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Slow down AC fan speed

kellys_eye

Jun 25, 2010
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I just don't agree with the statement that I contradicted myself...
OK, I misinterpreted that - sorry. But my offered solutions still apply and I'd like to know if there is anything about the solution that doesn't meet with your needs. If you want to deliberately complicate matters (install, design or build an interface that does 'whatever') then I'm sure we could come up with an Arduino and coding to drive a variable frequency AC source to control the motor... (tongue-in-cheek, don't get upset!) but what's wrong with the KISS approach?

At 12V you won't need your inverter on, it won't take as much power from your batteries (no DC-AC-fan losses), it is easily controlled (DC speed controllers are 10-a-penny) and the versatility would - if you needed it - allow you to position multiple fans on the same controller at strategic locations that offered a better cooling solution.

As I previously said, if you are concerned for power wastage and 'distortion' then avoid the AC-controlling solution as there is no practical way to avoid all-of-the-previous.

There are many very efficient 12V fan units available - even from junk yards, maybe even an identical unit to one already fitted in your RV? If your RV has a distributed 12V supply (usual cigar-lighter socket arrangement) then powering one isn't going to be a cabling issue.

IMHO the key to any solution these days is (1) energy-saving and avoiding a conversion process is key to this as they rarely better 80-90% efficiency and (2) cost-reduction where 'used' or industry-standard devices are long term cost-savers. Nothing quite so effective as something the vehicle manufacturers trust to install in a machine expected to last 20 years.
 

crutschow

May 7, 2021
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An AC TRIAC lamp dimmer will reduce the fan speed, but introduce harmonics and often generates a buzz in the motor.
I am presently using a high-voltage capacitor in series with a fan to quietly lower its speed and the power it consumes.

Try a 1µF or more non-polarized (film type) 400V capacitors (example).
You can put more than one capacitor in parallel if you need more capacitance to get the speed you want (I think I used 2-3µF for my 12" fan).

You can mount the capacitor(s) in a plastic electrical box for safety.
In my case, I put the box in series with a standard AC extension cord.

Note that this is inherently safe since, even if a capacitor were to short, all that would do would cause the fan to go to its full speed.
 

wjbell

Jul 27, 2023
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An AC TRIAC lamp dimmer will reduce the fan speed, but introduce harmonics and often generates a buzz in the motor.
I am presently using a high-voltage capacitor in series with a fan to quietly lower its speed and the power it consumes.

Try a 1µF or more non-polarized (film type) 400V capacitors (example).
You can put more than one capacitor in parallel if you need more capacitance to get the speed you want (I think I used 2-3µF for my 12" fan).

You can mount the capacitor(s) in a plastic electrical box for safety.
In my case, I put the box in series with a standard AC extension cord.

Note that this is inherently safe since, even if a capacitor were to short, all that would do would cause the fan to go to its full speed.
Thank you thank you! The tinkerer in me says thank you sir! That's what I wanted to hear, I knew there had to be some sort of way that this could be done. Not discounting anybody else's suggestions! Even though other suggestions of going with DC fans might be better and simpler I still wanted to know how this could be done just for the sake of doing it, if that makes any sense. I'll probably follow this example and go out and buy these couple of components and dig into it and try it... Just for the sake of experimenting. That's just me. If by chance in the process I ruin a $16 fan, perfect! To me that's what this is all about... Lol
 

bertus

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Nov 8, 2019
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Hello,
Will this work?
That is a TRIAC based circuit like the light dimmer that crutshow mentioned.
It will work, but introduce harmonics and possible a buzz in the motor.

Bertus
 

Bluejets

Oct 5, 2014
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(I think I used 2-3µF for my 12" fan).
That is fairly close to most ceiling fans as well.
Range is normally around 1.8 to 2.5uF for the two standard lower speeds and direct for full speed.
Adding a 1uF like you said should be close, but the Op expects it to lower current consumption which is not going to happen (not noticable at least)
 

wjbell

Jul 27, 2023
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...but the Op expects it to lower current consumption which is not going to happen (not noticable at least)
That would be a nice benefit but as I suspected whatever is cutting down fan speed does not remove the energy being pulled but rather limits it from passing... but the energy has to go somewhere. Unless it has a way to get back to the source it's still getting used somehow (heat), it's still being spent.

What I don't understand is why one fan with three speeds has a slower speed and less power consumption than my fan with three speeds?
 

wjbell

Jul 27, 2023
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Probably something to do with the specs which have not been presented.
It definitely has something to do with the specifications of the fan.... I guess a better way to put it would have been what the relationship between a speed switch and the motor causes a motor to spin slower and pull less energy at the same time. Are different motors designed to run at different speeds or frequencies and how does the switch trigger those different speeds?

