120 VAC fans - is it O.K. to undervolt them?

Control: management of fans, temp/rpm monitoring via soft/hardware

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Bartender
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120 VAC fans - is it O.K. to undervolt them?

Post by Bartender » Wed Apr 19, 2006 5:30 am

Good morning -
The computer desk that I built includes a large cabinet with roll-out shelves for the printer and scanner. I'm thinking of dismantling my Sonata and putting all the parts inside the cabinet on a 3rd roll-out.
One thing I'll have to address first is permanent ventilation in the back of the cabinet. Since I have the whole back wall to work with, fan size isn't an issue.
A coupla days ago a neighbor gave me two stout 120VAC fans out of some industrial server enclosure. I've read the sticky about 120 VAC fans but have an electrical question.
I've always thought that undervolting AC motors was a no-no? Amperage and heat goes up, etc. etc. I tried wiring mine in series and they spun (a little too slowly perhaps) and I didn't feel any heat at the motor hubs but I'm still concerned. Can someone explain whether this is kosher or not?
Thanks!

blandoon
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Post by blandoon » Wed Apr 19, 2006 3:58 pm

Generally speaking, trying to "undervolt" an induction (AC) motor is at best a waste of effort, and at worst a good way to wreck the motor. Mostly this is because the motor is specifically designed with a certain number of windings and poles to work on a certain voltage and frequency - speed control is done by much more complicated means than just reducing voltage.

Check the following:
http://home.comcast.net/~jmccomas2/wiri ... 4.gif.html

(I'm not an electrical engineer, so maybe someone else can explain this better than I can.)

mg1394
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Post by mg1394 » Wed Apr 19, 2006 4:53 pm

Odd answer from blandoon :? The article is just fine, and you'll notice the most common method of controlling speed of fans is to adjust the voltage. And that's just what you do. In normal household use (like a ceiling fan) it's called a reostat or dimmer. Fine units for lab use are called a variac.

The only thing (it's in the article) to be aware of - since the windings are designed to provide a certain speed at line voltage, the coils can physically vibrate at other speeds (similar to coils in a power supply with varying loads) and cause a hum - bet you've heard ceiling fans that hummed when slowed via the reostat.

Anyway, a good quality reostat from Radio Shack is the most common way of controlling 120v 120mm fans that are in A/V racks. Go for it and enjoy.

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Post by Elixer » Wed Apr 19, 2006 8:41 pm

In real use most AC fans that you would ever think of putting in a computer can be undervolted. However this is not an easy task, some people have placed two fans in series (like you) for similar results. Note also that dimmer switches (rhetostats) do not undervolt in the conventional sense. What they do is only apply voltage to the device it's controlling for part of the AC cycle. Works well in my experience with ceiling fans, but for smaller fans the on-off switching can produce additional noise in the fan. Wiring them in series shouldn't be a problem.

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Post by Tibors » Wed Apr 19, 2006 9:05 pm

Elixer wrote:Note also that dimmer switches (rhetostats) do not undervolt in the conventional sense. What they do is only apply voltage to the device it's controlling for part of the AC cycle.
A rheostat is a variable resistor. Dimmer switches based on rheostats do undervolt in the conventional sense. Modern household dimmer switches are not based solely on rheostats, but on a triac + rheostat. They do indeed control the power by switching on and off rapidly. Both methods will work with the AC fans.

http://home.howstuffworks.com/dimmer-switch.htm
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Bartender
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Post by Bartender » Thu Apr 20, 2006 4:11 am

Good morning -
Jeez, I never thought of a ceiling fan control. I've got one layin' around here somewhere. I'll wire the 2 AC fans back in parallel, then put the ceiling fan control in the circuit and see if I get clicking noises. Hope not. I could just wire them in series but they seem pretty ineffectual at that speed. If I can find that rheostat it won't cost me anything to experiment!
I saved that link for further study. How do you guys find this stuff!?

BrianE
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Post by BrianE » Thu Apr 20, 2006 3:39 pm

You may want to check out a local hardware store anyway. There is another type of AC fan speed controller that works using capacitors and (supposedly) do not create additional noise like the varible knobs you usually see. The downside is that they typically have only 3 speeds and aren't continuously variable. I've seen a 4 speed one once, but it's pretty uncommon. They usually say something like "quiet fan speed control" or something.

