Three-Phase Thyristor Bridge Missing One Thyristor

I'd guess you have a defective gate circuit, probably the pulse transformer (those always go first). No pulse, no gating, no conduction. In my experience, it's generally easiest to just replace the entire firing board, because if you fix the one bad pulse transformer, the others are all the same age and will likely fail later.

Open SCR or leg fuse - possibly - not that familiar with welders, mostly DC motor drives.

Bad firing circuit - distinct possibility.

Bad pulse transformer - possible - but funny thing - in 40 years I never had one bad pulse transformer - perhaps just rubbing shoulders with different manufacturers.

Thank you all for the replies.

I will try to see the gate signal for each thyristor on the scope, I do not know if my scope is capable of displaying such low voltage and high frequency signal (not actually a scope, it's a power analysis meter with an added function to see the waveform but not much settings to play with). I will post a picture of the waveforms when I have the chance.

So, as I expected, my scope wasn't able to view the gate signals. bu I measured them with a normal multi meter. here are the measurements (thyristor numbers according to the picture):

thyristor 1 113mv 100Hz

thyristor 2 121mv 135Hz

thyristor 3 120mv 100Hz

thyristor 4 120mv 100Hz

thyristor 5 no readings (multi meter reads 00.0)

thyristor 6 110mv 100Hz

So it obvious that there is no gate signal going to thyristor 5. note that the voltage readings is low because, I suppose that, I am reading AC voltage and the multi meter reads RMS value.

here is a picture of the output DC voltage:

I didn't check the thyristors because they are placed very awkwardly and need lots of work to get to. should I be 100% sure that it is the main board?

Durguy, thank you for a very informative post, I am sure that some of the members could learn something from it.

So when are you going to work for Miller or Lincoln as a welder repair man? Oh heck - even better idea - in their design department.

I was a temporary maintenance worker here until we got the panel / skid control design department busy year round, and I tried this once on a bad welder, with a whole lot less success than you have had. Excellent post.

Thank you for the nice comments.

lets take this a bit further, suppose we replace thyristor 5 (the one not getting a fire signal) with a diode of the same voltage and current rating. What would the output voltage and current look like? How would that affect the welding capability of the machine? Note that the machine can weld now fine for small jobs but when you increase the current or change the welding mode ( i guess that is what it is called, i am not a welder) you can feel and actually hear where thyristor 5 fails to conduct.

Not recommended. It may output rather large current spikes.

You have narrowed it down to the gate circuit --but probably will need a "real" oscilloscope to troubleshoot further.

1) check gate leads for continuity.

2) back track the gate circuit to the pulse transformer - is the pulse there on the secondary? - then go to the primary side and confirm.

3) If it is not on the primary side then you need to troubleshoot further. Or replace driver circuit and see what happens.

SCR is about same cost at diode - so no big saving there.

Can you explain further why a diode is not recommended.

It will put out a larger spike of current, because it will start conducting prematurely. (alpha = 0 degrees). When the SCRs are fired in turn they will all have a properly controlled (even) phase angle.

Yes I get that, but if thyristor 5 was replaced by a diode, it is paired with thyristor 4 and 6. So it will only conduct if thyristor 4 or 6 is on which is exactly what we want. So it will be kind of controlled by the firing of those thyristors. Right?

Guys, anyone can analyse the operation of a fully controlled bridge with a diode instead of one of the thyristors. I did my analysis and I think it might work, not as good as the original thing obviously. I just need confirmation, maybe you guys see something I overlooked.

spend your time fixing the root cause problem.

Well, you see, I am from a country where getting spare parts is extremely hard. the welder belongs to a company that works in oil fields maintaining crude oil tanks. Even for them getting spare parts is very hard.

they have come to me to repair the welder, they just need it to work. I am trying to make it work the easiest and fastest way. I could just tell them to buy new replacement thyristor controller, which I am sure they wouldn't be able to, and leave it at that. But I am trying to make a living and they are trying to just complete their job.

besides, I'm trying to think outside the box, challenge myself and, hopefully, create an interesting discussion for you guys. You never know what you can learn from somebody else.

so anyway, I did my research and I think a diode will work. It would be like a half controlled bridge. I have a 250A diode which I am going to connect to the bridge and see what happens.

I will keep you guys posted.

Durguy, the part that I wonder about is the fact that a Diode will actualy conduct for nearly 120 degrees and not be controlled to only conduct for the 60 degrees, only for the time it is needed. The Thyristor is controlled to only conduct from peak to 0 crossing while the diode will conduct from 0,6 V above 0 crossing and stay conducting up to 0,6 V before 0 crossing.

Durguy, if you draw the conduction path of your diode into your scope trace you will see that the diode will actualy switch on as a dead short into the peak of another phase and stay on into the peak of another. I think it will destroy the thyristors of two other phases.

This is my analysis: The diode in position 5 will only conduct if phase c is more positive in respect to phase a and phase b. If the conduction path is between phases c and b. The diode will not conduct until thyristor 6 fires. And similary, if the conduction path is between phases c and a, the diode will not conduct until thyristor 4 fires.

I do not see any "dead shorts"

I believe you will get one "extra large" current peak. The missing gap will fill in.

I still believe you need to troubleshoot the firing circuit - it is "nickels and dimes" for parts, whereas the large diode is dollars.

If you get the firing circuit working it will perform as "new".

I have on several occasions found it to be as simple as a broken gate lead.

you are right, I will look at the controller board tomorrow.

I know I keep bringing this up but it is bugging me, isn't each pair of thyristors fires at the same time, I mean T1 and T6 fires then T1 keeps conducting because it is still forward biased, while T6 becomes reversed biased and stop conducting, then T2 fires and the current runs through T1 and T2, and so on for other thyristors. can you please explain? I still don't get this. the diode will not fill the missing gap because it will not conduct by itself only, it will need a return path for the current, and this return path will only be created if T6 or T4 fire.

so, when it is D5 turn (the diode in place of T5) it will become forward biased as soon as Vc is more positive that a and b. but it will not conduct because both T6 and T4 are off. as soon as T4 fires, the path will be completed and the current will flow. then, when Vb becomes more negative the Va, T4 will be reverse biased and turns off and T6 will fire continuing the current path coming from D5.

I feel like I going in the right direction here, anyone?

well, I couldn't find anything obviously wrong with the thyristor controller, thyristor 5 itself was ok. I guess I will advice them to order a replacement controller.

meanwhile, can I connect a diode just to check the waveforms, no harm right?

It will be interesting and at the least educational.

UPDATE

I simulated the circuit in Mulitisim. It took me some time to figure out all the settings, I had to do the firing angle settings manually by setting a pulse generator for each thyristor and manually set the time delay of the pulses to correspond to the required angle. The most hideous job was calculating the time in msec for each thyrisor to fire at exactly. I think starting from Multisim 12, they added an integrated 6-pulse phase angle controller but I have version 10, so I did not have the privilege.

Anyway, this is the circuit (very messy, but works)

And here is the output waveform at a firing angle of 60 degrees, the red wave is phase A.

Here is the output waveform with an open circuit in place of thyristor 5. And the actual waveform from the output of the welder. They are exactly the same.

Now, here is the waveform with a diode instead of thyristor 5.

you can see where the diode just outputs the ac wave with no control (no phase angle control), this will increase the average value of the output voltage and current.

The input voltage is 50V ac, I set the load resistance as 1ohm. With everything normal i.e. all thyristors are working the output dc current was 75A, with thyristor 5 disconnected the current was 54A. With a diode in place of thyristor 5, the current was 81A.
This is the output waveform when I put an inductor of 5mH in series with the load, notice that the circuit here still has a diode instead of a thyristor. The waveform is much smoother. The welder has a rather big inductor in series with the output though I don't know its value.

So, I think I will put a diode in, set the welder in the minimum settings and test, will post an update here when I get the chance.

Durguy, thank you for a splended piece of education. You really went to a lot of effort to proof your thinking. I as one appreciate a splendid post with good science and experiments.

Oomborrie, thank you for your nice words. it looks like nobody is interested in this thread anymore, but I will keep updating it hoping it would help somebody looking for answers in the future.

So, I did some testing on Multisim. the dc current output with a diode in the bridge seems to be always about 10-15amps more than with an all thyristor bridge, no matter what the firing angle was. though the bigger the firing angle, the more portion of the input waveform the diode lets through comparing to the other thyristors. see this (firing angle is 120, and with a diode in place of thyristor 5)

the difference in the output current becomes more obvious the bigger the firing angle, this means that a difference of 10amps when the current is 100amps is way less obvious than a difference of 10amps when the current is 5amps.

maybe tomorrow or the day after I will test this on the machine itself, I will have a welder to test the machine in action and we'll see what happens. I will post updates then.

I am watching!

