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Slow down AC fan speed

wjbell

Jul 27, 2023
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Okay this is my first post to these forums hopefully it's in the right spot.

I have a 3-speed fan that the slowest speed it's not very slow. It's an AC current fan but I haven't hooked up in my RV through an inverter that provides me house power. The issue I'm having is that the speeds are not very far apart. When I have it on the slowest speed it still sucks down the battery. I haven't taken it apart yet but I'm thinking that after the switch the power going to the motor is probably reduced by resistors. I don't know I'm just guessing. So I want to put a more resistant resistor in between the low setting on the switch and the motor. I know it's kind of a relative thing and I wouldn't even know what size or capacity resistor to use. (And what RPM am I trying to slow it down too?)

One of the questions is, if I put a resistor in line that's cutting down the power to the motor therefore reducing the RPM is the motor still drawing the full power and the resistor is just creating heat as a way to dissipate that extra power? If that makes any sense. So in other words, with a modified resistor would the motor still be drawing the same amount of power from my battery in order to create the AC power? And what size resistor, or class of size of resistors, would I be looking at to splice in?
 

Delta Prime

Jul 29, 2020
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I have a 3-speed fan
Make, model number, link of your AC three-speed fan please.
It's an AC current fan but I haven't hooked up in my RV through an inverter that provides me house power.
"but I haven't" meaning (have not)?
I'm thinking typo.
Do you mean "but I have it"?
I think I know what you're saying but I do need clarification.
You're running your household electrical system through an inverter in your RV that's powered by batteries?
I know that sounds silly but that's how I read it.
As for the rest of your statements I have 10 questions & 10 clarification requests. So I'm not even going to touch those.
 

Bluejets

Oct 5, 2014
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When I have it on the slowest speed it still sucks down the battery.
Trying to slow it down will do next to nothing for you.
The inverter efficiency would attribute more to that than the fan.
Forget resistors, they will do nothing either.
Same goes for any capacitor on the ac side.

Get a bigger battery or, failing that, one of those manual ones with the palm fond.
 

wjbell

Jul 27, 2023
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Make, model number, link of your AC three-speed fan please.

"but I haven't" meaning (have not)?
I'm thinking typo.
Do you mean "but I have it"?
I think I know what you're saying but I do need clarification.
You're running your household electrical system through an inverter in your RV that's powered by batteries?
I know that sounds silly but that's how I read it.
As for the rest of your statements I have 10 questions & 10 clarification requests. So I'm not even going to touch those.
Here's a link to the fan:

Model # HPF820BWMV1

Yes typo, I meant I have it...

Sorry if it was confusing. It's an inverter that runs off the RVs 12 volt battery. I have this desktop fan plugged in to the inverter and that is meant to run off 120 volts ac but since I have an inverter that turns DC current into AC current I can run the household fan in my vehicle.

I'm just trying to slow down the speed of the fan so that it doesn't draw as much power. I'm trying to do this has a little project for learning. Yes, I could just go buy another fan. *OR* being the curious tinkerer that I am, I could go on to an electronics forum and maybe find out how I can do it with a couple of components and maybe learn something... (For those that replied things like, add more batteries to your RV or just buy another fan, or other such silliness) sometimes there's clever ways to do things if you put some thought into it. Which I'm sure is a reason a lot of people are here.
 

ivak245

Jun 11, 2021
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It's probably a shaded pole motor, so a light dimmer should work. To run it at low speed you will have to start the controller about midway and turn it down.
 

kellys_eye

Jun 25, 2010
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Helpful, thanks.
You discount my suggestion too quickly. If you are concerned for power consumption maybe it's time to look for a more efficient fan? One with better control as standard? Maybe even multiple fans (ex PC fans for example) that could run directly from 12V and thus bypass the need for running an inverter for 'just' the fans???
 

wjbell

Jul 27, 2023
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You discount my suggestion too quickly. If you are concerned for power consumption maybe it's time to look for a more efficient fan? One with better control as standard? Maybe even multiple fans (ex PC fans for example) that could run directly from 12V and thus bypass the need for running an inverter for 'just' the fans???
The inverter runs many things in the RV. I didn't buy a dedicated inverter just because I wanted to run some fans. I also want to slow down the fan speed because even on the slowest speed it's too fast. I feel like it needs some more variation between the slowest speed and the fastest speed.

I discounted your suggestion because I came on to an electronics forum to get some tips on electronics. If something is tweakable with a little component here or there I enjoy doing stuff like that. My mind goes that direction first. If I just thought I needed a better fan I would have just went on a fan forum. No offense.
 

wjbell

Jul 27, 2023
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Doesn't work like that, slowing it down by any means will not "save you power".

Well, that was part of my original question, if you had read it. Part of the question was if power is run through a resistor is the resistor going to do away with the power in its own way (as in creating heat) therefore maybe slowing down the motor but not saving any power. So, thank you for that answer. That's exactly why I'm here.
 

