Reducing AC fan speed

[STEEDA]

I have a 120V AC fan which is a bit "noisy" so I want to add a series WW resistor to reduce the speed a bit thus reducing the noise.

I am pretty confident I can calculate the resistor (value & wattage) needed but after reading "conflicting" results (on the web) I want to be sure I'm understanding exactly what is "happening" when a series resistor is added.

1st source states that E will remain the same (120v) after the addition of the series R and current (I) will be reduced.
2nd source states that E will be reduced (say from 120v to 100v for example).

Obviously both can't be right! I thought I understood this, but now I'm not so sure!


Thanks -

 

[carbonzit]

It will definitely reduce the voltage. Think about it: what you've got there, good buddy, is your classic voltage divider. (It might also reduce current as a side effect.)

It's probably not the best way to reduce a fan's speed, but it just might work.

 

[Reloadron]

1st source states that E will remain the same (120v) after the addition of the series R and current (I) will be reduced.
2nd source states that E will be reduced (say from 120v to 100v for example).

The only voltage that will remain the same is the applied voltage. When a resistor is added there will be a voltage drop across that resistor. This will reduce the voltage across the fan. Since the overall circuit resistance will increase (Fan + Resistor) the overall circuit current will decrease.

If the fan specifications are known a resistor can be chosen to reduce the voltage across the fan as a result of reducing overall circuit current. Current limiting. This is fine for fans that don't mind the reduced operating voltage.

If time and space permit, a simple lamp dimmer would likely work from a home improvement store like Lowes. They can be as cheap as about $6.00.

Ron

 

[canadaelk]

If it is a permanent-split capacitor motor (used in many fans) you can vary the speed by changing the capacitor value. E

 

[crutschow]

I've used a non-polarized capacitor in series with the AC line to reduce the voltage and fan speed. It has the advantage on not dissipating any power. You may have to experiment but a value somewhere around 1-10µF should work for typical desk fans. One source for such caps is speaker crossover capacitors such as these.

 

[STEEDA]

The only voltage that will remain the same is the applied voltage. When a resistor is added there will be a voltage drop across that resistor. This will reduce the voltage across the fan. Since the overall circuit resistance will increase (Fan + Resistor) the overall circuit current will decrease.

If the fan specifications are known a resistor can be chosen to reduce the voltage across the fan as a result of reducing overall circuit current. Current limiting. This is fine for fans that don't mind the reduced operating voltage.

If time and space permit, a simple lamp dimmer would likely work from a home improvement store like Lowes. They can be as cheap as about $6.00.

Ron

Okay, whew.... That's what I initially thought. The voltage is being shared (resistor & fan) so the voltage across the fan will be less than 120V. So, to be sure I understand completely, the fan (on its own) will draw (say) X amnt of current but the new circuit (fan + RESISTOR) will draw a current "smaller/lower" than X (due to the increased TOTAL resistance). Correct? However, since the resistor is in series, the current through the resistor will be the same as the current trough the fan, Yes? Basically the same idea behind adding a series resistor to limit current in an LED.

This is what confused me ----> https://www.instructables.com/community/How-do-I-calculate-what-resistor-I-would-need/

That's a cool idea (light dimmer). Not the same as a pot/rheostat, right? From what I understand, modern dimmers work by switching on and off (many times / sec)?

If it is a permanent-split capacitor motor (used in many fans) you can vary the speed by changing the capacitor value. E

Hmmmm, not sure if it is or not? Would I have to disassemble the fan and look for a cap?

I've used a non-polarized capacitor in series with the AC line to reduce the voltage and fan speed. It has the advantage on not dissipating any power. You may have to experiment but a value somewhere around 1-10µF should work for typical desk fans. One source for such caps is speaker crossover capacitors such as these.

Yes....Funny you mentioned this.... I thought about doing this a while back for a LED which was connected via mains (120V). I couldn't calculate the correct size and therefore asked for help. The members (another forum) advised against this and recommended using a series resistor instead. I ended up using a large 5W flameproof Res. but my initial thought was that a bi-polar electrolytic cap would eliminate the risk of fire (Heat dissipation w/the Res.).





Thanks-

 

[crutschow]

....... That's a cool idea (light dimmer). Not the same as a pot/rheostat, right? From what I understand, modern dimmers work by switching on and off (many times / sec)? ......................

Light dimmers work by using an SCR or TRIAC to turn on after the start of each line half-cycle. It then automatically turns off at the end of each half-cycle. The percentage of each half-cycle time the SCR is on determines the average output voltage. It basically chops at the line frequency.

I tried a dimmer for my fan, which did slow the fan, but the chopping caused a buzz in the fan that was annoying. That's when I went to the series cap (which leaves the fan perfectly silent of course).

 

[Reloadron]

As to your question (the instructables link) about:

How do I calculate what resistor I would need?
Ok, so I'm working on a project. I need to know how I can reduce 18volts down to 12volts. If someone would explain how to calculate this myself I can figure out what resistor (or series of resistors) I need.

Well some pretty interesting answers.

This was like saying if I apply 10 volts to a pair of 5KΩ resistors in series I have a voltage divider. I will in fact have 5 volts at the junction of the two resistors with reference to common. Works wonderful right till I place a load out there at my 5 volts. Now my load is in parallel with one of my divider resistors and it is no longer 5KΩ but something less. Looked real good right up until.......

Actually the question as posted has no answer as not enough info was given. Well it can be answered. You go to the store and buy a 12 volt regulator like a LM7812. That will get you 12 volts at about up to an amp but the question made no mention of current required. Instructables sometimes leaves a little to be desired.

Carl has another good approach that I had forgotten all about. The use of a capacitor.

Ron

 

[STEEDA]

Light dimmers work by using an SCR or TRIAC to turn on after the start of each line half-cycle. It then automatically turns off at the end of each half-cycle. The percentage of each half-cycle time the SCR is on determines the average output voltage. It basically chops at the line frequency.

I tried a dimmer for my fan, which did slow the fan, but the chopping caused a buzz in the fan that was annoying. That's when I went to the series cap (which leaves the fan perfectly silent of course).

Good to know (regarding the dimmer). I certainly don't want to increase the dB level; I'm actually trying to decrease the noise the fan already makes by slowing its speed a bit.

Okay, so on to the capacitor..... How would I go about calculating the proper value? What should the min. voltage rating for the cap be? And should it (the capacitor) be placed on the AC line or neutral, or does it matter? (My guess would be to place it on the AC line) but am not 100% sure.

 

[crutschow]

There's not an exact method to calculate the required cap since it depends upon the inductive reactance of the fan motor and how much you want to slow the fan. When I did mine I purchased two 2uF and one 1uF, 400V film capacitors to try (I used speaker crossover types from Parts Express). I then put them in parallel as needed in series with the fan power to get the fan speed I wanted.

A 400V cap rating should be adequate for 120VAC.

It doesn't matter whether they are placed in series with the hot or the neutral side of the AC line.

 

[STEEDA]

There's not an exact method to calculate the required cap since it depends upon the inductive reactance of the fan motor and how much you want to slow the fan. When I did mine I purchased two 2uF and one 1uF, 400V film capacitors to try (I used speaker crossover types from Parts Express). I then put them in parallel as needed in series with the fan power to get the fan speed I wanted.

A 400V cap rating should be adequate for 120VAC.

It doesn't matter whether they are placed in series with the hot or the neutral side of the AC line.

Thank you... I actually have a few film caps left-over from a recent crossover project. I'll try a few until I reach the desired fan speed.


Just curious.... How's a fan with several/multiple speed settings controlled? You know, your regular multi-speed portable household unit. Do they incorporate several different value capacitors OR is it controlled via a different method?

 

[crutschow]

I believe some multi-speed fan motors switch the number of windings to change the number of poles and thus the motor speed.

 

[Reloadron]

I believe some multi-speed fan motors switch the number of windings to change the number of poles and thus the motor speed.

My ceiling fans do.

Ron