Here is a really good page I came across that goes into this issue, but keep in mind it discusses full sized ceiling fans and not smaller 120mm thingies.
http://www.fancollectors.org/info/speed.htm

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Post by TomZ » Fri Apr 21, 2006 2:07 pm

Bartender wrote:I saved that link for further study. How do you guys find this stuff!?
google!

Bartender
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Post by Bartender » Fri Apr 21, 2006 7:35 pm

I bought a ceilling fan controller at Home Depot. Said right on the box "Quiet" and "Suppresses RFI". It has four distinct speeds rather than variable. The slowest speed is too fast.

Wiring them in series is too slow. Maybe I need to try a resistor in series w/ the fan controller, though I don't know how much resistance to shoot for.

Went ahead with the PC-on-the-shelf part, even though I don't have any forced ventilation in or out of the cabinet. Just a big hole in the back. It runs but you can tell the project needs more ventilation. After about a half hour the CPU is bouncing around a degree or two higher than in the Antec. If I open the door it drops 2 degrees C within a minute.
Built a little plywood rack for the HDD and DVD drive. The HDD is suspended with shock cord. It went up to 42 deg. C so I set a 90 mm Zalman fan (using the resistor extension cord that came with it) in front of the HDD. The Zalman is inaudible with the resistor. The HDD went down to 27 deg. C.

The whole thing is rough right now - need to do some serious cable mgmt., build a home for the floppy drive, decide what to do about all the bezel stuff - USB/Firewire/audio/LED's/power button - that I don't have anymore, etc.

Would anyone care to take a guess at how many ohms (ballpark figure would get me started) I should try if I put a resistor in series with the fan controller?

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Post by IsaacKuo » Sat Apr 22, 2006 3:37 am

You could try fashioning an exhaust duct from the computer's exhaust out the big hole in the back of the cabinet. How easy this is to do depends on the layout of your computer exhausts, of course. If it's just the PSU and a rear case fan next to it, then it should be pretty straightforward. This method has the benefit of not adding any fan noise, and it could even muffle some fan noise.

Another possibility is to simply run some power wires from inside the computer, and use this to run a plain old 120mm computer fan at 5v or 12v (or in between). Thus, you can use any number of quiet fan models we're familiar with.
Isaac Kuo

Bartender
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Post by Bartender » Sun Apr 23, 2006 5:34 am

Hi, Isaac -
I removed the scanner roll-out shelf, gaining another half foot of space above the PC roll-out shelf. Didn't want to do this but it gives me much-needed space for hot air and cable mgmt.
I'll blank off the big hole in the middle of the cabinet back (leaving enuf room for cables) and cut 2 holes right up at the ceiling for those AC fans wired in series.
There's a generous slot in the bottom of the cabinet back wall. If I can evacuate hot air out the top of the enclosure and cool, floor-level air streams in thru the slot that oughta make a big improvement.
Ducting the PSU output and placing a divider (a strip of neoprene or rubber) on the back of the cabinet which contacts the wall and impedes the flow of air between the intake at the floor and the outputs up high should also help.

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Post by IsaacKuo » Sun Apr 23, 2006 2:44 pm

Hey, yesterday while discussing an HTPC build with a friend, my friend came up with a really good way to independently power and adjust the speed of some cabinet ventilation fans. (He wants to make a cooled cabinet like you, to hopefully reduce the noise from his X-Box 360.)

His brilliant idea? An $11 power brick from Walmart. I just picked one up to try it out--rated at 1300mA, so that'd be about 15w at 12v. It's a universal power adapter, so it's got a switch for settings ranging from 3v to 12v. I'm going to do a brief write-up on this idea...
Isaac Kuo

depravedone
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Post by depravedone » Sun Apr 30, 2006 7:34 am

OK People. First OFF, VOLTAGE has nothing to do with slowing down an AC fan. To control the speed of an AC fan you must vary the frequency of the AC down from 50/60Hz using a frequency driver.


The varying of speed in ceiling fans is usually accomplished by using different taps in the motor windings.

