AC fan speed control - Page 1
Forum section: electronics / beginners

author: Sz_David
author_rank: Newbie
timestamp: June 28, 2017, 02:02:14 pm
content: Hi!I've got a standing fan with 3 speeds: Fast, Faster, Tornado.I thinking about how to speed control it. Googled it, found some datasheets, application notes, ebay listings, youtube videos, forum posts about it, and it seems to me, that the circuits for this, is basically a dimmer. STmicro recommends to do a soft start, and an inductor and capacitor (ACST1635-8FP datasheet), some other places recommends snubbers. On some places pleople says, it is okay to use a regular dimmer, because the fan is not so high power. Other places says its not okay, because it is not an universal motor. But it okay for a shaded-pole or PSC motor. I have a 50-300W dimmer in stock, and my fan is 50W, if it would work, that would be great. (I would later build a custom controller to remote control)Now I disassembled the fan to see what kind of speed control does it have. It seems like one common and 3 separate wires for the 3 speeds goes into the motor. And have a big capacitor. Doesn't seems like that kind of motor to me which can be speed controlled with a dimmer like circuit. But I don't know much about these things, so I think it's better to ask, than blow upSome pictures:http://imgur.com/a/fPCC7So the question is how can I control the speed of this fan?

author: Kjelt
author_rank: Super Contributor
timestamp: June 28, 2017, 03:02:47 pm
content: I used a variac three weeks ago, probably not the right way to do it but it worked okish

author: Sz_David
author_rank: Newbie
timestamp: June 29, 2017, 03:03:55 pm
content: Thanks!Checked that, it is just a transformer like thing. I may check my garage, there is a few transformer, maybe there is one that will do the job.What kind of other solutions are there? Maybe I should replace the motor?

author: MosherIV
author_rank: Super Contributor
timestamp: June 29, 2017, 03:27:06 pm
content: QuoteQuoteHmm, do you have Variacs lying around in your garage? You are a very lucky man.If you do not know, a Variac is a special type of transformer. It is a variable transformer.They are round and have a handle that allows you to adjust the overlap of the primary to secondary.They are also usually a 1 to 1 ratio.They are very specialised piece of kit.

author: Kjelt
author_rank: Super Contributor
timestamp: June 29, 2017, 04:21:43 pm
content: Quote from: MosherIV on June 29, 2017, 03:27:06 pmYes got 5 of them when they were thrown out at my previous job.Some are fully isolated which is very nice and are for in use in my labtable, the non isolated are used for halogen dimming or now fan dimmingProblem with variacs though is that the inrush current will trip the circuit breaker as my wife found out

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2017, 05:04:35 pm
content: Quote from: Sz_David on June 28, 2017, 02:02:14 pmIt will probably work with a lamp dimmer. The only risk is it might kill the TRIAC in the dimmer.

author: james_s
author_rank: Super Contributor
timestamp: June 29, 2017, 05:07:04 pm
content: I would use a motor speed controller, or modify the dimmer circuit to resemble one. Conventional light dimmers are usually very minimal and do not like inductive loads at all.

author: MrAl
author_rank: Super Contributor
timestamp: June 30, 2017, 11:45:08 am
content: Hi,Motors used for AC fans are set up to follow the frequency of the line voltage and they need a certain amount of power to do that and they get this power from the 120vac (or whatever) line voltage. If we just lower the voltage, the power goes down but the line frequency stays the same, and thus we start to see 'slip' where the motor can no longer follow the frequency. This introduces heat which could ruin the motor. To do it right we should use a variable frequency drive, which lowers the frequency as the voltage goes lower, and thus the motor can keep up with the lower frequency with the lower voltage.The word is not out yet on if every fan motor will burn out using a variac or lamp dimmer rather than a VFD. VFD's are generally expensive compared to lamp dimmers so lots of people try lamp dimmers. Variac's are not cheap either but they just lower the voltage too, although they do maintain the proper wave shape. I know of some fans that have burned out but im pretty sure others survived. I was not able to investigate this thoroughly because of too few cases to examine and variables in those cases such as amount of speed reduction actually used in each case.A variable frequency drive (VFD) would be an AC to AC converter that takes say 120vac 60Hz and converts it to DC, then back to AC at a frequency say 10Hz to 60Hz and at a variable voltage, and the frequency output is related to the voltage output but i dont remember the relationship for a fan (look itup on the web maybe).If the fan is not expensive though i'd probably try a lamp dimmer or control made for tools or something. Alternately buy a fan with a better speed range maybe.Long time ago i did use a small variac and the fan did not burn out. It was a regular box type fan maybe 18 to 22 inches.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 30, 2017, 01:24:27 pm
content: Quote from: Sz_David on June 28, 2017, 02:02:14 pmI Like your graphic descriptionIf all you want to do is reduce the speed across all three settings, you could use a simple auto-transformer to lower the voltage, let's say, 10%.You can use a common 240/24 volt transformer. The beauty of this connection is that only the secondary has to carry the full load current; therefore the transformer itself is small.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2017, 03:08:00 pm
content: Quote from: MrAl on June 30, 2017, 11:45:08 amI agree, the 'correct' way to do this is to use a VFD but I doubt it's necessary here. The motor's speed can be adjusted with a rotary switch which selects different taps on the motor's winding, therefore altering its impedance and the amount of slip.A variac will work but will probably work out more expensive than an VFD. A lamp dimmer will probably work, especially the one described by the original poster which is rated from 50W to 300W, yet the motor is only 50W.

