Adjusting Ac Fan Motor Speed With Added Resistors?

[Tim89531]

Hi,

I'm a DIY in the middle of Egypt, no easy access to parts.  I know a little about electrics, but that's a dangerous thing!

I have a floor-standing fan [pic attached] with 3 speeds + off.  On the lowest setting the fan is way too powerful for my needs and quite noisy.  My bad, I should have bought a smaller one.  

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My question is: can I slow it down by adding a resistor near the switch?

I attempted a fix which I detail below, but it didn't work.  I'd also be interested to know why it didn't.

This is what I did:

1) I disassembled switch.  Found that the 3 speeds seemed to be connected to 3 different wires going to the motor, plus 1 neutral.  I didn't open the motor itself [it was welded or glued shut].

2) Whilst fan disconnected, I measured resistance between each wire and the neutral.  The resistances were in the range of 30-70 ohms.  I had no idea why these values did not match the stated wattage (70W; V^2/R = 70, R should = 768.  

3) I disassembled an old piece of electronics and found a 1k Ohm resistor.  Figured this ought to slow the thing down.

4) I soldered this resistor in series with one of the wires coming from the switch.

5) Turned fan on, the resistor burst into flame and the fan stopped.  

6) Removed resistor, fan now working as originally.

I reckon now that the resistance measurement while the fan is off is probably useless, since the circuit is AC whose capacitors/inductors that don't figure well into Ohm's law.  

But even given this, I wouldn't have predicted that the 1k resistor would just burst into flame.  Is this because of a start-up capacitor suddenly delivering a lot of energy into it?

Any comments appreciated!

Tim

 

[IzzyS]

For an explanation, try working out the power dissipated by the resistor (P=V^2/R), then go and buy some ear plugs and/or move the fan further away you....

 

[kme]

The only two sensible options are:

1. Rheostat, rated at the correct wattage. The fan lifespan may be negatively affected - and it is worth noting that these fans, on lower speeds, often draw MORE power than mid- or high- speed.

2. An extra mechanical load or resistance ( steeper blade angle etc) would slow the rotation for a given motor power - this WILL negatively affect the lifespan of the motor.

The less-than-sensible option would be to gear the speed down - not easy to fabricate or fit, in your position.

Sorry - In your shoes, I`d get a smaller fan

KME

 

[Andy™]

ac motor speed is determined by frequency. adding a resistor will not work too well

 

[ProDave]

Sometimes a capacitor in series will work, Often refered to as a "wattless resistor"

 

[Marvo]

There's a good chance that the motor efficiency is dropping as the speed reduces so more heat is produced inside the motor. Add to this the reduced airflow across the motor and you've got a recipe for burned windings and bearing failure. Secondly it sounds like you're lucky to still have your eyebrows so maybe play it safe and just buy a smaller fan. Here's a link for Egyptian classified ads, you can sell your old fan to offset some of the capital outlay of the new one.

 

[Tim89531]

Thanks everyone for your input!

Tim