In other words, I had a similar three-speed fan that I bought from Amazon that I ran through my inverter. You could definitely tell the difference in RPM between the three speeds and also the power consumption in between the three speeds.
 
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crutschow

May 7, 2021
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the Op expects it to lower current consumption which is not going to happen (not noticable at least)
whatever is cutting down fan speed does not remove the energy being pulled but rather limits it from passing... but the energy has to go somewhere. Unless it has a way to get back to the source it's still getting used somehow (heat), it's still being spent.
Not sure how much the current will be reduced (since there may be some reactive current which dissipates no power), but the power certainly will be.
The capacitors dissipate no power, and since the fan will be using less power, then the power from the source must also be reduced.

There is no energy that has to go somewhere (what energy?).
 

Delta Prime

Jul 29, 2020
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There is no energy that has to go somewhere (what energy?).
I will do you one better!
Why pose the question?
To understand how electricity works ( no-one yet knows exactly how it works,but we have a very good idea) you need to get into quantum electrodynamics.
I'll be your Huckleberry.
Hehe!
 

wjbell

Jul 27, 2023
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Not sure how much the current will be reduced (since there may be some reactive current which dissipates no power), but the power certainly will be.
The capacitors dissipate no power, and since the fan will be using less power, then the power from the source must also be reduced.

There is no energy that has to go somewhere (what energy?).
I'm not sure I understand it, I'm trying to pretend that I do...

This image is kind of what I imagine going on. The volts are the power from the "wall", the fan speed sre the amps and the ohms are what I'm trying to do to limit the fan speed. (Not literally by the way just what this picture is illustrating) I would hope that with the ohms tightening the belt it wouldn't let as much energy through. But something in that scenario has to take some punishment. Some energy has to be doing something if it's not being allowed to turn the fan.
 

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crutschow

May 7, 2021
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The volts are the power from the "wall", the fan speed sre the amps and the ohms are what I'm trying to do to limit the fan speed.
Yes, but you need to understand AC power, which can be a little tricky.
There are resistive ohms and reactive ohms, and in this case they are reactive ohms from the capacitor, which do not dissipate power.
In a capacitor, the voltage and current are 90 degrees out-of-phase so if you multiply the instantaneous current by the instantaneous voltage, and average that over each cycle, there is zero average power.
In effect the capacitor stores the energy and then basically returns it to the source.

This is illustrated in the simulation below:

As you can see, the current through the capacitor (yellow trace) is 90 degrees out-of-phase with the voltage across the capacitor (green trace) while the capacitor is limiting the current to about 150mA peak (it would be 420mA peak without the capacitor).

The reactive power in the capacitor (red trace) is going between approximately plus 12W and minus 12W each cycle, thus the average power is zero, shown in small "Waveform" window.
(It indicates about 1mW due to the finite accuracy of the simulation calculation).

In contrast, the real power in the resistive load (blue trace) is always positive, and the net average power is about 4.4W (first Waveform window at bottom).

To verify, the second Waveform window at the bottom shows the average power from the V1 source, and it is 4.4W, that same as the resistive load.

So you can see that a capacitor is a lossless way to limit AC current.

All that make sense?

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1690758676181.png1690758395121.png
 
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Bluejets

Oct 5, 2014
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since the fan will be using less power,
When the voltage is reduced, the mechanical section is still creating a load, the friction of the bearings is the same friction level but more proportional to the applied power, the slip on the rotor increases and creates more current drain at lower voltages...on and on.
 

wjbell

Jul 27, 2023
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All that make sense?

Nope. It all sounds like Chinese to me.

Seriously though, I will reread and look at the illustrations and maybe it'll start to make sense. But at this moment I have to take your guys word for it and just try some stuff out. Which is another thing, I'm sure once I order a cap and then take the fan apart to try to figure out where I'm going to wire it in I'll probably have a dozen more questions..
 

crutschow

May 7, 2021
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the mechanical section is still creating a load,
Of course, but a fan load is greatly reduced as the fan speed is lowered.
the slip on the rotor increases and creates more current drain at lower voltages.
More slip only causes more current if the motor load is increased at a constant applied voltage.
 
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crutschow

May 7, 2021
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once I order a cap and then take the fan apart to try to figure out where I'm going to wire it in
Why not put it in a small electrical box, in series with the plug, as I did?
Seems like that would be a lot easier.
 

wjbell

Jul 27, 2023
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Why not put it in a small electrical box, in series with the plug, as I did?
Seems like that would be a lot easier.
Because then it slows down all the speeds. I don't want to slow down all the speeds I just want to slow down the slowest speed. So my thinking is that there's probably the main power feeding the switch which branches off into three separate power wires going to the motor. Fast medium and slow. So my thinking is I want to break into the slow wire and do the modifications there. Is that right or no?
 
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