author: james_s
author_rank: Super Contributor
timestamp: June 30, 2017, 03:30:55 pm
content: I've never seen a VFD used to control the speed of a single phase motor. Lots of infinitely variable speed fans exist, they are controlled by a circuit similar to a lamp dimmer. It's pretty standard practice, there's even a purpose built device to adjust the speed of the blower motor on the gas furnace at my mom's place so it works even on relatively large fan motors.

author: Red Squirrel
author_rank: Super Contributor
timestamp: June 30, 2017, 04:16:03 pm
content: Furnace motors have various taps for different speeds, though that got me thinking do lot of AC motors like that use brushes and work kinda like DC? I guess in that case a VFD would be more or less useless as those probably don't use frequency for rotation speed, so dimmer may very well be the way to go as far as easiest way as it will lower the average voltage. It's dirty but will work.

author: james_s
author_rank: Super Contributor
timestamp: June 30, 2017, 07:12:11 pm
content: While there are multiple taps on the furnace motor, the speed control is on the high speed winding and offers infinite control from full speed down to I would estimate about 20%. The most noticeable difference between a motor speed controller and a light dimmer is that the speed control clicks on at full and then you continue rotating to lower the speed.

author: Red Squirrel
author_rank: Super Contributor
timestamp: June 30, 2017, 08:07:23 pm
content: Also want to watch not to set it too low, don't want motor to stall.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2017, 08:41:39 pm
content: The thing with a fan is there's no torque at low speeds, so there's hardly any power dissipation. With such a small motor, the winding resistance might even be high enough to limit the current to a safe level if it stalls on the lowest speed setting.

author: MrAl
author_rank: Super Contributor
timestamp: June 30, 2017, 10:24:28 pm
content: Quote from: Hero999 on June 30, 2017, 03:08:00 pmHi,I havent priced VFD's in a long time. We can get smaller variacs for maybe 60 dollars USD.I agree though, i would try the lamp dimmer or similar first.

author: MrAl
author_rank: Super Contributor
timestamp: June 30, 2017, 10:28:34 pm
content: Quote from: james_s on June 30, 2017, 03:30:55 pmHi there,When i worked in the industry we had to design one. I dont remember all the specs, but maybe it was 100 amps. I remember we had to get the voltage and frequency just right according to a relationship but i cant remember what it was offhand, maybe a cube law. It's been a long time for me since then, that was almost 35 years ago now.The main issue might be the efficiency. Some of the stuff we did had to work with secondary power like a solar array, and ALL the stuff we did had to have good efficiency as a selling point. We did some stuff for Sandia Labs that was 90 percent efficient, and back then that was a big deal :-)

author: slurry
author_rank: Regular Contributor
timestamp: June 30, 2017, 10:28:43 pm
content: Is it possible to switch to a 24V AC fan?if so, you can use a multi tap transformer with 15V, 18V, 24V which would give you silence to tornado with a flick of a switch or relay control of some sort.

author: MrAl
author_rank: Super Contributor
timestamp: June 30, 2017, 10:33:53 pm
content: Quote from: slurry on June 30, 2017, 10:28:43 pmHi,Yeah you reminded me, i use a 12v computer fan sometimes. It's not your ordinary 12v computer fan though, it uses 36 watts at full power :-)My adjustment scheme is ever so simple: Three power resistors in series, and an alligator clip jumper to select the tap i want from taht series string. The closer to the 12v i get, the more power the more speed, and the farther from the 12v i place the jumper the slower and less power i get from the fan.It's kinda loud at the full 12v though but it's high speed above 4000rpm. I use a 4 amp rated wall wart.But my main point is that DC fans are pretty nice and of course you can control them easier.