Delta Prime

Jul 29, 2020
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Make model link to your inverter.
Have you tried plugging it in to mains utility power?
There should be a very distinct speed.Low,medium and high if not and you have recently purchased it take it back for either a refund or another fan. If there is a very distinct noticeable change of speed speed . Pure sine wave inverters mimic or reproduce the equivalent of utility grade power, the wires that come off a pole to your residential house.
Modified sine wave inverters suck at charging batteries.
Motors pumps and heaters can tolerate the poor Total Harmonic Distortion(THD) because of the wave shape itself it is distorted
Electronic circuits microprocessors controllers do not respond well they can't filter out the distortion good enough.
Those fans are popular and I have them and there's a big difference between low and high even medium.
Adding that little fan could be the straw that broke the camel's back there's only so much you can run with an inverter this is why I'm asking the questions.
And don't come back with questions like why then if I do this this works and this doesn't work how come it hasn't done it before it works fine with other things I don't have time for that. you want to tinker and learn we could do that as well but you have a bigger problem I think.
 

wjbell

Jul 27, 2023
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Okay. I don't think I've come back with any questions saying well if I did this why didn't this work or anything like that. I think my questions have been pretty straightforward.

Anyway, there is a difference in motor speed in the different selections but they don't seem like they were set from the factory to be great enough of a difference. That's why I came here to ask what I can do about it.

As far as the make and model of the fan I provided that the last reply I replied to you.

As for the inverter, it's a pure sine wave 3,000 watt inverter. I understand the different electronic components act differently with modified versus pure sine wave. I bought a pure sine wave inverter because I wanted sensitive electronics to operate as natively as they could. I don't think something as simple as a fan is going to be that touchy in the first place. So anyway I'll try somebody else's suggestion of a light dimmer. If that doesn't work I'll just return the damn fan. I didn't think this conversation would go on this long and be this difficult for a simple way to slow down the fan. I didn't think it was rocket science. Thanks anyway.
 

Delta Prime

Jul 29, 2020
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As far as the make and model of the fan I provided that the last reply I replied
Yes you did that's why I told you I have one or a couple of them.
Yes do as @ivak245 has suggested believe me.... you're going to learn. All these suggestions were made because everyone here has been there and done that and even designed that and this is your thread everyone here, except me are professionals and we're trying to save you the frustration of going through that process, but it is a learning process.
Good luck to you.
 

kellys_eye

Jun 25, 2010
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I didn't think this conversation would go on this long and be this difficult for a simple way to slow down the fan. I didn't think it was rocket science. Thanks anyway.
Without going back and quoting your first post your concern seemed to be for the wasted power (that a dropper resistor would create). You also mention the use of a pure sinewave inverter for the sensitive electronics you want to use.

If you use a triac motor speed controller you're introducing the distortion/harmonics your pure sinewave was meant to avoid - so that's out.

If you use a resistor you are correct in assuming the power isn't saved, just wasted in the resistor.

There is no reason to create an electronic solution when the KISS principle applies. A 12V fan is easily controlled for speed, it is more efficient in that you aren't creating losses in the DC-AC conversion process. If you want to 'play' electronics then get a DC motor speed controller (PWM) for a DC fan.

There is no 'magic' electronic solution. Even on an electronics forum we don't automatically create an electronic solution when a more practical one is readily available - we don't re-invent the wheel.

You have contradicted yourself and your reasons for discarding various offered solutions - including my perfectly practical, reasonable and KISS-principled offer.

But we, apparently, know nothing......
 

wjbell

Jul 27, 2023
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You have contradicted yourself and your reasons for discarding various offered solutions - including my perfectly practical, reasonable and KISS-principled offer.
??
But we, apparently, know nothing......
Knowing how to make a novice feel like his ideas don't make any sense with solutions that technically have absolutely nothing to do with the question is something. (Buy more batteries... )
 

Delta Prime

Jul 29, 2020
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All this is very dangerous with or without an inverter.
No one wants to see you get hurt.
Or anyone for that matter.
I am not so pretentious as to speak for others.
I am not trying to beat you up.
(K. I. S. S.) Is an acronym.
( Keep. It. Simple. Stupid) and no one's calling you Stupid or Simple.
It's commonly used in various disciplines.
In Electronics it is an reminder.
The simplest way is the best way.
(KISS method)
But I use the SWAG method
S. W. A. G.
(Scientific. Wild. Ass. Guess.)
Whatever you choose to do, if you have any questions please do not hesitate to ask
 

wjbell

Jul 27, 2023
34
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All this is very dangerous with or without an inverter.
No one wants to see you get hurt.
Or anyone for that matter.
I am not so pretentious as to speak for others.
I am not trying to beat you up.
(K. I. S. S.) Is an acronym.
( Keep. It. Simple. Stupid) and no one's calling you Stupid or Simple.
It's commonly used in various disciplines.
In Electronics it is an reminder.
The simplest way is the best way.
(KISS method)
But I use the SWAG method
S. W. A. G.
(Scientific. Wild. Ass. Guess.)
Whatever you choose to do, if you have any questions please do not hesitate to ask
I know what KISS stands for. I just don't agree with the statement that I contradicted myself... And thank you for keeping the welcome mat out..
 
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