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Electrical wiring question: fans, dimmers, and capacitors.

Jeff7

Jeff7

Lifer
Alright, situation - I've got a 120V box fan installed in a vent here, but it's kind of loud. I bought a nice fan controller that has three speeds - High, Medium, and Low. Problem: The only difference between the speeds is that Medium makes the fan hum a bit. Other than that, they're all exactly the same speed. Link, model number is FSQ-2F, most of the way down the page.
Best option - return the fan controller....but then I still need a controller for this thing.

A standard dimmer switch does not work - they just make fans hum oddly, as they are tailored for incandescent lights. Question:
I've got a nice motor-starter capacitor here. Might I be able to hook it up permanently to the fan, so that a standard dimmer would work?

Or do I go back to the store and say, "This is the fan, give me something that will make it spin slower."
 
if possible go with a squirrel cage blower style. i dont think you can avoid the hum with a cheap motor fan without extensive reworking to the motor.
 
This isn't a cheap fan - it's made by NMB, and I think it's pretty high quality. It doesn't have much motor hum by itself - it just moves a lot of air. Plus, the fan is kind of installed already. I got the fan controller because not everyone living here tolerates fan noise well.

I didn't think a fan controller would be a big deal - they're fairly cheap for PCs, so I figured a simple AC model wouldn't be that bad. The fan only pulls about 0.17 amps too.
 
I don't need over 600W. This fan is rated only 1.06 amps, 120VAC.....wait a sec, that isn't right. Lemme check, I think I remember semething now....

Ok, scratch all that. This thing draws 0.17 amps. I thought something seemed odd about 1 amp. This fan was in American Science and Surplus, brand new, because there was a misprint on the label. The amp rating was printed as "18/17" which I took to mean 18 divided by 17. This had a misprint - there was a decimal missing in front of the 17.


120VAC, 0.17 amps <-- correct rating of this fan.
 
You need a multi speed motor.

You cannot control the speed of a shaded pole motor with a triac based dimmer. It will hum you a tune of denial. 🙂

You can get a controller that can vary the speed of this motor but it isn't cheap. A discrete two or three speed blower motor is what you want along with the positional switch and wiring.
 
Either that, or find an efficient 12V power supply and get a Sunbeam fan controller. 🙂


I wonder if that might turn out to be the easiest solution. Come to think of it, I've got a cheapo fan controller I got on clearance at SVC. It made a quiet buzzing noise, enough to drive me nuts when it was mounted in the PC, right next to me as I used the computer. But if it's mounted in the house somewhere, that noise might not be noticable.


Or, an expensive solution: a variac.
 
A variac is not the proper way to control the speed of a shaded pole motor!

Want something quiet and effective? Build a reverse system with the blower as high as possible and run an oversized flex line to the opening in the lavatory. This will remove hundreds of CFM more than your typical nutone fan and in practically total silence.
 
I figured as much; the variac option was a bit of a joke too, guess I should have been cleared on that. 😱
Yeah, plus a variac is kid of a big hunking block, isn't it?

This isn't a bathroom fan (though it'd be nice to replace that one, as it's fairly loud) - this is to help move warm air from the cellar to a bedroom upstairs. The fan is mounted to the underside of a hole in the bedroom floor. I'll not be the one in the room - it'll be my mom sleeping in there. She does not like fan noise of any kind, and it's tough for me to know how quiet is quiet enough, as I find the sound of a fan to be fairly soothing. Quite the opposite with her. I'm pondering a Panaflo and 12V power supply. Be nice to have a 12V power supply that won't keep drawing power even when it's "off."
 
You can still cross ventilate with a 1/3 hp Emerson split phase + Lau squirrel cage combo. (GE and Dorr make nice high eff motors but avoid Dayton hehe) The key is creating a negative pressure within the room. Mounting the blower on resilant pads and using oversized flex from a collar on the suction side (can pan between two rafters too) will move a TON of air very quietly. Heck you will have a breeze going on! 🙂

Originally posted by: ThisIsMatt
Single speed brushless fans really aren't meant to be speed controlled.
Click to expand...


Not externally but a few facilitate control through a control line built into the electronic brushless commutator driver circuitry that works beautifully.
 
If you want to get rid of the hum, balance the blade. Remeber, any noise the fan makes will be tunneled though the vent, so it will sound louder as there are not any surfaces to disapate the sound energy.
 
How much does all that cost? I'd also like to keep the cost to a minimum. And I do mean minimum, like, using parts on hand would be nice. 😉 Kind of limiting I know....
The $17 fan control I bought initially was a bit pricey for this project.
 
Originally posted by: Evadman
If you want to get rid of the hum, balance the blade. Remeber, any noise the fan makes will be tunneled though the vent, so it will sound louder as there are not any surfaces to disapate the sound energy.
Click to expand...

😕

Humming caused by an inbalanced blade would be more of a hammering, thumping sound which would vary directly proportionate to shaft speed. The humming the OP was hearing was caused by an electrical change from using a TRIAC device intended for incandescent lighting.

Also if the fan blade is out of balance, it is damaged either from a foreign object coming in contact with one of the blades (pitch changed) or a weight was removed from the blades which is doubtful in this case as it's most likely a single piece stamped steel design.

Originally posted by: Jeff7
How much does all that cost? I'd also like to keep the cost to a minimum. And I do mean minimum, like, using parts on hand would be nice. 😉 Kind of limiting I know....
The $17 fan control I bought initially was a bit pricey for this project.
Click to expand...

Try a junkyard. There are plenty of knowledgeable operators out there that can point you in the right direction. You want a smaller air handling unit with a fractional hp split phase motor with FLA < 10 at 115VAC. Some of these will have multi speed motors so you can either wire them for a speed or use a relay/contactor and use a switch in a convenient place to change the speed as desired or connect to a thermo/humidstat, etc.
 
Originally posted by: C6FT7
Originally posted by: Evadman
If you want to get rid of the hum, balance the blade. Remeber, any noise the fan makes will be tunneled though the vent, so it will sound louder as there are not any surfaces to disapate the sound energy.
Click to expand...

😕

Humming caused by an inbalanced blade would be more of a hammering, thumping sound which would vary directly proportionate to shaft speed. The humming the OP was hearing was caused by an electrical change from using a TRIAC device intended for incandescent lighting.

Also if the fan blade is out of balance, it is damaged either from a foreign object coming in contact with one of the blades (pitch changed) or a weight was removed from the blades which is doubtful in this case as it's most likely a single piece stamped steel design.
Click to expand...

Sorry, based on his first post, it sounded like the original controller had 3 settings, and the medium hummed, so he tried a regular dimmer.

I used to have a 20" box fan that hummed on low but not on med or high, so I stuck it on the point of a nail and noticed it leaned to one side. I shaved off bits of the trailing edhes until it set level, and the humming went away.

You are correct in that the frequency would be totaly dependant on shaft speed. However, the frequency will effect the harmonics. It is entirely possible to 'hear' noise at once speed and ahve it go away at higher speeds. For example, if a driveshaft is out of balance on a vehicle, you may feel it only between 60 and 70 miles an hour. It is a very common happenstance.

You are also making the assumption that the blade was designed and manufactured correctly. This isn't a life critical fan, albeit it is a good one. A gram or 2 in the right place can screw up a perfect balance.
 
Let's not also forget that a lot of novices will "play" with the naked motor and the controller and think it's broken! It doesn't take much of a change in shaft speed to make a big change in the speed at the blades where the air movement takes place.

That's what amprobes and strobescopes are for. 🙂
 
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