Phase Angle Control Using Controller

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gauravkothari23

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Hi all.
I am trying to Control the Speed of 120W AC Motor using controller NUC029LAN, Traic BT136, Zero Detection Circuit.
Have also attached the circuit diagram and some wave form. In the waveform i am triggering the Triac after 7ms delay after zero crossing is detected.
my problem is when incandescent bulb is connected as load, it works perfectly (wave form attached at 7ms of delay).
But when 120W AC motor is connected, i am not able to control the speed, it rotates at full speed only. (Wave form Attached at 7ms of Delay).
The waveform when motor is connected, is completely changed.
Have also attached the waveform of MCU Pin which is driving the Triac with reference of 12V AC, just for reference to confirm that there is no issue with the software part.
Can anybody please let me know where the issue is.
 

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  • Motor-2.jpg
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  • Motor-3.jpg
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I'm not a motor expert, but I know that the speed of AC motors can be controlled by changing the frequency or voltage. Not sure that phase control will work. (apparently, it doesn't).
 

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barry said:
I'm not a motor expert, but I know that the speed of AC motors can be controlled by changing the frequency or voltage. Not sure that phase control will work. (apparently, it doesn't).
Click to expand...
Changing the frequency is a different logic, but can i change the voltage using the logic which i am using.
--- Updated ---

i have referred to one of the board which i got it from one of my vendor where the board was designed from china, where they have used only Triac to control the speed of the same motor. But i dont know how.
 

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Looking at your waveforms, I'm not sure what's going on. Is the first one showing the voltage across the bulb? Why is there ALWAYS voltage there? I assume the second waveform is voltage across the motor; THAT looks as expected. What kind of motor are you using-that might be your problem. If it's a synchronous motor the speed is a function of frequency only (until it gets overloaded).
 

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i am using

120W AC gear reduction motor 220V single - phase speed control motor forward and reverse motor​

--- Updated ---

motor1.jpg is the waveform when motor is connected as load
motor2.jpg is the waveform when bulb is connected
 

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I have no idea what a "single-phase speed control motor forward and reverse motor" is.

But it looks to me like your triac is not turning off. Not sure why. Hopefully there's somebody on here who has more experience with triac circuits than me.
 

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You need to know "EXACTLY" what type of motor you are using - only a true single phase motor with cap start / cap run can be phase controlled for speed ( at 120W ).
 

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Not sure why you are extending trigger pulse up to next zero crossing? This can be one reason why the triac doesn't turn off. Try with 100 us wide trigger. Other possible reason is self-triggering due to high dV/dt generated by inductive load. Stronger snubbering may be necessary.
 
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a triac cannot turn off while there is current in it, a spinning motor can provide this current at the zero crossing and a little way beyond.

You need to issue short gate commands ( ~ 50uS ) say 30 deg before the zero crossing to get lesser motor power and observe, while moving the short ( 50uS ) gate pulses more and more in advance of the zero crossing to see the effect, so +30 deg, then 40, 50, 60, 70, 80, 90 and observe ...

A light bulb will turn off at the zero cross as the current is zero ( as long as triac not gated on throughout )

the re-alpplied dv/dt from a motor can keep a triac on at the zero cross if there is insufficient snubber
 

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Thanks all.
Got your point.
what snubber value is recommened, or how can i make a test or any calculation for snubber circuit.

have came up with one more solution, just need to confirm if it is reliable,
Can i make any circuit where i can check. after the motor is Triggered OFF from MCU, is there any current available in motor or not. if any current is present, i wont start the next cycle of switching the motor ON.
will wait for the current in the motor to go 0.
 

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Hi,

No need to know motor current, the triac will do this for you.

The point is:
On a 50Hz one half wave is 10ms.
After zero cross signal wait a variable delay
Then trigger the gate, but only for a short, fixed time (50us, 100us)
You need to be sure the gate is off before the next zero cross. Mind: your ZCD signal may be delayed and unsymmetric with respect to mains zero cross point.

Example
For a headroom of 0.4ms make the variable delay from 0 to 9.5ms maximum. Then the gate trigger 0.1ms

With this values, there might be a new problem, when the variable delay is very short (close to zero), then mains voltage is still small ... thus the initial motor current may be too small to be above the triac_holding_current. The triac may not be/stay ON.
To avoid this you may set the range for the variable delay to 1ms .. 9.5ms.

Klaus
 

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Yes,
I Tried this, i made a variable delay as you said.
when i trigger the gate after approx 8.7 ms of ZCD, the motor is working fine at slow speed. it works well from 8ms to 8.7ms. but if i trigger the gate after 8.7ms the motor does not start. so that's not an issue.
But when i trigger before 7.8 or 7.5 ms, the motor rotates at full speed and the wave form is also changed. as per the attached image.
 

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Hi,

Again: the problem is to switch OFF your gate trigger signal before the next mains zero cross (not ZCD signal)
We don't see this in the diagrams nor in your text.

Klaus
 

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KlausST said:
Hi,

Again: the problem is to switch OFF your gate trigger signal before the next mains zero cross (not ZCD signal)
We don't see this in the diagrams nor in your text.

Klaus
Click to expand...
sorry but did not get your point.
what is the difference between mains zero cross and Zero crossing detection. basically zero crossing is detected when mains waveform is zero.
 

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Hi,
gauravkothari23 said:
what is the difference between mains zero cross and Zero crossing detection. basically zero crossing is detected when mains waveform is zero.
Click to expand...
As already said: the ZCD signal is not perfect with respect to the true mains signal zero cross. It may be unsymmetric and it surely is delayed. You have to care for this delay and uncertainties.

In post#10 you wrote:
gauravkothari23 said:
after the motor is Triggered OFF from MCU
Click to expand...
Maybe here is a misunderstanding in triac function.
* you can not trigger OFF the triac by the MCU
* you can trigger ON the triac by the MCU, but trigger OFF will happen only at "zero cross of current". (Not zero cross of voltage)

Klaus
 

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still the things are not clear to me.
KlausST said:
* you can trigger ON the triac by the MCU, but trigger OFF will happen only at "zero cross of current". (Not zero cross of voltage)

Klaus
Click to expand...
how can i OFF the triac then. because zero detection is for voltage and not current. how will i know that the current has gone to zero. because once the motor starts rotation, it will hold the current and keep the triac always ON because of back EMF.
--- Updated ---

As KlausST said in post #11
Then trigger the gate, but only for a short, fixed time (50us, 100us)
but how can i trigger the gate for short perioed of time, when making it OFF is not the MCU Part.
 

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You’re just not listening. The triac will turn itself off when the current through it drops to zero. Post 9 and post 15 and now this post tell you that.

Again: YOU CANNOT TURN THE TRIAC OFF; ZERO CURRENT TURNS IT OFF.
 

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barry said:
You’re just not listening. The triac will turn itself off when the current through it drops to zero. Post 9 and post 15 and now this post tell you that.

Again: YOU CANNOT TURN THE TRIAC OFF; ZERO CURRENT TURNS IT OFF.
Click to expand...
Yes, thats my question, as KlausST said in post #11
Then trigger the gate, but only for a short, fixed time (50us, 100us)
how can i do this, because to trigger it for a short peried of time, i have to switch it ON and again OFF for 50us, so here, how can i switch it OFF.
--- Updated ---

i have a Sample PCB with me (Image Attached), which my vendor provided me for my reference. they have used the same circuit but without snubber. and it works well. how is it possible,
 

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That picture is as useful as a picture of your house You dont even say WHAT that board is.

You need to generate a trigger pulse of about 50uS. What don’t you understand about that?
 

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