Hi,

Thanks for your comments. So, this journey started out with trying several diac-triac controllers- they caused the fan to hum at low speeds. Next I bought a small Variac on Ebay. (Staco 201) I boxed it up as shown below:
View attachment 213651
It works fine even when I can see the blade rotation- no hum. With some time on my hands with the Covid thing, I thought I'd try to reduce size and make it solid-state.

The basic circuit that you commented on came from: https://www.electronicshub.org/pwm-based-ac-power-control-using-mosfet-igbt/
It looked simple to simulate and I relaced the Arduino with my own PWM circuit. But, as you can see, it doesn't work as expected with the measured motor inductance.

Next choice may be Danko's suggestion if I can get the drive implemented. The question that I won't know until I try it is whether the fan will hum at low PWM duty cycles.

Thanks,
Neko

Hello,

It is good to see a pic of this setup as that helps understand what you are doing better.

After seeing that, i tend to go with Max's idea where a low voltage DC fan would work great for this application. You can get very very quiet fans that are made for computer cases.

The reason for the hum was because the fan motor gets banged with rather sharp 60Hz spikes which power wise are 120Hz spikes which is not hard to hear with the human ear.
PWM on the other hand is usually much higher and when the sine wave is reasonably free from distortion the hum will be the same as with the variac ... virtually not there anymore.

The key is to make a low distortion synthesized AC controller. That is one of the reasons i suggested the H bridge configuration. It's a known and tried idea that has been used over the years. I actually worked for a company that made this power supplies and the output on some units were very clean, and every one of them used an H bridge configuration.

But the DC fan idea is good too because the fans often dont make any noise even at full speed, which BTW is not that fast anyway. Sometimes this is a hard thing to specify though because you probably dont have any way to measure the air flow you really want to get. So some trial and error comes into play.
The 12v fans are nice to work with though. You can get a variable output wall wart that has settings like 5v, 6v, 8v, 9v, 10v, 12v, and just select one. If you choose say 8v and you think it is still too fast, you can add a power resistor to get it to work slower, or you could change one of the resistor values in the wall wart circuit (if you care to).

Also BTW, i hope you got a good deal on that variac. I see them being sold but way overpriced. It's only rated around 240 watts and i see prices of over 150 dollars USD. You can get 20 amp units for less than that (at least in the USA).

 

For one, AC 1phase induction motors in general are notoriously poor at being speed controlled.
For the most part it is a compromise.
Apart from AC 3ph induction versions, which, VFD's work well, the general choice is with the PM motors of some kind as the prefered format for RPM control.
Max.

Continuing my search for a PWM topology that's somewhat easy to implement, I ran across an IEEE paper containing the following circuit:

The complete text is contained in the full article which is attached. I think this may have legs. Any thoughts from this thread group?

Thanks,
Neko

 

That circuit may well be a viable option.
But I still like the simplicity of the DC version.
Max.

 

Continuing my search for a PWM topology that's somewhat easy to implement, I ran across an IEEE paper containing the following circuit:

That schematic shows it being used with an induction motor, but most all of the desk fans I've seen are a shaded pole motor. What motor does your fan actually have?

Quote, "Moreover, they are compatible with TRIAC-based variable-speed controls, which often are used with fans." From - https://en.wikipedia.org/wiki/Shaded-pole_motor

 

That schematic shows it being used with an induction motor, but most all of the desk fans I've seen are a shaded pole motor. What motor does your fan actually have?

Quote, "Moreover, they are compatible with TRIAC-based variable-speed controls, which often are used with fans." From - https://en.wikipedia.org/wiki/Shaded-pole_motor

When I first started this project, I opened the fan housing to see how the control switches for slow, medium and fast hoping that there would some control electronics that I could tap into. What I found were three motor windings, the neutral return, and a NP cap. Don't know how the cap was connected because I couldn't get to the connection points. I tried a lamp dimmer from Home Depot and three fan controllers from Amazon. All slowed the fan but caused it to hum. Compatible? Not for hum. Whatever the motor type, these controllers didn't work for me.

,

 

That circuit may well be a viable option.
But I still like the simplicity of the DC version.
Max.

Max, can you build one and send it to me?

 

Continuing my search for a PWM topology that's somewhat easy to implement, I ran across an IEEE paper containing the following circuit:
View attachment 213691
The complete text is contained in the full article which is attached. I think this may have legs. Any thoughts from this thread group?

Thanks,
Neko

This circuit looks intriguing, and may be worthwhile to investigate further.
The disadvantage I see is that it requires three isolated auxiliary voltages.
The keyword here is isolated.

A better solution, in my opinion, is shown in figure 12 of the paper.

Although the paper shows a motor with a permanent capacitor run winding, it should also work for shaded pole motors.

 

most all of the desk fans I've seen are a shaded pole motor.

Which is a type of induction motor.

 

Max, can you build one and send it to me?

The DC version I referred to was using a 12vdc Radiator fan for example.
This just requires a simple 555 PWM controller of which there is an abundance of circuits out there and via ebay.
Max.

