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Old 06-17-2021, 10:44 AM   #1
Dannyx
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Question AC Fan motor speed control ideas

Good day folks. What I'm trying to achieve may seem pretty simple and although I found lots of discussions on the topic, it's hard for me to settle on a definitive answer, so I thought I'd ask for more opinions.

I have a small inline duct fan which looks like this, which I want to slow down, the reason being overall noise. I have no idea what type of motor this is, because I can't exactly take it apart to inspect it without ruining it, but perhaps we can work it out by examining its behavior.

Two things I've tried: a variac and a cheap light dimmer (triac+diac inside of it). The variac worked great: as I lowered the voltage, the fan slowed down and didn't give off any immediate signs that I shouldn't be doing what I was doing.

The dimmer was less than ideal: other than the fact that the motor slowed down considerably as soon as I moved the slider a small fraction, which made the slider virtually useless, the motor buzzed quite noticeably and I was afraid to let it run like this for too long, fearing the triac might pop.

I also have one of these that I could try, but I reckon it's the same exact thing as the dimmer I just tried, since I see the same exact components, with the exception of the inductor. The light dimmer also has a large inductor, but other than that, they seem virtually identical: diac, triac, potentiometer, capacitor. I heard about different kinds of dimmers and stuff like "phase angle control", but wouldn't know how to measure and analyze any of this stuff without a scope, which I don't have. I think all garden-variety hardware store dimmers will be like this and wouldn't be much better...

Based on these observations, I believe we're dealing with a shaded pole motor ? The variac (transformer) solution seemed to work the best, but I'm definitely not going to dedicate a big variac to this one sole use, neither am I willing to invest in VFDs to drive a small motor, so I think I'll have to compromise.....
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Old 06-17-2021, 10:52 AM   #2
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Default Re: AC Fan motor speed control ideas



http://electronics-diy.com/schematic...controller.png
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Old 06-17-2021, 11:07 AM   #3
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Default Re: AC Fan motor speed control ideas

I belive that is a "dimmer", is it not ?
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Old 06-17-2021, 11:13 AM   #4
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Default Re: AC Fan motor speed control ideas

M1 appears to be a motor
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Old 06-17-2021, 11:15 AM   #5
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Talking Re: AC Fan motor speed control ideas

But it could also be a lamp in the case of the dimmer

Here: I drew the schematic for the ones I got off Aliexpress. They look about the same, save for the snubber (?) network on the left and the dual pot setup on the right (RV1 is a trimmer). I didn't bother to add values for the components too, so they're just as kicad spat them out....
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Old 06-17-2021, 12:19 PM   #6
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Default Re: AC Fan motor speed control ideas

Yes these tiny boards all tend to be TRIAC phase control lamp dimmer circuits just like the wall mount.

If cheap is the only way to go, that's what you get with phase control. The buzzing is expected, you can stick a inductor in series to dampen out some of the buzzing. Probably have to hack the pot/series resistor for more reasonable values for your motor - system behavior depends on the motor, control device, and also the load... together...

There are specially made "motor" "dimmers" that you can get too... usually they're like "high" "medium" "low" "off" and not a continuous function in between.
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Old 06-17-2021, 12:23 PM   #7
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Default Re: AC Fan motor speed control ideas

I guess I could also try a regular transformer ? Not as cheap as the dimmers, but not as expensive and overkill as a full-sized variac either. Of course, I'd be stuck with the single fixed output voltage, so I'd have to find the sweet spot where the noise/speed are both acceptable. I think this was around 70v in my case.

I also found these things but I have no idea what they are and how it would be possible to hook one up with all those wires. I guess they're for a specific type of motor, since they mention a capacitor....

Last edited by Dannyx; 06-17-2021 at 12:25 PM..
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Old 06-17-2021, 03:37 PM   #8
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Default Re: AC Fan motor speed control ideas

A phase-controller (dimmer) will always be noisy due to the sharp pulse, which makes the motor buzz. I use one in summer on my window fan.

You could use a variac, or transformer to cut back to say 160VAC, or put a series capacitor to the motor to slow it down. If you have any motor caps, try a 5uF or something.
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Old 06-17-2021, 05:37 PM   #9
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Default Re: AC Fan motor speed control ideas

VFD = correct solution!
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Old 06-17-2021, 06:07 PM   #10
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Default Re: AC Fan motor speed control ideas

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Originally Posted by eccerr0r View Post
VFD = correct solution!
Yes but not for 120 volts these are rare
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Old 06-17-2021, 06:56 PM   #11
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Default Re: AC Fan motor speed control ideas

Rheostat?
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Old 06-17-2021, 10:20 PM   #12
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Default Re: AC Fan motor speed control ideas

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Rheostat?
I thought of that too, but have no idea what value it should have and what I should look out for so it wouldn't go up in flames overnight
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Old 06-18-2021, 01:21 PM   #13
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Default Re: AC Fan motor speed control ideas

Another approach is to put a transformer (primary) in series with the motor, then load down the transformer's secondary with a smaller rheostat or resistor. It's safer and not as much smoke.
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Old 06-18-2021, 01:38 PM   #14
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Default Re: AC Fan motor speed control ideas

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Originally Posted by redwire View Post
Another approach is to put a transformer (primary) in series with the motor, then load down the transformer's secondary with a smaller rheostat or resistor. It's safer and not as much smoke.
Sounds nice and hacky, just the way I like it, though as you suggested in the very last sentence, I think there's a little bit more to it than just randomly picking out a transformer out of my junk bin
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Old 06-18-2021, 05:17 PM   #15
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Default Re: AC Fan motor speed control ideas

With a rheostat or resistor, you are wasting the extra (voltage) energy as heat. Using a transformer the voltage to a load resistor is just lower voltage and isolated from mains.
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Old 06-19-2021, 12:00 AM   #16
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Default Re: AC Fan motor speed control ideas

Quote:
Originally Posted by redwire View Post
If you have any motor caps, try a 5uF or something.
Do NOT do this!!!


