Reducing fan speed

[355F1]

Hello,

My name is Orazio and, even though I have been a member for quite a while, this is my first post.

I have an old Dayton boxer fan [110V, .16A and 13W]. I would like to half the fan speed by reducing the voltage to, let's say 50V and, according to my calculation, I need a 4.3 watt resistor.

Any idea where to get just one 110V 4.3w resistor?

Thank you very much in advance.

Regards,

Orazio

[richie00boy]

You've not quite got it right, resistors are specified in ohms not volts.

Fan equivalent resistance R = V / I = 110/0.16=687.5 ohms

Therefore if you put a 687.5 ohm resistor in series with the fan you will halve the voltage the fan sees.

With approx 55 volts across the resistor it will dissipate P = V^2 / R = (55x55)/687.5=4.4 watts

The closest practical component is 680 ohms, 6 watts. Depending on the fan though it may not necessarily turn at half speed, you may have to experiment to find a suitable resistor to give you the speed you require.

[355F1]

Sorry, my mistake, I should have written 825 ohms and 4.3 watts per the following table:

Original fan voltage: 110V
Original fan current: 0.16A
V=IR (Ohm's Law), so 110 = 0.16R => R = 110 / 0.16 = 687.5
Fan resistance: 687.5 Ohms
Target voltage: 50V
V=IR (Ohm's Law), so 50 = 687.5I => I = 50 / 687.5 = 0.073
Target current: 0.073A
Voltage drop across resistor: 110V - 50V = 60V
V=IR (Ohm's Law), so 60 = 0.073R => R = 60 / 0.073 = 825
Target resistance: 825 Ohms
Power dissipated by resistor: 60V * 0.073A = 4.364W

Thanks a lot.

Orazio

[star882]

You cannot reduce the speed of an induction motor by decreasing the voltage.

[355F1]

I could use a dimmer but it is not very convenient and wanted to simplify things.

[boywonder]

Originally Posted by star882

You cannot reduce the speed of an induction motor by decreasing the voltage.

I've decreased the speed of my CPU fans on my computers with resistors, are they induction motors? They are certainly brushless. It works great.

[nigelwright7557]

I have used dropper resistors on PC fans too.

I use the running current of the fans to determine the resitor required.

I use two 12 volts fans in series with a resistor dropped from 60 volts.
It does need a 10 watt resistor tho.

[star882]

Originally Posted by boywonder

I've decreased the speed of my CPU fans on my computers with resistors, are they induction motors? They are certainly brushless. It works great.

Brushless DC fans are basically induction motors with built in inverters, but the frequency changes with voltage.

[cbdb]

If its a 110v ac induction motor with a speed thats 3600 rpm (or multiple) than its speed is based on the 60 hz (60x60=3600 rpm)of the line voltage, adding a resistor will probably increase the current the motor draws, keeping the speed approximately the same. A dimmer might work.

[355F1]

Chaps,

I have two of these fans, NOS - 25 yr old, cast iron, USA made - Dayton square AC axial fans, air flow 75 CFM, voltage rating @ 50/60 Hz 115, 4 11/16" square, 1 1/2" depth. Current rating is 0.16 Amp, power rating is 13W, speed 1800 RPM.

Regards,

Orazio