Reducing power of a stand fan with a permanent split capacitor (PSC) induction motor

Thread Starter

LMF5000

Joined Oct 25, 2017
47
I just bought a new wall fan (the oscillating kind with plastic blades with 3 speeds) and it's too powerful on the lowest speed setting (too much noise and wind for the room). How can I reduce the power and speed below the lowest setting?

I've measured an input of 53 Watts at 240 V in the lowest speed. The other fans in the house have low speeds ranging from 35 to 45 watts, so I want to lower this one to that power level by shaving off 5-7 watts.

I've tried a triac-based dimmer in series, and it worked (was able to start and spin the blades with the dimmer turned down to 40 watts of input power, at which point the output from the dimmer was 204 Volts) but it made the motor buzz audibly at mains frequency (50Hz).

Are there any alternatives that don't make audible noise? Maybe resistors and/or capacitors in series? I can't return the fan because I've already changed the plug from the 2-prong EU plug to the 3-prong UK type to match the sockets in my house.
 
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crutschow

Joined Mar 14, 2008
24,967
I had the same problem with noise using a dimmer to reduce the speed of a fan so I added a non-polarized film (not electrolytic) capacitor in series to reduce the speed.
I think I used about 4μF for my 120V fan, but you would likely need less for a 240V one.
I bought 1μF ones and put them in parallel to get the desired speed reduction.
The capacitors should be rated for a least 400V.
Motor Run (not start) capacitors would be a good option.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
47
I had the same problem with noise using a dimmer to reduce the speed of a fan so I added a non-polarized film (not electrolytic) capacitor in series to reduce the speed.
I think I used about 4μF for my 120V fan, but you would likely need less for a 240V one.
I bought 1μF ones and put them in parallel to get the desired speed reduction.
The capacitors should be rated for a least 400V.
Motor Run (not start) capacitors would be a good option.
Great idea! Now to find a source of 1uF capacitors during lockdown hehe. Is this kind suitable? https://www.fabian.com.mt/en/products/webshop/5902/capacitor------polyester-1uf-400v.htm

Do I just put them in series (between the plug and the fan)? Is there a power rating or ventilation concerns I should worry about or could I put the caps in a waterproof electronics box?
 

crutschow

Joined Mar 14, 2008
24,967
Interesting solution! Can someone explain why the capacitor in series reduces the fan speed?
Sure.
The capacitor has a impedance equal to 1/(2pi*f*C) where f is the frequency and C is the capacitance, so this impedance reduces the voltage to the fan.
Edit: The capacitor value is selected to generate the desired impedance at the main's frequency to give the desired fan speed.
It's similar to having a resistor in series except the capacitor dissipates no power in the process.
 
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Thread Starter

LMF5000

Joined Oct 25, 2017
47
Could electrolytic capacitors be used? I basically have to harvest the capacitor from failed electronics because of current supply chain issues. Most SMPS I disassemble have a low-value polyester capacitor (I found one that's 0.1uf 250V in a battery charger) and two or more electrolytic (I have two that are 4.7uF 400V from the same charger). Would it be safe to make a circuit with a pair of electrolytics and possibly some diodes for reverse polarity protection?

Also, how do we calculate the necessary capacitor value?
Here's how I tried:
Currently the fan draws 53W at 240V so its impedance is 1086.8 ohms
I want it to draw 45W at 240V, so my target impedance is 1280 ohms
That means I need an extra 193.2 ohms of impedance.
Putting that into X = 1/2*pi*f*C where f=50Hz and X = 193.2 gives a C value of 16.4uF.

Does this sound about right? Of course I'm neglecting the non-ohmic nature of the induction motor. If you can suggest where that means I would need more or less capacitance than my simple calculation above that would be very helpful :)
 

crutschow

Joined Mar 14, 2008
24,967
Could electrolytic capacitors be used? I
No, that's why I stated "not electrolytic" in my post.
You'd have to put two of them back-to-back to handle the AC, but they can't handle the continuous current from the fan..
You'd likely end up with an exploded electrolytic, and that's no fun. :eek:
....gives a C value of 16.4u

Does this sound about right?
It sounds way too high.
You are forgetting that the motor impedance is not constant, and goes up as the fan load goes down.
I estimate you will only need in the neighborhood of 1 to 4 μF.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
47
Could you explain how you arrived at the correct capacitor value? Just for educational purposes, I'm interested in being able to make these sort of calculations myself, as AC theory isn't my forte (being a mechanical engineer and all :)
 

cmartinez

Joined Jan 17, 2007
6,971
Could you explain how you arrived at the correct capacitor value? Just for educational purposes, I'm interested in being able to make these sort of calculations myself, as AC theory isn't my forte (being a mechanical engineer and all :)
@Danko once helped me set up a capacitor in series for a fan motor, so as to limit its torque. I honestly don't know how he calculated its value, but it worked wonderfully.
 

crutschow

Joined Mar 14, 2008
24,967
Could you explain how you arrived at the correct capacitor value?
It's difficult to calculate accurately because of the unknown motor load at the desired fan speed, unless you have a device such as the Kill A Watt to measure the power consumption or current at that speed.
From that you could do a phasor calculation to determine the desired capacitor value.

I just made a rough calculation and bought several capacitors to connect in parallel until I got the desired fan speed.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
47
It's difficult to calculate accurately because of the unknown motor load at the desired fan speed, unless you have a device such as the Kill A Watt to measure the power consumption or current at that speed.
From that you could do a phasor calculation to determine the desired capacitor value.

I just made a rough calculation and bought several capacitors to connect in parallel until I got the desired fan speed.
I did use an inline power meter (cheap ebay version of a kill-a-watt) to get wattage at the desired fan speed - I had a plug-in dimmer connected in series and turned the knob until it sounded and felt right, and that's where I got the target value of 45W (that was the kill-a-watt reading of input power at 245V. Output power from dimmer was about 42W at 205V).

I remember doing phasor calculations at university - but not enough to actually solve this for myself.
 

crutschow

Joined Mar 14, 2008
24,967
Based upon the above data, I calculated a value of ≈4.8μF for a series capacitor to give the same current at 245V as you were getting with the dimmer at 205V.

I calculated an equivalent resistance of 1.001Ω for the motor at 205V, giving a current of 205mA.
The impedance to give the same current at 245V is then 1.196kΩ.
The vector impedance of the series resistor and capacitor is the square-root of the sum of the squares of their impedances.
Solving that gives a desired capacitance impedance of 654.9Ω.
At 50Hz this is provided by a 4.8μF cap.
My LTspice simulations verified this value.

But note that the accuracy of the meter is suspect for the chopped sine-wave that the dimmer generates to reduce the voltage.
Don't know if it measures true-RMS or not for such a waveform.
 
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