using zero crossing optocoupler as zero cross detector

Thread Starter

mtalent

Joined Oct 2, 2017
30
I am trying to build a circuit that will allow me to control voltage to a single phase induction motor with capacitors for speed control.

Here is what I am proposing I use a zero crossing opto to an input pin on mcu as a zero crossing detection circuit. I throw in a delay based on the amount of voltage I am trying to limit then fire the triac as usual until the next end of the cycle and repeat.

So as I understand it as the sine wave approaches zero again the triac will not have the voltage to remain on and therefore turn off then my zero crossing opto will see the zero crossing I will delay a certain amount of time and repeat.

In my mind this should limit the voltage and in turn slow the fan speed.

I am trying to figure the delay's necessary to make this work. So in my studies I have found that 110 is running at 60hz

with this in mind each cycle (360 degrees) is 16.67 ms.

In my mind for calculations then half of the cycle 180 degrees is going to be 8.3 ms.

So if you wanted lets say half the voltage you would delay firing the triac 4.15 ms, the only problem I see is that you are not going to get a smooth transition with that because at 4.15 ms the wave will be at it's highest would it be better to fire the triac on at maybe at 2.075 ms after zero and off again at 6.225 ms or does it matter and then adjust these numbers until you get the desired result?????

Thanks again

Mark
 
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Alec_t

Joined Sep 17, 2013
10,355
What you are proposing sounds like a fairly standard phase-controlled speed control, as used for universal motors, but be aware that induction motors generally cannot be controlled over a wide speed range, or some even at all, that way.
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
There are several previous posts and app notes on this such as Fairchild AN-3006 and pic micro AN958.
As per @Alec_t you may have an issue with cap operated induction motors, some PSC motors will be OK with it, but usually require starting at full voltage.
Max.
 

AlbertHall

Joined Jun 4, 2014
8,330
would it be better to fire the triac on at maybe at 2.075 ms after zero and off again at 6.225 ms
Once the triac is triggered it will stay on until the current through it falls below the holding current, usually at the end of the cycle.
 

Thread Starter

mtalent

Joined Oct 2, 2017
30
There are several previous posts and app notes on this such as Fairchild AN-3006 and pic micro AN958.
As per @Alec_t you may have an issue with cap operated induction motors, some PSC motors will be OK with it, but usually require starting at full voltage.
Max.
Thanks Max and others

So the part about starting at full voltage just means I run the fan on high for as long as necessary and then initiate the speed reduction after fan is running?

And the some psc motor portion of this I am going to read the posts you quoted I was just wondering if there was a rhyme or reason as to which motors work and which don't.

I have also been researching the workings of these motors is the problem that the windings are set to 90 degrees off phase for proper functioning and what I am proposing causes this phase to be changed resulting in less then efficient results.
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
AFA larger induction motors that have both start & run caps, they do not control well even with VFD controller, they tend to drop out of run at low rpm and under load.
The small motors that typically are used with simple phase angle control generally are intended for light loads such as fans etc.
These are PSC and shaded pole motors.
These two methods are used to provide a form of split or two phase, if anything tends to interfere with either of these designed components, it starts to impinge on their performance.
Ceiling fans often require full voltage initially in order to overcome this decreased performance of this artificial phase at low rpm.
PSC can also be reversed, whereas shaded pole require a physical reversal of the rotor end to end.
Max.
 

Thread Starter

mtalent

Joined Oct 2, 2017
30
You guys are awesome,

I am getting the parts tomorrow for cmartinez zero cross detection circuit only because it seems to have less of them. And the description on the designers page helped give me an understanding. However a big thank you for max taking the time also....... we shall see what happens.

The parts are only around 12 bucks so no harm no foul right.

Thanks again you guys

Mark
 

Thread Starter

mtalent

Joined Oct 2, 2017
30
One more question ....

It seems I only want to delay and fire the triac on the positive side of the wave (this from another forum I read). And since my power on/off switch optocoupler triac driver is no longer zero crossing how know which way the wave is crossing when I start.

I figure if I know where I am when I start I can programatically keep track with a switch skipping crossings but it's apparent there is no way to tell where you are when you start.

Thanks

Mark
 

cmartinez

Joined Jan 17, 2007
6,461
One more question ....

It seems I only want to delay and fire the triac on the positive side of the wave (this from another forum I read). And since my power on/off switch optocoupler triac driver is no longer zero crossing how know which way the wave is crossing when I start.

I figure if I know where I am when I start I can programatically keep track with a switch skipping crossings but it's apparent there is no way to tell where you are when you start.

Thanks

Mark
There's a tweak to the circuit I posted that lets you do just that. Hang on a few minutes and I'll let you know.
 
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Thread Starter

mtalent

Joined Oct 2, 2017
30
Cmartinez .....

I was just told on another forum that it doesn't matter with a triac just zero cross - delay - fire triac - repeat.

I have been getting some bad info ..... (not here) so please let me know if it matters

thanks my friend

Mark
 

cmartinez

Joined Jan 17, 2007
6,461
There, I've just checked.

About 5 years ago, I needed a circuit just like the one you've described. Its purpose was to control a 90 vdc motor using a conventional, non-rectified 110 vac source. What I did was build a couple of zero-crossing detectors. One would detect zero-crossing on the rising edge of the sinewave, and the other one on the falling edge. All I did was remove diodes D1 and D4 for the former, and D2 and D3 for the latter. Also, I had to change C1 to 0.47uF, and C2 to 2.2nF. Otherwise the circuit wouldn't work.

I was warned by the designer (I contacted him by email) that such a change would put a stress on C1. But as I said, the circuit's been working fine for 5 years every day, without a glitch. I haven't simmed it to corroborate it though. This is probably a good opportunity to do so.
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
Cmartinez .....

I was just told on another forum that it doesn't matter with a triac just zero cross - delay - fire triac - repeat.

Mark
The circuit I used has a positive going pulse every 800μs as per the Fairchild App fig 4.
Max.
 

Thread Starter

mtalent

Joined Oct 2, 2017
30
Good Morning All (It's morning here anyways)

I am having a heck of a time getting my head around how to set up the diodes on the breadboard (cmartinez circuit). I am new to reading schematics and when I first saw this it looked easy it wasn't until I started wiring it up that I started getting a bit confused as to how to do it. I have been doing some reading and trying to understand series and parallel and these look like both.

Not sure what you could do to help me to understand but I'll take what I can get

Mark
 

Thread Starter

mtalent

Joined Oct 2, 2017
30
Thanks Guys,

I am 1hr away from any parts and my guy works on cell towers all night and doesn't open up shop until after noon. I will most definitely get one this afternoon. However I am here now and the diodes are here with me so any advice as to how to think about this in a way that makes sense. I should add I can definitely see why the devices above were created because my head is spinning on this thing.

Anyhow now that I know it's called a diode bridge I might find more info on these things.

Here is what I found look good.

 

cmartinez

Joined Jan 17, 2007
6,461
Thanks Guys,

I am 1hr away from any parts and my guy works on cell towers all night and doesn't open up shop until after noon. I will most definitely get one this afternoon. However I am here now and the diodes are here with me so any advice as to how to think about this in a way that makes sense. I should add I can definitely see why the devices above were created because my head is spinning on this thing.

Anyhow now that I know it's called a diode bridge I might find more info on these things.

Here is what I found look good.

Just remember, the circuit I posted pulls down at its output when a zero-volt condition is detected. That is, its normal state is 5V, and it will output 0V when triggered. If you need to invert that, let me know.
 
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