Add a little inductance on the output for that last simulation so the current becomes continuous - the result will be a little more spectacular.

It's not unusual to see three tiristor and diodes at DC bridge and it working well . I did not see good your data but I think that two tyristor at your welding aparat is not working .

I know it is not unusual to use three thyristors and three diodes, that is called half controlled three phase rectifier.

if you look at the oscilliscope graph, from both the simulation and the actual graph, you'll see that the wave has four "bumps" and then goes to zero. If everything works properly, you'd have six bumps and repeating so you see it continous. Each bump has two thyristors working, and the next bump has one of the thyristors from the previous one with a new thyristor (look at the thread from the start, people have explained this). So what i am trying to say is, if there are two thyristors not working, the wave would be missing three or four bumps at most, not only two. If the two thyristors not working are consecutive, three bumps would be missing. If they are not consecutive, four bumps would be missing.

So, the diode I have is 275A stud mount. And the thyristors in the welder are disk type (see picture). So they are not exactly direct replacement, mounting-wise.

So, as I was about to go install the diode (just put it wherever and connect for testing), it occurred to me, cooling! Since the thyristors are different type, the cooling in the machine(heat sinks and fans) are designed for this type of thyristor.

so I tested the diode at 200A with a loading unit at the workshop, and it easily reached about 120 degrees celsius in a matter of seconds. Now I do not know what the maximum operating temperature is, but I am sure that is too much.

so I am now in the process of making a heat sink for my diode before installing it in the machine and test, because I want to test it at all conditions, from minimum to full load.

note that the welder maximum current is 600A, the diode will handle a third of that, which is 200A.

I will keep you updated.

Diodes are available in the same puck size.

You can also strap the SCR to turn on like a diode - it only takes a few parts.

this is the only diode I have with such current rating. stapping the SCR is a very good idea. why ddn't I think of it. however, the anode voltage which is the phase voltage needs to be stepped down before connecting the gate or else the SCR will be damaged. how would you go about doing that?

Yikes! I haven't done that in 30 years.... did it for a transit rectifier for a subway train - it was just a couple resistors and maybe a diode - since the SCR turns on the forward anode-cathode voltage drop is small - reverse blocking you have to protect the gate against a reverse voltage. On most larger SCR's the gate-cathode resistance is about 20 ohms (unless it is a sensitive gate). I think the circuit was simply adding from the anode a 1N4007 diode in series with about 180 ohms to the gate - (but check that the gate-cathode resistance is about 20 ohms measured either polarity with your meter.) If the g-c resistance is much higher than 20 ohms, then add a 50 ohm resistor across the g-c leads.

Warning - this is an experiment - do it at your own risk.

Thank you for the information.

but what is the anode voltage here? I mean like the phase voltage? Because the 180 ohms resistor has to drop that to the a voltage that is tolerable by the gate. So do I change the resistance value according to the voltage?

No need - the diode conducts and turns the SCR on - so the fwd voltage ends up around 2 volts.

Reverse blocking the diode takes the voltage,

Oh ok, I get it now.

but what do you mean "Reverse blocking the diode takes the voltage"?

When the SCR is reverse blocking, the internal GC resistance (say 20 ohms), plus the external resistance, plus the reverse (blocking) diode is the conduction path. The diode because it is "blocking" takes 99.99% of the total reverse voltage, so the GC is only slightly reverse biased.

Durguy, could you get the Welder to operate successful at the end using the Diode?

Just today I had the chance to work on the welder, before trying strapping the thyristor as GW suggested I thought I might get a closer look at the board itself.

I will post a detailed update when I finish.

Just wondering if you managed to conclude on the fault you had with the welder. I'm into welding machine repairs and would obviously benefit from your knowledge. Great posts on this thread. Keep the discussions going!

I will also like to know what happened at the end and if you were successfull.

I also found one issue while working with a bridge having 6 SCR picture is attached

https://postimg.cc/image/gf7gqzvuf/

I don't see a problem with the last scope shot - it is a condition known as discontinuous conduction and is simply low enough current with the circuit LRC time constants theat the current stops flowing between the gating of the SCR's. increase the current and it will become continuous with the saw tooth appearance the other traces have.

Hi guys,

Can anyone help me to understand how to measure the firing voltage to a thyristor using any measurement tool like a multimeter ?

I want to troubleshoot a welding machine from Esab which works on thyristors.

See if this will help you.

https://www.eng-tips.com/viewthread.cfm?qid=259725