstsobur on June 22, 2016, 07:26:42 amI like this idea!