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

Thread Starter

LMF5000

Joined Oct 25, 2017
87
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.
Bingo, found my mistake. I was thinking in DC so I simply added resistance with impedance. I forgot that it has to be vector sum (sqrt R^2 + X^2).

I've cobbled together a circuit with two paralleled lightbulbs in series with the fan and by changing the wattages of the bulbs was able to get it to run well (the cold resistance of the paralleled bulbs was 120 ohms, when fan switched on the bulbs glowed dimly and I measured 55V across the bulbs at just under 200mA so hot resistance of about 1kohm). This made it run a bit slower than ideal but it's a reassuring proof of concept until I can get the capacitive circuit set up.

I've determined that I will have to buy capacitors on eBay as local availability is spotty. Can you suggest capacitors to buy? If the correct value is about 4.7uF I was thinking of buying a pack of 10x 1uF 400V caps. Then I could parallel them until I get a comfortable speed out of it (giving me a range of 1-10uF). What do you think of that strategy? Am I correct in assuming that more capacitance = less impedance = more speed?
 
AC fan motors are super cheap and use the minimum copper. This means the fewest turns on the windings and so low speed is always quite high. TRIAC phase-control (dimmers) work for a speed control but the buzz is to noisy for a bedroom fan.

Note there is a capacitor value that creates series-resonance and you can get more than applied mains voltage to the motor! I've used this to make surplus 240VAC fan motors work on 120VAC. So sometimes using more or less capacitance gives you the same lower speed- which seems counter-intuitive.

Don't forget to put a bleeder resistor across the capacitor bank as it will bite you with stray charge.
I would try cascade 1uF or 2.2uF film caps to get a value then put in a CBB60, the designation for cheap motor polypropylene film caps used in ceiling fans, pumps, appliances for start/run use.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
AC fan motors are super cheap and use the minimum copper. This means the fewest turns on the windings and so low speed is always quite high. TRIAC phase-control (dimmers) work for a speed control but the buzz is to noisy for a bedroom fan.

Note there is a capacitor value that creates series-resonance and you can get more than applied mains voltage to the motor! I've used this to make surplus 240VAC fan motors work on 120VAC. So sometimes using more or less capacitance gives you the same lower speed- which seems counter-intuitive.

Don't forget to put a bleeder resistor across the capacitor bank as it will bite you with stray charge.
I would try cascade 1uF or 2.2uF film caps to get a value then put in a CBB60, the designation for cheap motor polypropylene film caps used in ceiling fans, pumps, appliances for start/run use.
I actually noticed that! The motor in this fan had a coil construction unlike any fan I'd ever disassembled before, using far less copper. The coils were small and isolated - just wire on a plastic-like bobbin. The traditional fans had coils that overlapped in the front and back of the motor held together with thread.

However in this fan the connections are much neater, the motor coils terminate in a PCB and then the connection from the control box connects to a PCB; in the traditional construction the wires from the control box are soldered directly to the thin motor wires which makes the assembly more fragile.

I've attached a pic of this fan's construction compared to traditional fan motors.

What's the advantage of a CBB60 over the rectangular plastic-cased capacitors like the one already in the motor?
 

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Thread Starter

LMF5000

Joined Oct 25, 2017
87
Can anyone suggest suitable bleeder resistor values? I have 100kohm and 38kohm quarter-watt resistors in the parts bin.

Also my electrical engineer colleague had the idea of putting the capacitor bank in series with the low speed wire (I think it's the yellow one according to the silkscreening on the PCB) - that way it would only slow down the low-speed setting, and leaves medium and high speeds at design power (gale and hurricane respectively).

Is that a good idea? The caps would fit in the empty space at the top of the fan body (near the wires going to the motor). Would I still need a bleeder resistor with that configuration?
 
Last edited:

crutschow

Joined Mar 14, 2008
26,400
You could also put a switch across the capacitor to bypass it for the full speed range.
I like having three low-speed ranges, along with the original three high-speed ranges.

