I am trying to diagnose a circuit failure at my workplace designed by one of my coworkers for driving a particular Tubular Motor that will be part of an automation system in a bioclimatic pergola installation. The motor specs are as follows:

and the circuit schematic for the Tubular Motor driving circuitry is this:

AC-MOTOR-1 and AC-MOTOR-2 nets are going to an MCU onboard and are controlled by 3.3V IO pins. J9 is a screw terminal for connecting the motor. The neutral wire is always connected to the motor, and two live wires determine the spinning direction. TRIACs are there for connecting one of them to the live wire to turn the motor in either direction.

The driver can start, change direction, and stop the motor over 1000 cycles without abrupt behavior or excess heating, but there are adjustable mechanical limit switches inside the motor for limiting the travel distance. Sometimes, when the motor hits one of those switches and stops like that, one or two of the TRIACs become damaged and continuously conductive, which renders them useless.

From what I can observe, the circuit should have MOVs in parallel with each TRIAC according to ST's design documents, but my coworker apparently skipped that detail. I also noticed that the optotriac circuit is looking a little off, but I am not sure if that is causing this issue. The circuit works normally after changing the TRIACs with new ones, and nothing else seems to be affected by this issue.

I tried adding a MOV with clamping voltage around 600-700V, and this is probably big for T405 TRIAC, but that was the only one I had at the moment. This helped, but only to some extent. TRIAC survives a couple of times after hitting switches, but it still gets shorted eventually and doesn't seem reliable at all yet. I also tried adding a bigger TRIAC BTA16, but again, this seems to be helping a little, but only to some extent, and the fault rate is still quite high and not ready for a demo.

I am asking for any suggestions for what might be the reason for this problem and things I can try for further diagnosis or if there is anything off with the schematics or with the particular motor in question.

 

I see that you’re using 220VAC. Procure MOVs with a withstanding voltage of 250VAC. . Also called maximum AC volts.
After you have chosen a MOV, look for the Varistor voltage rating. It should be lower than the MOC3063’s 600 volt rating. Your Triacs are rated for 800 volts, you should be fine there.
Also you don’t mention the motor’s locked amp rating. The provided data sheet only lists the full load rating. Again, this locked rating should be lower than the Triac’s maximum current rating of 4 amps.
EDIT;
If all of this fails, it is time for you to purchase or borrow a hall-effect current probe. With a DSO, capture the current waveform as the motor hits the end stop. If you can also get a hv diff probe, also simultaneously capture the voltage waveform.

 

Try it with the BTA16-600BW triac, remove C2 and C4, and change R7,R8, R10,R11 to 47Ω.
The MOC3063 is a zero-cross driver, so you need to make sure that there is enough current to trigger the triac early in the cycle.
A triac with a higher gate current requirement is likely to be tougher than a sensitive gate triac, as sensitive gate triacs are more prone to dV/dt triggering, and the -W (snubberless) version is less prone to accidental triggering. You need as much gate current as you can get to make sure it switches quickly and cleanly.
If both circuits drive the same capacitor-run induction motor, make absolutely sure that they can't both switch on at the same time. It may be worth using a relay to set the direction, then switching the motor with a single triac. That way the relay never switches under load, and you can't get both forward and reverse switched on at the same time.

 

Sometimes, when the motor hits one of those switches and stops like that, one or two of the TRIACs become damaged

The T405_800B triac has a current rating of only 4A rms. Your motor rated current is 1.6A, but its stall current (when it hits an end stop) will be several times that, so likely exceeds the triac rating.

 

I see that you’re using 220VAC. Procure MOVs with a withstanding voltage of 250VAC. . Also called maximum AC volts.
After you have chosen a MOV, look for the Varistor voltage rating. It should be lower than the MOC3063’s 600 volt rating. Your Triacs are rated for 800 volts, you should be fine there.
Also you don’t mention the motor’s locked amp rating. The provided data sheet only lists the full load rating. Again, this locked rating should be lower than the Triac’s maximum current rating of 4 amps.
EDIT;
If all of this fails, it is time for you to purchase or borrow a hall-effect current probe. With a DSO, capture the current waveform as the motor hits the end stop. If you can also get a hv diff probe, also simultaneously capture the voltage waveform.

Yes, after replacing the TRIACs with higher-rated ones and purchasing more appropriate MOVs, if the system keeps failing, then my next plan is only to do some tests on the motor itself to determine some other specs that aren't mentioned in the technical documents, such as the locked current rating. I don't have the necessary probes for measuring transients in 220VAC yet, but I will be looking to get my hands on them in the meantime.

I do wonder if using ICLs such as this one in series with the TRIACs is feasible for limiting high inrush currents at the starts and stops at limit switches instead of relying on an over-rated TRIAC.

 

Try it with the BTA16-600BW triac, remove C2 and C4, and change R7,R8, R10,R11 to 47Ω.
The MOC3063 is a zero-cross driver, so you need to make sure that there is enough current to trigger the triac early in the cycle.
A triac with a higher gate current requirement is likely to be tougher than a sensitive gate triac, as sensitive gate triacs are more prone to dV/dt triggering, and the -W (snubberless) version is less prone to accidental triggering. You need as much gate current as you can get to make sure it switches quickly and cleanly.
If both circuits drive the same capacitor-run induction motor, make absolutely sure that they can't both switch on at the same time. It may be worth using a relay to set the direction, then switching the motor with a single triac. That way the relay never switches under load, and you can't get both forward and reverse switched on at the same time.

I will definitely be changing these TRIACs to something similar to what you have advised. I do not think these small DPAK TRIACs are designed or tested for such medium to high watt motor tasks that involve sudden stops caused by mechanical limit switches. My coworker was blaming the motor capacitor inside the chasis and told me that removing it almost fixed the issue so I am suspecting we are dealing with a very high current capacitor discharge on TRIACs caused by a faulty firmware, so that's on the check list too.

I really liked your last suggestion. I always prefer preventing problems on the hardware level instead of relying on the software to prevent it, and I think this should always be the common practice because code can change but hardware can't change. However, the whole purpose of this new design is to replace our previous working system that used electromechanical relays to do the same thing, and our project leader/boss doesn't want us to use relays. Would it be possible to implement a similar failsafe system by using something like a NAND gate IC to trigger only one of the TRIACs with a single input so any software glitch won't cause them to be activated at the same time, but I think this still won't guarantee that the active TRIAC will be fully closed when the direction change signal arises and opens the other TRIAC.

 

The T405_800B triac has a current rating of only 4A rms. Your motor rated current is 1.6A, but its stall current (when it hits an end stop) will be several times that, so likely exceeds the triac rating.

Yes the TRIAC on the schematic is really not cut for this kind of task and will be replaced to allow more margin of error...

 

Yes the TRIAC on the schematic is really not cut for this kind of task and will be replaced to allow more margin of error...

Triacs will withstand a serious amount of overload. 10 times the rated current for one mains cycle is common. If your triac fails, it is getting more than that!

 

these tubular motors can rotate in 2 directions, when driving in one direction, the voltage on the other side is TWICE the mains voltage. So, for 230V AC +10% gives a peak voltage of 358V, twice that is 716V. Even a 800V triac does not have enough headroom to handle the voltage spikes that occur when switching the motor off.
I think that a relais is a better choice, also make sure that the relais can handle about 1000V between the contacts.