[SOLVED] How to determine minimum fire angle in inductive load

[Masoud85]

Hello dear friends
I'm designing a soft starter circuit for inductive loads specially single phase motors.
I've used an AVR microcontroller and mains zero crossing detector and a Triac.
As we know from power electronics references the fire angle in inductive loads can not be smaller than phase angle of load.
If phase angle is ɸ and fire angle is α then ɸ<α<π . So if α=ɸ voltage and current of load is sinusoidal and load gets maximum power.
My question is how can we determine phase angle of unknown inductive load? Because users maybe connect an asynchronous squirrel-cage motor or maybe a universal vacuum cleaner motor to the soft starter. Both of them are inductive loads but with different phase angles!
Is it necessary that i use a current transformer to detect load current zero crossing and after first fire pulse applied to triac calculate phase angle of load? if yes how can i calculate it?
Thanks

 

[KlausST]

Hi,

Are you using pulse fitirng or DC firing.

Often with AVRs one does DC firing, then there is no problem at all.

Klaus

 

[Masoud85]

Are you using pulse fitirng or DC firing.

Dear Klaus
Thank you for your answer. I have not decided yet witch firing method must be used. Of course pulse train losses on triac gate is less than DC firing. But I want to know the the minimum fire angle not minimum pulse width. When soft starter function starts to drive motor, fire angle (α) will decrease from π radian to around ɸ and in α=ɸ maximum power would apply to the motor. if we reduce fire angle less than load phase angle (α<ɸ) it may cause unbalanced waveform in motor current and generates DC current certainly.

 

[aliarifat794]

There should be proper signal conditioning for both voltage and current signals to avoid noise and ensure accurate detection.

 

[Masoud85]

There should be proper signal conditioning for both voltage and current signals to avoid noise and ensure accurate detection.

Detection of what???
Imagine we used proper signal conditioning circuits and detect voltage and current zero crosses. So how can we calculate ɸ?

 

[BradtheRad]

I once built a fan-speed controller using a triac. I believe it was a squirrel-cage type (mounted on a wood stove). I may have tried it with the circuit printed on the package. (At the same time I had to discover which pin was for the load side and which was for the control circuit.)

Then seeing the principle that a triac triggers when the gate reaches a high enough voltage...

It gave me the notion to simplify things. I installed a mere potentiometer for my control circuit. It worked. Its ohm value was several megohms (to operate 120 VAC). I had it run that little squirrel-cage fan for years without a problem. I didn't have to be concerned about zero crossings or power factor.

More recently I plugged in a window fan. Immediately something in my triac controller was destroyed and I haven't tried to repair it since.

 

[Easy peasy]

How to determine minimum fire angle in inductive load ?​

The minimum firing angle is at the start of the AC cycle otherwise known as 0 degrees.

 

[KlausST]

Hi,

But I want to know the the minimum fire angle not minimum pulse width.

DC vs single pulse vs pulse train:

I think you should decide this first.
Pulses often are used when an ignition transformer is involved.

You ask for minimum firing angle:
* it does not hurt if you go down to zero.
In case of inductive load the triac is still conductive at low phase angle. So a triggen does not change the situation.
Example:
So if the triac switches OFF at 10° ... then an iginition at 5° simply does nothing.

But if you want the triac to switch ON you need to be sure that it triggers the triac after 10°. Where "after" means at least that long that the load current is above triac_holding_current.
While you say "you don´t want to know minimum pulse width" .. still the minimum pulse width is very important for proper operation, especially with inductive laods.
If pulse width is too short it will not trigger the actual half wave .. current will go down ... and thus it will trigger the next half wave.
...resulting in DC offset in load and wrong load power.

So to answer the question in technical terms:
The minimum firing angle depends on
* the load´s nominal phase shift
* the actual firing angle (because it determines the load current and thus how long the inductive load will be ON after voltage_zero_cross)
* the triac´s holding current
* the load´s behaviour on how fast the triac´s holding current is met (to avoid that triac switches OFF immediately after trigger pulse is removed)
* maybe more

So in my opinion at a low firing angle the firing pulse should be stretched at least to 90° after voltage zero cross .. but it should be OFF before next zero cross.

