Transformer output control with Triac - Problem - Page 1
Forum section: electronics / projects

author: sbdada
author_rank: Contributor
timestamp: October 19, 2020, 03:04:22 am
content: Hello,I'm trying to control the output power of a 1000VA transformer with Triac. I'm using BTA16 along with MOC3021 and an Arduino UNO to drive the Triac. Here's my diagram.This circuit is taken from a already available commercial product, and it's pretty reliable. But the program part is done by me. For the zero cross, I'm using a separate 9V transformer, which driving a transistor to pull-down an interruption pin of the MCU. In my program, I'm firing the Triac from the maximum angle, and then decreasing it gradually according to load demand. That means, output voltage of the transformer is zero or near zero at the beginning, and then increases according the Triac firing angle. Now please see this wave-shape of my actual circuit operation.This is just bellow 5ms (10ms for each half cycle). Everything is working well here, though the wave shape doesn't looks good.The yellow line is the voltage taken across the Triac, and the blue one is taken across the Transformer.This one is taken whe firing angle is near about 3ms.Now look at this one, if I fire the Triac beffore 3ms after the zero cross, this thing happens. In this time, the transformer makes a loud buzzing noise, and within a moment the Triac become shorted. And this 3ms lower limit varies with input voltage or load.Here, the yellow line is the rectified AC signal for zero cross detection, and the blue line is the gate pulse. This is the lowest pulse delay. If I fire it earlier than this, the previous scenario happens.I'm not an expert, so don't know what is going on here. How can I prevent this? Or at-least how can I detect this, so MCU won't fire not before this delay? (As it changes according to load and line voltage)Can anyone please give a solution for this problem?Thanks in advance.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 19, 2020, 03:51:06 am
content: As much as possible, avoid controlling the primary of a transformer by triac.Any dyssimetry saturates the transformer, which causes a very high current which destroys the triac.Instead, place the triac in the transformer secondary, you will no longer have any problem.The control of the primary of a transformer requires a near perfect control circuit, a slow variation of phase angle and a gradual input

author: sbdada
author_rank: Contributor
timestamp: October 19, 2020, 05:00:10 am
content: Thanks for your attention Mr. akimpowerscr. For some reason, secondary control is not possible in my project. Moreover, this technique is pretty common in Bangladesh, India and Pakistan. Those inverter with builtin charging system (IPS) use this circuit to charge the battery. Both analog and digital version is present. So, if there is no problem in the circuit, then it is in my program. Should I post it here?

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: October 19, 2020, 07:00:44 am
content: is there enough holding current for the triac at all firing conditions?

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 19, 2020, 09:12:26 am
content: When using phase control at the primary of the transformer, this transformer is usually of special construction and has a small air gap to be less sensitive to direct current saturation.Maybe this is the problem with your project?Test your circuit on resistive load to see if there is any anomaly in the waveform applied to the load.Also place a 100W incandescent lamp in parallel on your transformer to check if this fixes the problem or not.

author: Alti
author_rank: Frequent Contributor
timestamp: October 19, 2020, 09:43:56 am
content: I think the problem here is that you used rectified input voltage for plotting results.Had you used original sine on input, DC drive would have become evident.QuoteThe transformers shift a phase and this phase shift is load-dependent. You have incredibly complicated the whole control sequence. What is wrong with using a voltage divider and a proper comparator?When driving an inductor, make sure there is no DC on input.QuoteAs for 1kVA transformer driven by BTA16, either you guarantee some slow ramp-up of current on turn-on is applied, or you have to make sure the inrush does not overwhelm a triac. If the transformer is left magnetized in one direction on switch-off and you turn it on at exact same half then the inrush is ~ 20x the rated current. We are talking like >80A hereQuoteThat is a reasonable approach. Just make sure you apply this seuence aftereveryshut-down. Even after a 10ms one.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 19, 2020, 11:37:28 am
content: Regarding fig 4, there is an obvious error in the triac triggering strategy.Either a 180 ° long pulse or a train of pulses must be used.What happens here is that the trigger pulse is applied before the delayed inductive current has passed through 0 A.For this reason, the triac no longer triggers at the next alternance, which causes a strong voltage dysimetry applied to the transformer primary and its saturation.Use a long trigger pulse of 180° duration, you can't use a short pulse as you do, that's your problem !!!!!