Certainly efficiency is not one of the top 20 requirements given to those who design these motors for this type of product. And with plenty of airflow heating is seldom an issue, until the inferior grade lubricant fails to lubricate after the warranty runs out.
Many of these fan motors are returned to operation just by adding a few drops of motor oil.

Hi,

I had one fan I had to take apart about once every three months and clean the shaft and bearings and then re-oil. I found using synthetic motor oil lasted longer than regular oil or sewing machine oil. It was a pain in the neck though because the cage had to be unbolted to get the shaft out after taking the blades off. Super pain in the neck. Eventually I got rid of that dang thing.

 

Mobile-1, or, Valvoline-Synthetic Automotive-Engine-Oils are definitely worth
their high-prices for use in Electric-Motors with Bronze-Bearings.
.
.
.

 

However, due to the larger size of the motors you seem to be dealing with, the capacitors would have to be quite large, at least for AC capacitors.

I tried a few capacitor values on a smallish fan that only uses 46W when plugged straight into the wall.

No Capacitor
Current 0.378 A
Active Power 46 W
Apparent Power 46 VA
Reactive Power 0 VAr
Power Factor 1.00

40uF
Current 0.367 A
Active Power 43 W
Apparent Power 44 VA
Reactive Power 10 VAr
Power Factor 0.97

26uF
Current 0.359 A
Active Power 40 W
Apparent Power 43 VA
Reactive Power 16 VAr
Power Factor 0.93

11.6uF
Current 0.324 A
Active Power 31 W
Apparent Power 39 VA
Reactive Power 24 VAr
Power Factor 0.79

10uF
Current 0.297 A
Active Power 26 W
Apparent Power 36 VA
Reactive Power 24 VAr
Power Factor 0.73

But the fan sounds just as noisy as when it is controlled by a triac. Is this expected?

 

I tried a few capacitor values on a smallish fan that only uses 46W when plugged straight into the wall.

No Capacitor
Current 0.378 A
Active Power 46 W
Apparent Power 46 VA
Reactive Power 0 VAr
Power Factor 1.00

40uF
Current 0.367 A
Active Power 43 W
Apparent Power 44 VA
Reactive Power 10 VAr
Power Factor 0.97

26uF
Current 0.359 A
Active Power 40 W
Apparent Power 43 VA
Reactive Power 16 VAr
Power Factor 0.93

11.6uF
Current 0.324 A
Active Power 31 W
Apparent Power 39 VA
Reactive Power 24 VAr
Power Factor 0.79

10uF
Current 0.297 A
Active Power 26 W
Apparent Power 36 VA
Reactive Power 24 VAr
Power Factor 0.73

But the fan sounds just as noisy as when it is controlled by a triac. Is this expected?

Hi,

I would not think so, but myself and maybe others were assuming that you were hearing the effect the mid-half-cycle fast voltage rise, or the unusual turn off near zero. If the noise is due to another problem, then i don't think this, by itself, would work. That is, when the power is reduced.

What kind of noise are you hearing, is it a very low sounding noise or something else?
I know it is hard to describe some sounds, but you may be able to make a recording.

What kind of capacitors were you using? If you have a part number for the 10uf that would be great.

Normally capacitors just drop some of the voltage and that drops some of the power in the load. Since this is inductive, maybe there is something happening that is non-linear and messes up the sine wave. Do you happen to have an oscilloscope?

I guess i also have to wonder if we are seeing something like resonance.
Does the sound get louder or less loud as you move from a big value cap to a small value cap?
There is the possibility that this may also require a resistor to damp out the oscillations, if in fact they exist.
If you want to try another test, try hooking a light bulb in series with the fan and see what noise it makes. Maybe a 40 or 50 watt if you have one. It has to be incandescent though. You can use a regular lamp and clip leads to the AC plugs to connect for the test, being careful of the voltage of course.

 

Indeed the nature of the sound is vital in determining the source of the sound. It might be a friction noise caused by something rubbing, either constantly or once per revolution. The noise might also be a vibrating lamination sort of noise. If there is a defect in that rotor then it would be fairly constant but vary once each revolution. Is the noise a mains frequency buzz, or is it a higher frequency??
I have fixed a fan that squealed because a bearing was dry. And one that rattled because a bearing was worn.
So without a much more detailed description there is no reason to attempt a diagnostic process.

 

The noise sounds like an impure sine wave. I put this fan on an oscope with a current clamp and it does look somewhat jagged even with no in series capacitors. It's making noise even on a variac, so it's the fan itself. Maybe the run capacitor is getting old.

When controlling fan speeds using capacitors, is this method basically 100% efficient? I have a transformer that I use on one fan that is about 90% efficient. So losing 10's of watts to heat it not that cool, literally.

 

If the capacitor does not get hot, or very warm, then it is doing the job rather efficiently. In simple motor systems the power not delivered to the load is wasted as heat. So if there is not much heat then it must be fairly efficient.

 

The noise sounds like an impure sine wave. I put this fan on an oscope with a current clamp and it does look somewhat jagged even with no in series capacitors. It's making noise even on a variac, so it's the fan itself. Maybe the run capacitor is getting old.