Klaus

 

[Masoud85]

DC vs single pulse vs pulse train:

I think you should decide this first.
Pulses often are used when an ignition transformer is involved.

Hi dear Kluas. I appreciate you again.
As I use optotriac MOC3052 I don't concern about pulse transformer. But you're right about short pulse width in inductive loads to ensure triac turning on correctly. I've used DC firing for a resistive load in my past project from fire angle up to end of half cycle. I'll use that in this project probably.

* it does not hurt if you go down to zero.

I disagree with you. According to the STMicroelectronis application note AN308, it would cause problem if fire angle reduces less than phase shift of load. Although the control circuit is not based on microcontroller, but it is implied to maintain θ > ϕ
Please check the AN308 and consider
"Figure 6. Synchronization by the mains voltage - asymmetrical operation θ < ϕ"

 

[KlausST]

Hi,

I disagree with you. According to the STMicroelectronis application note AN308, it would cause problem if fire angle reduces less than phase shift of load.

This application note tells exactly the same what I have written.

But you completely ignore what I as well as the AN wrote about the pulse width.
You say you don´t are about pulse width .. but both I and the AN say you need to care.

Fig. 6 talks about the problem with a very narrow pulse width.
I also addreseed the same problem.
And I also wrote:

If pulse width is too short it will not trigger the actual half wave .. current will go down ... and thus it will trigger the next half wave.
...resulting in DC offset in load and wrong load power.

This is exactly depicted on Fig. 6.

--> "If pulse width is too short" --> the solution is: make it longer ... I even gave the informations how long it needs to be.
And my answer was more detailed and more precise in this regard.

Just waiting for the current is zero is not enough. The trigger needs to be long enough for the load to build up a current bigger than the triac´s holding current.

Klaus

 

[FvM]

it would cause problem if fire angle reduces less than phase shift of load.

This is only the case if a short trigger pulse ends before current zero crossing. You should read the application note completely.

 

[Masoud85]

But you're right about short pulse width in inductive loads to ensure triac turning on correctly. I've used DC firing for a resistive load in my past project from fire angle up to end of half cycle. I'll use that in this project probably.

Dear Klaus, I've told you that I'm agree with you about pulse width and I'll use DC firing from fire angle up to end of half cycle. You're right definitely. But does it(DC firing) solve the θ < ϕ problem?
what is your opinion about this recommendation of AN308 exactly under the figure 6 (To prevent this fault, it is necessary to limit the turn-on angle to maintain θ > ϕ.)?

 

[Masoud85]

This is only the case if a short trigger pulse ends before current zero crossing. You should read the application note completely.

So if I understand you and Klaus truly, in DC firing (long pulse from fire angle up to End of half cycle), asymmetrical operation and DC component generation in load's current does not occur even fire angle reduces less than phase shift of load.

 

[KlausST]

Dear Klaus, I've told you that I'm agree with you about pulse width and I'll use DC firing from fire angle up to end of half cycle. You're right definitely. But does it(DC firing) solve the θ < ϕ problem?

I´ve already told you. Three times now: In post#2, post#8 and in post#10.

what is your opinion about this recommendation of AN308 exactly under the figure 6 (To prevent this fault, it is necessary to limit the turn-on angle to maintain θ > ϕ.)?

Did you read my posts? I´ve explained it already .. rather detailed.

Fig. 6 talks about the problem with a very narrow pulse width.

Now a third time: Don´t use a narrow pulse. Extend the pulse width. --> Problem solved.
****

I don´t understand at all, why you refer to figure with a very narrow trigger pulse, while your application uses DC firing. Which is quite the opposite to a narrow pulse.

Klaus

 

[Masoud85]

I don´t understand at all, why you refer to figure with a very narrow trigger pulse, while your application uses DC firing. Which is quite the opposite to a narrow pulse.

Hi
Because I thought that in both cases (DC & narrow pulses) minimum fire angle is limited in ϕ.
Now with your explanations i got that it is considerable only in narrow pulses.
So I appreciate you and other friends that helped me to know this.
Thank you.