 

The DC version I referred to was using a 12vdc Radiator fan for example.
This just requires a simple 555 PWM controller of which there is an abundance of circuits out there and via ebay.
Max.

Max, I get your point. By the time I retrofit that DC motor into my existing fan frame and figure out the mechanical connection to the existing 16" fan blade, it's not only an electrical but mechanical journey. What did they teach us in Engineering School? KISS!
As we always said where I've worked: "How many PhDs does it take to change a light bulb.
No offense meant to you- I just want to try some version of a PWM albeit Danko's or something I've morphed.
Always open though to suggestions!-Neko

 

This circuit looks intriguing, and may be worthwhile to investigate further.
The disadvantage I see is that it requires three isolated auxiliary voltages.
The keyword here is isolated.

A better solution, in my opinion, is shown in figure 12 of the paper.

Although the paper shows a motor with a permanent capacitor run winding, it should also work for shaded pole motors.

Based on the Infineon app note link that Danko posted, There are limitations in Figure 12, namely, the pulse transformers. From the app note, "The volt-second product applied across the magnetizing inductance must always be symmetrical for any two consecutive half-cycles in order to reset the core flux. This limits the dutycycle to a maximum of 50percent when using the same driving voltage for turn-on and turn-off. For the LLC topology operating with constant 50percent dutycycle, this is not a significant drawback. However, for a FBSR on the secondary side, operating with dutycycles in the range of 75percent to 85percent, a solution based on pulse transformers cannot be used[12]. "
It would seem then that the circuit in Figure 6 may be a way to get the full PWM range.

 

Which is a type of induction motor.

But not the capacitor start type shown in the schematic.

 

All slowed the fan but caused it to hum.

What speed did you have the fan switch on? That switch probably selects the way the number of motor poles are selected. The more poles used the slower the motor turns, but with the speed controller and a larger number of poles being used there could be a better chance of the pulses from the controller causing the hum. Try keeping the fan selector on high(lowest number of poles) and then controlling speed with the dimmer.

 

What speed did you have the fan switch on? That switch probably selects the way the number of motor poles are selected. The more poles used the slower the motor turns, but with the speed controller and a larger number of poles being used there could be a better chance of the pulses from the controller causing the hum. Try keeping the fan selector on high(lowest number of poles) and then controlling speed with the dimmer.

Good point. I always used the slowest speed setting not the fastest. I'll give it a try.

 

Good point. I always used the slowest speed setting not the fastest. I'll give it a try.

ked
Good point. I always used the slowest speed setting not the fastest. I'll give it a try.

OK, trucked over to Home Depot and bought a dimmer. (had returned the dimmer and fan controls. Shortbus, your thinking about the windings was directional but didn't provide enough hum relief. I ran a comparison as shown below:

Fan Speed	Dimmer	Hum Amp (dB)
L	1/3	58
M	1/3	56
H	1/3	54
Fan Speed	Variac	Hum Amp (dB)
L	1/3	Inaudible
M	1/3	Inaudible
H	1/3	Inaudible

 

OK, trucked over to Home Depot and bought a dimmer. (had returned the dimmer and fan controls. Shortbus, your thinking about the windings was directional but didn't provide enough hum relief. I ran a comparison as shown below:

Fan Speed	Dimmer	Hum Amp (dB)
L	1/3	58
M	1/3	56
H	1/3	54
Fan Speed	Triac	Hum Amp (dB)
L	1/3	Inaudible
M	1/3	Inaudible
H	1/3	Inaudible

Oops- made a goof in the table; Triac should be Variac. Neko

 

Thats better!
I was about to post, a dimmer and a triac, usually the same in this application.
Max.

 

Thats better!
I was about to post, a dimmer and a triac, usually the same in this application.
Max.

Yah, Max, sounded liked Shortbus was was on to a simple solution but it turned out to be nada. :-( Looked again at Danko's circuit which is the simplest to date which provides the PWM performance. Does not work at 60Hz- also don't understand the relationship between the PWM and Back signal drives.
The isolated driver shown below looks like it could work:


Another option posted in another thread in this Forum shows the following shows the following:

I think that with some love and changing the IGBTs to FETs (drive posted for FET version is overly complicated) could have legs. Danko's circuit has impressive results, but don't see how create a simple practical implementation.
Comments welcome. :-} Neko

 

Take a look

 

sounded liked Shortbus was was on to a simple solution but it turned out to be nada

Sorry about that was just trying to help. The hum is probably from the motor coils and laminations vibrating, as the voltage switches directions. If you can get to the coils you could possibly put lines of hot melt glue or even some Epoxy on them tying them better to each other and the laminations.

This must be one very loud fan. Back years ago I worked midnight turn and had to sleep during the day. I would have a window fan on in the bed room, summer and winter for the hum it created to help me sleep. Are you sure the noise is coming from the motor and not the air getting disturbed by the movemnts of the blades themselves? Some fan blades are very loud on there own due to the shape of them.