When controlling fan speeds using capacitors, is this method basically 100% efficient? I have a transformer that I use on one fan that is about 90% efficient. So losing 10's of watts to heat it not that cool, literally.

Hi,

As MisterBill2 says if the cap does not get hot then it is probably helping to keep the efficiency higher. Low efficiency is associated with hotter parts, high efficiency with cooler parts. At the lower voltage if the motor itself uses more power than it should also depends on how the motor reacts to the lower voltage

One other thing worth mentioning. It is a common belief that using a cap in series with an AC motor to lower the speed decreases the normal life of a capacitor. I don't know for sure how true this is, but if it is true, you may have to replace the cap every 2 years or so.

 

Hi,

As MisterBill2 says if the cap does not get hot then it is probably helping to keep the efficiency higher. Low efficiency is associated with hotter parts, high efficiency with cooler parts. At the lower voltage if the motor itself uses more power than it should also depends on how the motor reacts to the lower voltage

One other thing worth mentioning. It is a common belief that using a cap in series with an AC motor to lower the speed decreases the normal life of a capacitor. I don't know for sure how true this is, but if it is true, you may have to replace the cap every 2 years or so.

Using a series capacitor does reduce capacitor life compared to sitting on a shelf unused. But if the capacitor is selected with adequate ratings for that application then it will often outlive the associated motor.

 

Using a series capacitor does reduce capacitor life compared to sitting on a shelf unused. But if the capacitor is selected with adequate ratings for that application then it will often outlive the associated motor.

Hi,

Ok then maybe you should follow up with some advice as to select a capacitor type and ratings so knows what to get.

 

The capacitor Voltage rating should be at least 20% greater than the supply peak-to-peak voltage, the capacitive reactance should be enough to reduce the effective motor power to the value to provide the desired reduced RPM. The capacitor power rating, often in amps, should be a least as much as the calculated power at speed. The temperature rating should follow the manufacturers guidelines.

 

some advice as to select a capacitor type and ratings so knows what to get.

For my money, either oil filled paper, motor-run rated, or for smaller motors, the metalized polypropylene, again, motor run rated.
One source of the metalized poly, motor run types are the WIMA MKP4 series.
The latter are also good for use in Triac/SCR phase angle controllers etc

 

So far I'm getting pretty good results. The capacitors that I had floating around in the parts bin are all HVAC parts, so they either ran compressors or fans, and usually on 220. The fans are dead quiet, at least as quiet as a Variac, and I can't feel any warming of the capacitors. Maybe I should get the thermal camera out. So it should be even more efficient than a Variac.

Question about motors with multiple taps: for effeciency which winding should be used if the fan is turned down via voltage to speeds that are slower than the "slowest" tap?

 

May be up for experimentation, but I would start with the highest winding.

 

So far I'm getting pretty good results. The capacitors that I had floating around in the parts bin are all HVAC parts, so they either ran compressors or fans, and usually on 220. The fans are dead quiet, at least as quiet as a Variac, and I can't feel any warming of the capacitors. Maybe I should get the thermal camera out. So it should be even more efficient than a Variac.

Question about motors with multiple taps: for effeciency which winding should be used if the fan is turned down via voltage to speeds that are slower than the "slowest" tap?

Hi,

Sounds good. Literally

 

Which tap to use on a tapped variable speed motor is an interesting trade-off, if you are also using a series capacitor. One thing to avoid is creating a resonant circuit, because the current and voltage will be higher if it achieves a series resonance condition.
On a normal furnace blower motor the lowest speed has the most winding turns and the thinnest wire. An unforeseen result is that when a higher speed is used, the voltage across the lower speed connection is quite a bit higher. So if a transformer for a humidifier is connected across the low speed winding, which is used for heating, then powering the high speed winding will produce enough voltage to burn out the connected transformer, and possibly damage the motor. Who would have guessed that a multi-speed motor would act like a step-up transformer???
But if the multi-speed motor changes speed by changing the number of poles, the picture is totally different.

 

May be up for experimentation, but I would start with the highest winding.

The difficulty in this experiment is measuring actual air flow output at any given watts input, to determine ultimate efficiency. And I unfortunately don't have the right measuring tool for air flow. Unless effeciency can be measured by proxy in some other way?

 

A simple pinwheel can provide an RPM indication, which is proportional to mass air flow. And an inline ammeter will give a fair indication of input power if the mains voltage at the current measuring point stays constant. Thus you would have both air flow and power, the variables needed to figure efficiency.

 

It is a common belief that using a cap in series with an AC motor to lower the speed decreases the normal life of a capacitor. I don't know for sure how true this is, but if it is true, you may have to replace the cap every 2 years or so.

If you use a quality capacitor intended for and rated induction motor run, then it should last the life of the motor easily,
As opposed to a motor that uses a start capacitor also, these are only intended for very short operation and will fail if the means of disconnect fail.

 

Tap	Watts	Power Factor
Low	328	0.89
Med-Low	362	0.87
Med-High	398	0.81
High	461	0.75

So I rigged up a 4-pole rotary switche to one of my fans, and got these reading when it's connected without any capacitors.

It has a run capacitor. Might changing the value of that run capacitor shift the power band?