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

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

Last edited: