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IMPORTANT UPDATE (Update 6), scroll down to see.

I have been trying to make an AC (air conditioner) soft starter for my home AC unit.

I have built the following circuit and connected it to my AC unit compressor. As soon as the AC starts the TRIAC fails and becomes a short between T1 and T2 pins. I have tested the circuit with smaller loads like a small 1 HP air compressor and it worked fine.

Schematic:

enter image description here

Code:

https://www.codepile.net/pile/wXkBew0R

Any ideas on why the TRIAC keeps failing?

Update 1:

I forgot to mention that the AC unit has a big capacitor on it but I couldn't read it's value.

Update 2:

AC unit specifications: Phase 1, Volts 220-240V, FREQ 50Hz, Max Input 15.8A

Update 3:

Start-up waveform:

The sequence starts like this and the on time keeps getting longer until it gets to the end which is always on.

Green is the Zero cross signal from the optocoupler. Yellow is the TRIAC_PIN

enter image description here

Later on:

enter image description here

Update 4:

The 'Soft start' effect on the scope with an air compressor: The differential probe is on a 1 ohm shunt.

Without soft starter: enter image description here

With the soft starter:

Current rises slowly over time. enter image description here

Update 5:

Here's the current (yellow) and voltage (green) waveform on the compressor:

enter image description here

Update 6:

Okay, so I decided to scope the AC unit without the starter and results were quite different, have a look the following:

The current shunt is 0.1 ohm.

AC unit startup: enter image description here

Normal Operation: enter image description here

So, apparently the startup current is almost around 100A (70A RMS)! with 150 - 160 phase angle between current and voltage. Under normal operation it consumes 23A (16A RMS) but the current wiggles! have a look at this video.

Update 8:

I sacrificed another TRIAC to check it whit scope and see what is destroying it but, I couldn't find anything.

Current = yellow x 10, voltage = green x 10 enter image description here

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3 Answers 3

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Sounds like the triac is underrated for the surge. There are somewhat larger triacs such as CMA80MT1600NHR, but good luck finding any in stock at the moment.

Two SCRs are also a possibility (and likely much more available in hefty module form), and you could trigger them with your single opto. See Jack Creasey's answer here

enter image description here

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Your circuit is not a "soft starter". In other words "zero detection" is not a "soft starter"

Compressor inrush of 100+ amps sounds about right for a compressor with RLA of 15-16.

The compressor will have a large inrush current for at least 160ms... until the compressor has gotten up to speed.

You are exceeding the "inrush rating" of the BTA41. You can pound it for an full AC cycle or two. Not for 100+ cycles of 100 amps.

Soft starters for home AC units (for 2-5 ton condensers) cost $300-$500. They are designed to "learn" how to startup (using a current ramp map) and detect when the load has change. In other words they have a built in relay "just in case". When the present inrush load exceeds the current ramp map used by the processor in the soft starter it "bails out" to a trusty relay.

Soft starters I install need 4 or 5 compressor starts to learn/develop the current ramp map.

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Complementing the answers from Spehro Pefhany and pianoman, here is the datasheet for your TRIAC (BTA41) from ST.

The relevant part is figure 5:

enter image description here

Note the two curves: the top one is for non-repetitive surges, with the internal chip temperature at 25°C; the bottom one is for repetitive surges with case temperature at 75°C. Your actual situation could be represented by a curve inbetween.

Anyway, assuming your TRIAC is at room temp, it could handle 100A peaks only for about 250 cycles (20 ms each), so about 5 seconds. Most probably you are in worse conditions, since the circuit is probably encased and mounted somewhere where even if just starting "cold" the real internal temperature could be higher than the 25°C of the top curve.

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