Bringing V and I in phase

moc3020 with triac scheme

hello

I have a problem with an inductive load (230/12 transformer) running via TRIAC BTB16600BW. The TRIAC is controlled via PIC MCU optically coupled with MOC3020. Even though the TRIAC is a 'snubberless' version from ST...i have still added a snubber network across the TRIAC anodes. The TRIAC is controlling the phase angle on the transformer HT (input side) and the transformer LT side is powering a 12V halogen lamp.

The above setup described is actually a PIC dimmer and works well if an electronic transformer is used instead....but i need to use simple transformer.

The problem happening is that with the SIMPLE transformer....it is heating up very quickly even though the rated VA of the transformer is powerful enough and my halogen lamp is only 25W. Of course this is happening due to the inductive load nature and V and I are not in phase. In order to bring the V & I in phase i connected accross a 100W/230V normal light bulb accross the transformer HT. This has fully solved the problem and now every thing is running smoothly with no over heating anywhere in the network circuit.

However having a 100W bulb ballast is not practically feasible for long term...So please can someone advice on a better alternative to correct the V and I phase out problem? I hav also tried power resistor to simulate the same 100W bulb ballast but it over heats! I hav also tried to remove the 'snubber' network, but that made no difference.

Im also enclosing my diagram web link for the actual circuit arrangement......

**broken link removed**

Please can someone help?


hhssmm

phase control triac pic microchip

Hi friend,
check your firing angle, snupper suppresed the heat.

in phase current and voltage waveforms

Excessive heating of transformer typically indicates DC input voltage applied. You may want to check with a multimeter. This happens easily by incorrect phase control operation with inductive load. Triac re-ignition (typically occuring at one half wave only) due to voltage transients is another possible cause.

Basically, you can't "Bring V and I in phase" in case of inductive load. You have to operate the controller considering the inductive current instead. You should be able to learn about appropriate control schemes by watching the current and voltage waveforms in your circuit.

phase angle control with pic

Hi

Thanks for replying.

Currently my PIC MCU gets interrupted @ every 10ms (zero crossing) with normal non-inductive load. However when I connect the transoformer inductive load then I do not get interrupts at every 10ms since the zero crossing is now occuring at every 5ms due to the V & I not being in phase....(by 90 degree I lags V)

Please see the enclosed attachment.

So from your suggestion, to make the firing of the TRIAC symmetric during both halves of the AC wave.....i have come up with a software fix for my PIC MCU which I will now describe as below.

As per the enclosed attachment, lets suppose the first time the PIC is interrupted at zero crossing at -ve 180 degree. I then shut off the zero crossing interrupt. I fire the TRIAC as normal and let the TRIAC conduction continue to my desired level. Then at point 'A' which is approximately at 6ms I then turn back on the zero crossing interrupt. Hence the next zero crossing will be occuring at 0 degrees which is exactly after 10ms. The same logic then repeats over. By this method I predict that the TRIAC voltage wave form will be symmetical as shown in my diagram. This is how i intend to over come my problem of V & I not being in phase in an inductive load.

Please comment/suggest or modify or correct my logic if im wrong.

Thank you
hhssmm

snubberless triac inductive load

FvM said:

Excessive heating of transformer typically indicates DC input voltage applied. You may want to check with a multimeter. This happens easily by incorrect phase control operation with inductive load. Triac re-ignition (typically occuring at one half wave only) due to voltage transients is another possible cause.

Basically, you can't "Bring V and I in phase" in case of inductive load. You have to operate the controller considering the inductive current instead. You should be able to learn about appropriate control schemes by watching the current and voltage waveforms in your circuit.

Click to expand...

Why do you say we can't correct the angle between I and V? If it's an inductive load, you should use a cap in paralel. If it's a capacitive load, you should use and inductive load in paralel. of course, it all depends the values of H and F you apply on your power correction.

and that's it!

best regards

Why do you say we can't correct the angle between I and V? If it's an inductive load, you should use a cap in paralel.

Click to expand...

Compensation of reactant load impedances is a generally an option, but not for a triac controller. In a short, it's neither helpful nor necessary in this case.

FvM said:

Why do you say we can't correct the angle between I and V? If it's an inductive load, you should use a cap in paralel.

Click to expand...

Compensation of reactant load impedances is a generally an option, but not for a triac controller. In a short, it's neither helpful nor necessary in this case.

Click to expand...

hello,

if it overheats, you should.. i think that this is what we're talking about.

by the way, could you explain why you don't have to correct the cos fi when using a triac?

The transformer doesn't overheat due to reactant current, but because inappropriate control leads to false triggering. It's no problem to modify the control algorithm in a way, that even unloaded transformers (e.g. with a defective lamp connected to it) can be phase angle controlled. That's state of the art in lighting control (with analog trigger circuits) since decades.

Compensation of reactive loads only works with sine waveforms, but you don't have it here, furthermore you get high current spikes with phase angle control and connected compensation capacitors. That's why I said, it's not helpful. But mainly, it's not necessary. The apparent power consumed by small transformers can be usually ignored for power quality considerations.

transformer saturation?

Hi hhhsssmmm,
It is happening due to the DC offset in the Transformer due to the unequal phase cutting in the 2 half cycles in each cycle, which is amply clear from your shown waveform. You have to control the triac firing in such a way that phase angle controlling should be equal in both the half cycles, so that no DC offset is present in the Circuit. If you have problem in calculating triac firing delay from ZeroCrossing, you can try Pulse Skipping Method. Thanks.