Controling a fan with a triac

Thread Starter

dragilla

Joined Jan 15, 2012
18
Hi,
I'm doing a small project, in which I control a regular home fan from my processor. I use a triac (BTA12-600BW3G) to shift the phase of the current and this way I can control the speed of a fan. The triac is controled by an optoisolator with triac driver output (MOC3021) which is controlled directly from the processor. I get a zero crossing signal through LTV-354T and based on that event I start a delay function which turns on the triac after a desirable time.
I'm attaching my schematic.
The whole thing works perfectly, but only with one of my home fans. With a second one it works until a certain delay level (about 50%, so ~5ms). Once go over this limit (below 5ms), the fan goes full speed and the triac locks - I cannot control it anymore. I'm guessing the second fan must use an induction motor? The fans don't differ much on paper - the first one is 65W, the second - 95W.
I will try to build a snubber circuit for the triac, but based on my initial calculations it seems that this triac should not be affected by this. The triac has (dI/dt)c = 2.5 A/ms
Can you please help me find a reason behind this behaviour? Maybe it's more a problem with the MOC3021? But as I understand a snubber circuit for it would be neccessary only to protect it from high, induced voltage.
The zero-crossing detection works - I tested it with an oscilloscope when trying to control this second fan.

PS: please be patient with me, as I'm not an electronics engineer, just a hobbyst :)
 

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Ian0

Joined Aug 7, 2020
3,281
Fans tend to have induction motors. Induction motors run at a speed equal to the mains frequency less "slip". You are varying the voltage not the frequency with your phase-control circuit. This tends to work with most fans, as you reduce torque and create more slip and thus reduce the speed, but it doesn't always work.
 

Thread Starter

dragilla

Joined Jan 15, 2012
18
Fans tend to have induction motors. Induction motors run at a speed equal to the mains frequency less "slip". You are varying the voltage not the frequency with your phase-control circuit. This tends to work with most fans, as you reduce torque and create more slip and thus reduce the speed, but it doesn't always work.
Ok. Can you say more? Why would it not work? What's the cause? Is there a way to fix it univesally (more/less, for say 90% of home fans)?
 

MaxHeadRoom

Joined Jul 18, 2013
23,649
The two types of motors usually used in fan applications are the (smaller) shaded pole version or the PSC or permanent Start Cap style.
Check out the App notes by Fairchild, AN-3006 & 3003, are you using a random phase triac driver opto?
 

crutschow

Joined Mar 14, 2008
27,761
A common problem with such speed control of induction motors is a buzz sound from the motor due to the sharp turn-on of the waveform from the Triac, so be prepared for that.
To eliminate that, I used a few µF of non-polarized capacitance in series to slow the fan.
 

Ian0

Joined Aug 7, 2020
3,281
Ok. Can you say more? Why would it not work? What's the cause? Is there a way to fix it univesally (more/less, for say 90% of home fans)?
Look up "torque speed curve of induction motor". The torque vs. speed curve varies in a complex way depending on how the motor is designed. For a fan, the required torque varies with the cube of speed (or is it the square?) Depending on how those two curves interact, there may be points where an induction motor would stall.
Universal motors can be speed controlled much more reliably with a phase-fired circuit, but they are not often used in fans.
Induction motors would really benefit from variable-frequency drives.
 

MaxHeadRoom

Joined Jul 18, 2013
23,649
1ph induction motors have a very poor record where VFD's have been implemented, they tend to drop out of run at low speeds etc, mainly why the 1ph supplied 3ph motor/VFD is more popular.
If designing along the lines of the Fairchild Triac designs, I found they work quite well for the average/simple fan applications.
There is also some app notes from Picmicro for the more sophisticated Triac design.
 

Thread Starter

dragilla

Joined Jan 15, 2012
18
The two types of motors usually used in fan applications are the (smaller) shaded pole version or the PSC or permanent Start Cap style.
Check out the App notes by Fairchild, AN-3006 & 3003, are you using a random phase triac driver opto?
I checked and they seem similar to the one I use MOC3021, which is random phase. Those application notes don't include the snubber circuits for the Triac, which I think I need. Do I need a snubber for the MOC as well?
 

Thread Starter

dragilla

Joined Jan 15, 2012
18
Look up "torque speed curve of induction motor". The torque vs. speed curve varies in a complex way depending on how the motor is designed. For a fan, the required torque varies with the cube of speed (or is it the square?) Depending on how those two curves interact, there may be points where an induction motor would stall.
Universal motors can be speed controlled much more reliably with a phase-fired circuit, but they are not often used in fans.
Induction motors would really benefit from variable-frequency drives.
I don't have a problem of a stalling motor. My problem is that it won't stop :)
Also, I'm lookig for a uiversal solution or at least one that would work for most of home fans. Would variable-freq driver solution work for all of them?
 

Thread Starter

dragilla

Joined Jan 15, 2012
18
A common problem with such speed control of induction motors is a buzz sound from the motor due to the sharp turn-on of the waveform from the Triac, so be prepared for that.
To eliminate that, I used a few µF of non-polarized capacitance in series to slow the fan.
Hi, thanks. Yeah, I get the buzz, but it doesn't bother me. The more I delay the phase, the more the buzz, to the point where the fan stops turning and only buzzes ;) But that's fine. As long as I can preset the lower and upper limit - a useful limit of operation. I do that in my android app for controling the fan. Anyways, still looking for Ideas on how to make it work for most fans.
 

Ian0

Joined Aug 7, 2020
3,281
I don't have a problem of a stalling motor. My problem is that it won't stop :)
Also, I'm lookig for a uiversal solution or at least one that would work for most of home fans. Would variable-freq driver solution work for all of them?
VFD is only really a solution for three-phase motors.
Not stopping is usually a triac commutation problem.
Does it the snubber make a difference?
 

Thread Starter

dragilla

Joined Jan 15, 2012
18
Ok. I've just soldered the RC (39ohm and 10nF) and the result is the same as before. I can't see any difference. Anything else you think I could try?
PS: correct me if I'm wrong, bu the order of R and C does not matter here, right?
PS2: see attached diagram - this is how I've made the change.
 

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Thread Starter

dragilla

Joined Jan 15, 2012
18
Additional questiosn, please:
1. Can a snubber for MOC change this behavior? Or is it suppose to protect the optocoupler only?
2. Does it make sense to try different values of RC for the Triac snubber? What to change first and in what "direction" (smaller cap, res, or bigger)?
3. Is there anything I can measure that would tell me what happens? I still don't understand the reasons behind it, so I don't have an idea what to fix.
 

DickCappels

Joined Aug 21, 2008
7,746
Ok. I've just soldered the RC (39ohm and 10nF) and the result is the same as before. I can't see any difference. Anything else you think I could try?
PS: correct me if I'm wrong, bu the order of R and C does not matter here, right?
PS2: see attached diagram - this is how I've made the change.
Right.
 
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