Lower fan speed?

[daz]

Sorry, i know this isn't music related but i don't know where else to ask and there are a lot of knowledgeable people here. I have a box fan with 3 speeds and the slowest speed is much too high. The 3 way switch is sealed and theres no way to open it short of breaking it, so i don't know how the different speeds are accomplished. Can anyone tell me how i can lower the sped, even if the overall speed so all 3 speeds are lower? Thanks

[Jazz P Bass]

Short of 'dumping voltage' there is nothing that you can do.
The three way switch is selecting specific windings inside the motor.
It is what it is.

[daz]

Short of 'dumping voltage' there is nothing that you can do.
The three way switch is selecting specific windings inside the motor.
It is what it is.

Damn. Well, thanks.

[Bruce / Mission Amps]

AC or DC motor?

[daz]

AC or DC motor?

Didn't know there WERE any DC house fans. I guess AC

[Bruce / Mission Amps]

My mistake... you said box and I was thinking Boxer fan for cooling off your power tubes.

[tedmich]

a cheap "router speed control" may work, Harbor freight has one for $20
Router Speed Control
about the price of another smaller fan

[eschertron]

I haven't put a tach on any of my house fans, but I agree that the three speeds available by using the winding selector method are unsatisfactory. The only fan I have that offers a good variety has all its controls in the base of the unit, and also features a capacitive kill switch that stops the fan when anyone touches the fan blade cage. Definitely more than just a set of contacts inside.
Hypothetically, would a half-wave rectifier make an AC device see ~55VAC at 30Hz? Would the non-sinusoidal waveform wreak havoc on the fields generated inside the fan motor? daz, I'm not suggesting that you try that, just wondering out loud. Might be something I'd be willing to try out on a fan that I didn't mind seeing catch on fire...

[daz]

Yeah, i'm just going to buy another fan. $30 down the tubes. What really pisses me off is i have bought this exact same model/brand for 20 years or more, and with this last one the cost went up $10 from what they always used to cost and they ruined it by making the slow speed nearly like the fast was.

[olddawg]

Depending on the current involved you might try just using a triac light dimmer switch like you replace the light switches in your house to be able to dim the light. These control the phase conduction of the AC waveform. If it isn't a huge fan it will probably work. Just get the one that's for multiple floodlights in a room. They have them in any hardware store.

[J M Fahey]

Triac dimmers work well (and are meant for) carbon/brush "universal" motors (such as what you have in your drill).
Not very useful on these induction, capacitor ones, which love only "real" AC (meaning sinusoidal).
And they have *huge* phase shifts, as part of the design.
If you have built a neat safe Lamp Bulb Limiter you can plug your fan there.
Problem is this kind of "permanent capacitor" induction motors (not sure about the US name) have low starting torque to begin with, and it might even not start unless you push the blade a little with your finger.
Of course, if you have a spare Variac, it will work very well.

EDIT: check "Permanent-split capacitor motor" , that's what you have.
http://www.allaboutcircuits.com/vol_2/chpt_13/9.html
Probably they are being built cheaper and cheaper every passing year (said built, not sold) and latest ones have very poor bearings which make them easy to stall with just a little dirt trapped , so they do not dare to allow lowering torque below a certain value. Oh well.
PS: *maybe* you can add a switch and halve the present capacitor value .... remember it's a *motor* rated capacitor, unlike anything you may have in your parts bin.

[R.G.]

There's a really useful way using your audio equipment. Set a signal generator to 60Hz. Feed this into a BFA (Big Freakin' Amplifier ) and feed the output of the amp to the fan's line cord. The amp has to be big enough to run the fan, naturally. You can use power transformers backwards to up-vert the voltage as needed.

With the fan running from the amplifier turn the signal generator frequency down. The fan motor is almost certainly a squirrel-cage induction motor. As such it's speed is dependent on the frequency of the line. The voltage has to be enough to run enough current through it to give it enough torque to start and run under load, but the frequency is what sets the speed.

This is the scheme used for variable speed on large machine tools. They use horsepower + motors running from 3-phase electricity, and Variable Frequency Drives to vary the actual speed.

[J M Fahey]

Absolutely true, congratulations.
That's why dimmers do not work on this type of motors, they are (or try to be) syncronic with line frequency.

[Steve A.]

Problem is this kind of "permanent capacitor" induction motors (not sure about the US name) have low starting torque to begin with, and it might even not start unless you push the blade a little with your finger.

Cheap box fans don't have run capacitors- they don't need a high starting torque (like a garbage disposal.) We call them "shaded-pole" motors here in the US. They can have multiple taps for the different speeds. Broan makes a variable speed controller for their low HP exhaust fans. I believe that the circuit is basically the same as a light dimmer BUT there is a trim pot that you use to set the minimum speed. If you set that control too low the motor will just buzz but not start.
If you add a few drops of oil on the shaft next to the bearings every year or two the motor can last a very long time.

Steve Ahola

[Steve A.]

Problem is this kind of "permanent capacitor" induction motors (not sure about the US name) have low starting torque to begin with, and it might even not start unless you push the blade a little with your finger.

Cheap box fans don't have run capacitors- they don't need a high starting torque (like a garbage disposal.) We call them "shaded-pole" motors here in the US. They can have multiple taps for the different speeds. Broan makes a variable speed controller for their low HP exhaust fans. I believe that the circuit is basically the same as a light dimmer BUT there is a trim pot that you use to set the minimum speed. If you set that control too low the motor will just buzz but not start.
If you add a few drops of oil on the shaft next to the bearings every year or two the motor can last a very long time.

Steve Ahola