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2017, 03:24:11 am
content: I have a capacitor run fan like that and a variac works fine for controlling it. A dimmer will work also but is electrically noisy.

author: Sz_David
author_rank: Newbie
timestamp: July 02, 2017, 01:48:42 pm
content: Hi!Oh, received quite a few answerVariac: I don't have any variac, but have multiple transformers with multiple output below 24V, and one 220V->380V (or 220V->140?). (But found some fun stuff, 120A germanium diode, 25A silicon diode in home made car battery charger, a few transformers, etc).Dimmer: Some of you said, a Dimmer sould be okay, but dimmers doesn't like inductive loads. What sould I modify on them? Maybe can it work without (the dimmer is up to 300W, fan is 50W, but I don't about starting current, etc..)VFD: Googled it, haven't found anything cheap, so I think I may build one. But how? Probably an H bridge with an arduino wouldn't be enoughMotor replacement: Maybe I could find a universal 24-12V motor but I'm not sure that is powerful enough or is even near the right RPM.DC Fan: I don't say that a standing fan looks cool, but computers fan are less cooler, and cools less. And I have to get some better PSU than a 1A charger. (For that 2 relays for my speakers are good enough) But cool idea, I'm considering getting some. But the AC fan is still fun, I'm curious that can it be solvedThe thing I afraid of is that the motor has a big capacitor. In the garage there are some old motors, which have capacitors that uses the starter winding, if the motor can't reach the operating RMP, those starter windings can be burnt for too long on time. Maybe this motor is similar?And there is 4 wires going into the motor, with no external circuit. I don't know what it is, and I afraid it is neccesary for the "built in speed control" like separate windings or something. (Grandfather says, the motors in the garage can't be speed controlled, because they'll stall or continously need to use the starter cause they are not commutator type - and yes, they aren't universal, they'rewhere the armature is shortedkind of motor)Thanks for your help

author: David Hess
author_rank: Super Contributor
timestamp: July 02, 2017, 08:31:28 pm
content: Quote from: Sz_David on July 02, 2017, 01:48:42 pmThe capacitor is physically large but low capacitance and high voltage used for starting and running so it is continuously connected.https://en.wikipedia.org/wiki/AC_motor#Permanent-split_capacitor_motorIf you do not want to use a variac or triac dimmer, then a bucking transformer will work to create several voltage taps for a switch to select from. I usually find inexpensive variacs at local sales or on Ebay; they are very handy.

author: Sz_David
author_rank: Newbie
timestamp: July 03, 2017, 04:46:41 pm
content: Quote from: David Hess on July 02, 2017, 08:31:28 pmOn afraid I mean I'd like to look up things before I blow up them. In the hungarian wiki the PSC part is not there, or maybe I'm blindThanks for the useful link!Did some research, PSC motors seems to speed controllable by lowering the voltage. So the bucking transformer should be good, thanks, I'll get some proper wires and try it.And probably try the dimmer one too, but first I want to make sure that I can still get 300W dimmers, it is neccesary for my lighting.james_s said I should modify the dimmer circuit, but how?

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2017, 05:29:48 pm
content: chinese fans are always super fast because copper is expensive- adding turns to give lower speed costs money.I bought a $4 eBay 4000W triac/scr phase control module and it works but the fan motor buzzes and is noisy, and the potentiometer makes sparks sometimes. They are cheap but not so safe.Now I'm using my variac, lol.

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2017, 05:55:50 pm
content: Quote from: floobydust on July 03, 2017, 05:29:48 pmNo this is an AC motor. The speed is dependant on the frequency of the supply and slip. Less copper would make it run hotter, not faster.In the case of a DC motor, the speed can be reduced, without adding any extra copper by using thinner wire, thus enabling more turns with the same amount of copper.QuoteThere's nothing inherently dangerous about a thyristor power controller. It's how it's physically designed which makes it safe or a deathtrap. The picture of the one you're using appears to be dangerous. There probably isn't enough creapage and clearance, even if the heat sink and potentiometer were earthed.Never run the speed controller with a load equal to its full power rating. De-rate decent speed controller modules slightly and cheap ones by a factor of two or more.

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2017, 06:50:06 pm
content: Quote from: Hero999 on July 03, 2017, 05:55:50 pmFans are high-slip motors, B-field strength is set by the winding taps. More copper= more turns higher impedance, lower flux and higher slip.Less copper, runs hotter = perfect chinese design.