Depending on the load & leakage inductance and other characteristics of the motor, there's the very real risk of series resonance. It is possible (likely?) to end up with greater-than-normal voltage across the motor, with excess current draw and core saturation, even though the calculations for Xc suggested otherwise.

Do NOT assume the motor's reactance is inert for at least part of the fan speed/torque curve.

In testing, a "C-frame" shaded pole motor (1980s MWO fan) in series with a ~6u cap ended up receiving ~160V while drawing close to an amp, vs its normal .6A @ 120V.


OP, your fan uses a similar "round" shaded pole motor:


Use a triac based motor speed control; looks similar to a light dimmer but has off/hi/lo markings.
The buzz is normal as the fast risetime is audible via the core&coils.
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Old 06-19-2021, 05:14 AM   #17
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Red face Re: AC Fan motor speed control ideas

Just for reference, here's also the first dimmer I tried - the one with the slider, whose buzz I wasn't too satisfied with. It actually does have the coil you chaps mentioned, so that's probably one positive thing going for it. I haven't analyzed it yet, but I reckon it's the same exact circuit thing as the one on Aliexpress, just with the coil and the little neon lamp, which I'm not sure affects the functionality in any way, other than to offer a visual indication of the setting.

Trouble is, one faint nudge on that slider down and the motor already slows down a lot, so I'd probably have to modify it a bit...

Still, I'd much prefer a transformer since it appears to run more smoothly, but what specs should the transformer have ? Should it be rated close to the same power as the motor ? Does the secondary voltage matter ? Do I just wire the primary in series with the fan ? If so, then with no load on the secondary (open circuit), would the fan run at its full speed ? My fan draws 14w according to its label, by the way.

EDIT: I just got another idea, which I actually tried out, since it's really simple to do: low wattage incandescent bulb in series with the thing. I tried 20w, the smallest one I had around and it slowed down quite a lot, close to how I want it. I'd then use a relay or a switch to bypass the lamp to enable full speed. True, this is highly counterproductive, since I'm now wasting power as heat which the lamp gives off, which is the opposite of the whole purpose of a fan to begin with, but assuming I place the bulb somewhere remote where temperature doesn't matter, it works quite nicely
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File Type: jpg 20210619_140917.jpg (605.5 KB, 8 views)
File Type: jpg 20210619_140925.jpg (444.4 KB, 3 views)

Last edited by Dannyx; 06-19-2021 at 05:43 AM..
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Old 06-19-2021, 12:34 PM   #18
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Default Re: AC Fan motor speed control ideas

A surplus dealer had a bunch of cheap German 240V fans, but normal is 120V.
We added a series capacitor to get resonance to 240VAC with 120V supply, it worked well. But trying different cap values it was not going to make a Tesla coil, the Q is low due to the motor's DC resistance and load of the rotor, it is a lossy circuit. I found the motor's inductance alone was enough.
I thought of OP using a small motor cap below resonance as extra impedance to slow the fan down. It won't make buzzing noise.

For better control range, a triac dimmer with the extra (cap reset) diodes works much better. I use the Ali "4,000W SCR" board (hint: it's a triac) with motors and fans and it works IF the pot is the right value for your mains voltage and other resistors are not just copied. This schematic is after I fixed it. No EMI inductor on it. Look for a small bridge rectifier to tell it's the right one.
Your dimmer might just be junky and need resistors or trigger cap size tweaked. App note on dimmers AN1003

P.S> A light bulb would work too.
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File Type: jpg eBay 4000W dimmer with mods.JPG (121.0 KB, 8 views)
File Type: jpg eBay 4000w dimmer.jpg (35.5 KB, 6 views)
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Old 06-19-2021, 12:53 PM   #19
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Default Re: AC Fan motor speed control ideas

I don't really need a range of speeds, but rather two settings: full speed and low speed ("quiet"), which I plan to use a time relay to switch between at certain hours when it's "nap time", so once I'm satisfied with the noise level, I would no longer adjust anything anyway, hence why fixed solutions like series resistance are also acceptable. A continuously adjustable circuit just makes it easier to find said sweet spot, as opposed to trying out different solutions which yield a single fixed speed at any given time, plus it's more professional and possibly more efficient.

I came across those SCR devices like in the last picture, but thought they're the same thing as my triac setup and wouldn't perform any better in the long run, so I disregarded them...
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Old 06-27-2021, 08:48 AM   #20
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Default Re: AC Fan motor speed control ideas

Speaking of these motors: any way to get them to spin backwards ? I was planning on mounting one on the door of a storage closet to suck the hot air out from the top, but it then became obvious that all these fans suck from the front and blow out the back. You'll immediately suggest mounting the fan on the inside of the door, but the point is the faceplate has to be on the outside of the door so it looks somewhat nicer and I don't just have a hole in the door where you see the back of the fan, so I'd need the blades to turn the other way somehow...I think I can take the rotor out and flip it 180 degrees...
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