If you do not understand, or think I'm full of it, look at the link below


http://www.answers.com/topic/variable-f ... ?method=22
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TomZ
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Post by TomZ » Sun Apr 30, 2006 5:26 pm

depravedone wrote:OK People. First OFF, VOLTAGE has nothing to do with slowing down an AC fan. To control the speed of an AC fan you must vary the frequency of the AC down from 50/60Hz using a frequency driver.
No, you're wrong about that. AC induction motors can be controlled either by varying the voltage (typically TRIAC/phase control) or by varying the frequency.

With all due respect, please don't chime in and take an authoritative tone unless you have deep knowledge and/or experience.

depravedone
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Post by depravedone » Sun Apr 30, 2006 5:47 pm

First off, I don't appreciate your tone TomZ, as you obviously have NO idea who I am or what my experience is. Varying the speed of an AC motor is not normally done simply by changing the voltage with some sort of rheostat. The problem with using TRIACs, in my experience, is noise. Just as you will likely get with motors ran off a VFD. The point is, you're pretty much always going to get more consistent, quieter results with simple DC motors.
TomZ wrote:
depravedone wrote:OK People. First OFF, VOLTAGE has nothing to do with slowing down an AC fan. To control the speed of an AC fan you must vary the frequency of the AC down from 50/60Hz using a frequency driver.
No, you're wrong about that. AC induction motors can be controlled either by varying the voltage (typically TRIAC/phase control) or by varying the frequency.

With all due respect, please don't chime in and take an authoritative tone unless you have deep knowledge and/or experience.
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Post by qviri » Sun Apr 30, 2006 6:44 pm

depravedone wrote:First off, I don't appreciate your tone TomZ, as you obviously have NO idea who I am or what my experience is.
And he still doesn't, since you weren't kind enough to provide said credentials. Why don't both of you get off your high EE horses and hammer out a consensus on this one. Obviously there's just one right answer.
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TomZ
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Post by TomZ » Sun Apr 30, 2006 7:07 pm

qviri wrote:And he still doesn't, since you weren't kind enough to provide said credentials. Why don't both of you get off your high EE horses and hammer out a consensus on this one. Obviously there's just one right answer.
Besides the question of credentials, I don't see any open issue here. depravedone first said that frequency control is the only method of speed control, and then said in the second post that TRIAC control and probably also said that rheostat also works (although is not commonly used). Case closed?

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Post by Butcher » Mon May 01, 2006 3:03 am

depravedone is correct - simply reducing the voltage isn't a good way to reduce AC fan speed. AC fans spin at a speed based on the AC frequency, not on the voltage.

A rheostat works, but that's because at lower voltage the motor has less torque so slows down because of friction and air resistance. If you ran it with friction free bearings in a vacuum it would be the same speed. The problem is that just relying on torque to slow your motor is somewhat unreliable, and not particularly good for the motor either (can induce a lot of extra heat).

Triac controls are a bit different, because they chop the sine wave of the AC rather than reducing voltage. When the triacs kicks in and chops the signal the fan will simply lose power for that part of the cycle. During that time the motor will coast and you'll get a lower average speed than if you ran it flat out. It's more or less PWM for AC motors.

To get a nice linear control such as you'd get with variable voltage on a DC motor you'll need a variable frequency AC supply. I'm not sure on commercial availablity but it's certainly possible (though not neccessarily cheap) to make one.

Personally I think you're best off with DC fans, they're just much more suited to the purpose. If you must use AC and want to slow it a triac based dimmer or a variable frequency supplies are the only sensible options.

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Post by jaganath » Mon May 01, 2006 3:56 am

Triac controls are a bit different, because they chop the sine wave of the AC rather than reducing voltage. When the triacs kicks in and chops the signal the fan will simply lose power for that part of the cycle. During that time the motor will coast and you'll get a lower average speed than if you ran it flat out. It's more or less PWM for AC motors.
Won't that lead to a kind of "jerky" fan motion, whereby at some parts of the cycle the fan is pushing more air than when it's coasting? Obviously this won't be noticeable to the user, but as you say it seems kind of sub-optimal from an engineering point of view. Anyway Butcher, thank you for that succinct explanation for the layperson.

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Post by TomZ » Mon May 01, 2006 5:00 am

Butcher wrote:depravedone is correct - simply reducing the voltage isn't a good way to reduce AC fan speed. AC fans spin at a speed based on the AC frequency, not on the voltage.

