Slowing Down a Fan (not in a computer)

[SMurf]

Hello,

Recently I bought an 18-inch desktop fan, ready for the summer. It has a rotary switch on it for three speed settings, but even the lowest setting is a lot like the back of a hovercraft.
The whole point of my buying a largish fan was that it could run more slowly and quietly, while still moving a decent volume of air.

With this in mind, I would prefer not to crack the control unit open in case I still want to return it for a refund.

Is it possible to in some way constrain the AC (110-230V, 50/60Hz) input so that it runs more slowly? Or would I need to work on the DC side?

[phantomotap]

O_o

I'm not much of an electrics guy so I don't even know if you could reasonably do this on the AC side without spending more than what a new fan would cost.

That said, I think it would be safer to operate on the DC side in any event so that has my vote.

Soma

[jimblumberg]

You could use a reostat to control the voltage to the fan, thereby reducing the velocity. Or you could use PWM to reduce the duty cycle. But probably the least expensive approach would be to return the fan for a smaller, lower velocity fan.

Jim

[cyberfish]

It depends on the design of the fan. If it uses an AC synchronous motor, there's not much you can do. They run at a fixed ratio of the AC line frequency, and you can't change the frequency easily (short of moving to Japan). Decreasing the voltage will just make the fan run rough/noisy/break.

If it's a DC motor decreasing the voltage may or may not work. It may not work because if the fan has a closed loop regulator it will just draw more current at the lower voltage to get the same voltage to the motor, and spin at the same speed.

In short, there is probably no easy way.

[megafiddle]

A smaller fan is probably your best solution.

Multispeed fans typically use various taps on the field winding to select speed. Each tap effectively
provides the fan with a motor of a different horsepower.

A light dimmer type control reduces the effective voltage supplied to the load. There is a couple
problems with that. If the horsepower is reduced below some certain value, the fan motor will
be running in an overloaded condition, and will overheat. Also, dimmer type controls have a high
frequency component in the power supplied to load. It is not a sinusoidal voltage, and fan motors
are not typically designed to handle this high frequency voltage.

-

[Elkvis]

Chances are it's just a typical AC induction motor - the most common type of AC motor. Synchronous AC motors are actually quite uncommon. even 3-phase motors are much more common. With an induction motor, you can reduce the voltage and it will run at a lower speed. Look for a 5-amp variac on ebay. That would work really well for your purposes. Set the fan on high, and then use the variac exclusively to control the speed.

[Matticus]

Perhaps the simplest (safest, cheapest) solution would be to mount a vent to the front of the fan...

[grumpy]

It may not work because if the fan has a closed loop regulator it will just draw more current at the lower voltage to get the same voltage to the motor, and spin at the same speed.

Drawing more current doesn't produce the same voltage at the motor. It simply means the motor will draw the same power (and hence spin at comparable speed).

In any event, it would probably be easier and safer to get a fan that meets the need, rather than trying to coerce a fan to behave differently. Also easier to check the fan in operation before buying it.

[cyberfish]

Drawing more current doesn't produce the same voltage at the motor. It simply means the motor will draw the same power (and hence spin at comparable speed).

In any event, it would probably be easier and safer to get a fan that meets the need, rather than trying to coerce a fan to behave differently. Also easier to check the fan in operation before buying it.

Unless it has a switching regulator, that will give the motor the same voltage, and the motor will draw the same current when it sees the same voltage.

[SMurf]

It is an AC induction motor. 50 Hz.
I am itching to open the control box, because I would be interested to see what the rotary switch is doing to the circuit.

[jimblumberg]

Probably just switching to different "taps" of the motor windings, thereby changing the resistance of the windings to chang the speed of the motor.

Jim