I see no particular reason for a bleeder resistor unless you plan on poking your fingers around the circuit when it is off.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
You could also put a switch across the capacitor to bypass it for the full speed range.
I like having three low-speed ranges, along with the original three high-speed ranges.

I see no particular reason for a bleeder resistor unless you plan on poking your fingers around the circuit when it is off.
So there's no risk of the plug shocking anyone who handles the bare prongs shortly after unplugging the fan? Well, I suppose the fan side of things would be open circuit in theory since the fan is "off".

About switch across capacitor, I was thinking of doing one better. I could put the caps in a box with some switches and have a number of settings, where each setting parallels more capacitors. Which gives me multiple levels of "attenuation" in addition to the 3 speed settings of the fan (kind of like an offroad vehicle with two gearboxes in series).

I've attached crude drawing of my simple idea. Is there a clever way of switching in/out parallel capacitors with very few switches (or multi-way switches)? I can think of a simple way for caps in series (just string them all in a line and use a multi-way switch to connect to the points between the caps), but my idea for parallel caps will require one switch for each setting.
 

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Thread Starter

LMF5000

Joined Oct 25, 2017
87
You obviously need one switch less than the number of capacitor combination.
How many combinations do you want?
Hmm, probably three. I guess I will know when I start putting capacitors in line and feeling the performance of the system. I could probably manage three levels with a single 2-way switch (with center-off position).

I just bought these, are they suitable? https://www.ebay.co.uk/itm/282654567700
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I've done a bit of a simulation of the power with different capacitance values, using the observed resistances of the motor at low, medium and high speed (with lightbulbs in series to get it down to 40W, which is where it should be operating when a capacitor is swapped in). The V/A/W numbers at the top were all measured with the kill-a-watt. The ohm numbers are V/A.

For the table, I used this formula (I assumed the motor was a resistor and calculated real power across it):
Code:
P = I^2 * R
Where:
I = V/Z
Z = sqrt (R^2 + X^2)
X = 1/(2*pi*f*C)

So P = (V/sqrt(R^2 + (1/(2 * pi * f * C)^2))^2 * R
P = (V/sqrt(R^2 + (2 * pi * f * C)^-2))^2 * R
P = (V^2/(R^2 + (2 * pi * f * C)^-2))*R
Lastly, the no-cap value was measured with the kill-a-watt with the fan connected directly to the supply (no lightbulbs) to see how close I was.

The most interesting result was that for capacitors of 1-3uF I predict the fan speed control will actually work in the opposite way to the usual.

Anyway I bought a CBB60 4uF capacitor to complement the 10-pack of cheap small ones. Hope 4uF is not too small.

1591345370209.png
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
Hello again everyone, OP here.

The capacitors have arrived and I've finished my experiments. Here are the results (power in watts at low/medium/high speed with capacitance values from 1 to 9 uF). I had an extra 4uF CBB60 cap so I tested that as well as 4x of the cheap 1uF caps in parallel, the wattages were identical in both:

Capacitance (µF)LowMedHigh
1Does not spin
213.013.314.4Insufficient power
319.821.524.1Insufficient power
4 x 1uF34.237.239.1Too little power
4uF CBB6034.237.239.0Slightly buzzy and unreliable start at low power
534.437.340.0OK
634.237.441.0OK
735.138.443.1Nice
835.339.344.3Slightly too much power at low speed
935.039.446.0Ditto
These wattage values were measured with a kill-a-watt at the input (wall outlet - before the capacitor) so they should be taken with a pinch of salt. The power factor of the fan alone was 99 but with the capacitor in series around 75-80.



In conclusion, the 7uF gave the perfect power output (around 38-40W input in the final circuit) so I put the 4uF CBB60 in parallel with 3 of the cheap 1uF capacitors, put a 100k 1/4-watt bleeder resistor in parallel and sealed it all in a waterproof box with plug for input and socket for output. Put that between the fan plug and the wall plug and problem solved! Thanks everyone for the inspiration :)
 
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