A rheostat works, but that's because at lower voltage the motor has less torque so slows down because of friction and air resistance. If you ran it with friction free bearings in a vacuum it would be the same speed. The problem is that just relying on torque to slow your motor is somewhat unreliable, and not particularly good for the motor either (can induce a lot of extra heat).

Triac controls are a bit different, because they chop the sine wave of the AC rather than reducing voltage. When the triacs kicks in and chops the signal the fan will simply lose power for that part of the cycle. During that time the motor will coast and you'll get a lower average speed than if you ran it flat out. It's more or less PWM for AC motors.
So first you are saying that AC motors have to spin at a speed determined by frequency, but then you also state that TRIAC and rheostat controls can also work. Am I the only one noticing the contradiction?

Your statement that AC motors having to spin at a speed stricly determined by the frequency - where voltage control would not work - is correct for Permanent Magnet AC Motors. These motors must always turn at a frequency synchronous to the operating frequency. To change the speed, you have to change the frequency of the power input - not an easy job.

But for AC Induction Motors, the type of motor commonly used in fans, there is what is called "slip" which means that the rotor (core) does not keep up with the rotating magnetic field of the stator (outside). These types of motors can rotate at any speed up to just below (80-90% of) their synchronous speed (speed if the motor was a permanent magnet AC motor). As I said before, the speed of AC induction motors can be reduced by reducing the effective voltage applied to the windings. I agree with your description of TRIAC speed controllers - they are effectively lowering the voltage applied to the windings, which slows the motor.

The reason that rheostats are not commonly used isn't due to heating of the motor, but due to heating of the rheostat. Effectively, if you are reducing the power to the motor by, e.g., 1/2, that means that the rheostat is absorbing the same amount of power as the fan. That makes for a large and/or hot rheostat. For small motors, this is okay, but for large motors becomes impractical.

In the end Butcher decided to control the motor speed using a residential ceiling fan control. This will most likely be a TRIAC-based speed controller, and should work fine for this application assuming there is no extra noise due to the low-frequency TRIAC switching control (as you mentioned). This is not an issue for ceiling fans because they have a lot of inertia, but it might be more of a concern for a smaller motor.

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Post by TomZ » Mon May 01, 2006 5:06 am

jaganath wrote:Won't that lead to a kind of "jerky" fan motion, whereby at some parts of the cycle the fan is pushing more air than when it's coasting? Obviously this won't be noticeable to the user, but as you say it seems kind of sub-optimal from an engineering point of view.
The idea is to switch the power to the fan fast enough so that it doesn't have time to mechanically react to change in magnetic field. What is fast enough depends on the motor characteristics. From an engineering point of view, this is not sub-optimal at all - nearly all motor speed controls are based on this principle.

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Post by Butcher » Tue May 02, 2006 2:52 pm

TomZ wrote:In the end Butcher decided to control the motor speed using a residential ceiling fan control. This will most likely be a TRIAC-based speed controller, and should work fine for this application assuming there is no extra noise due to the low-frequency TRIAC switching control (as you mentioned). This is not an issue for ceiling fans because they have a lot of inertia, but it might be more of a concern for a smaller motor.
I did? Funny, there was me thinking I used all DC fans in my setup. Guess all my DC fans turned into AC fans without me noticing, and a dimmer switch magically appeared in my setup to boot!

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Post by TomZ » Tue May 02, 2006 3:59 pm

Butcher wrote:
TomZ wrote:In the end Butcher decided to control the motor speed using a residential ceiling fan control. This will most likely be a TRIAC-based speed controller, and should work fine for this application assuming there is no extra noise due to the low-frequency TRIAC switching control (as you mentioned). This is not an issue for ceiling fans because they have a lot of inertia, but it might be more of a concern for a smaller motor.
I did? Funny, there was me thinking I used all DC fans in my setup. Guess all my DC fans turned into AC fans without me noticing, and a dimmer switch magically appeared in my setup to boot!
Sorry, I meant Bartender!

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Post by j-frog » Tue May 02, 2006 7:57 pm

I sucessfully slowed down an AC driven fan in a watercooling setup with one of those wall dimmers used for incandescent lights. The fan did not hum excessively.

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