# How to slow down a fan.

**URL:** <https://boards.straightdope.com/t/how-to-slow-down-a-fan/414082>\
**Category:** Factual Questions\
**Created:** [August 2, 2007, 4:04am UTC](https://boards.straightdope.com/t/how-to-slow-down-a-fan/414082 "2007-08-02T04:04:10Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\
**Post date:** [August 3, 2007, 1:54am UTC](https://boards.straightdope.com/t/how-to-slow-down-a-fan/414082/21 "2007-08-03T01:54:05Z")

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[QUOTE=cornflakes]  
You mean the theory of this, right?  
[/QUOTE]  
No. I mean how effective is it? The diode results in a DC component to the input which does nothing but heat the motor. The frequency is the same and so the speed will be the same, or nearly so, as long as the available torque can turn the load.

As **Kevbo** said, if a fan has an induction motor there isn’t much you can do about the speed. They pretty much run at near synchronous speed (3600 rpm/number of pairs of poles) with some slip or they don’t run at all.

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**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\
**Post date:** [August 3, 2007, 4:43am UTC](https://boards.straightdope.com/t/how-to-slow-down-a-fan/414082/22 "2007-08-03T04:43:19Z")

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I guess I should add that you can’t do much about the speed without a sophisticated controller.[This article describes three methods.](http://www.designnews.com/article/CA486922.html) One uses a microprocessor and there are two variable frequency units.

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**Author:** ![EvilGhandi](https://avatars.discourse-cdn.com/v4/letter/e/f17d59/32.png) [@EvilGhandi](https://boards.straightdope.com/u/EvilGhandi)\
**Post date:** [August 3, 2007, 7:22am UTC](https://boards.straightdope.com/t/how-to-slow-down-a-fan/414082/23 "2007-08-03T07:22:26Z")

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**[t-bonham@scc.net](mailto:t-bonham@scc.net)**

You’re shitting me, right?

Quoth  
“The typical dimmer switches are really cheap, and produce a lot of stray electrical noise”

True dat, is the plug right behind the stereo? I didn’t ask. I was guesing it was at least a foot or two away. If the stereo picks up clicks and pops from that then the solution lies in the stereo itself.

Shit i run a really big fan (enough to cool my house in Hawaii) less than two feet from my audio system. The only noise I get is from the fan itself. That’s why I installed the celing fan switch on the cord. Damn thing can wake the dead on low.

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**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\
**Post date:** [August 3, 2007, 9:24am UTC](https://boards.straightdope.com/t/how-to-slow-down-a-fan/414082/24 "2007-08-03T09:24:56Z")

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[QUOTE=EvilGhandi]  
**[t-bonham@scc.net](mailto:t-bonham@scc.net)**

You’re shitting me, right?

Quoth  
“The typical dimmer switches are really cheap, and produce a lot of stray electrical noise”  
[/QUOTE]  
No, I’m not.

The electrical noise is injected into the electrical supply, and travels thru the house wiring. Physical distance of a few feet from the stereo system is not relevant.

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