Weird stuff with 3 phase transformer

[Ignoramus2330]

You can consider this post to be venting, as such, possibly I do not
give enough information to solve my problem, and I am not trying to
avoid spending time with a multimeter.

So, I have this welder, with three phase transformer, where there are
two secondaries on every leg.

I have five contactors that switch from paralleling two secondaries on
every leg, and delta connecting them, for welding, to wiring them in
series Wye connected.

Put in the microcontroller, made some fixes regarding voltage feedback
and could run some stick electrode beads.

And then, after a short while, this thing stopped working because the
SCR firing board detected an error. As it turned out, there is
markedly lower voltage on one of the legs leading to the SCRs.

Voltage between legs 1 and 2: 68 volts, 1-3: 25 volts, 2-3: 25 volts.

So the SCR firing board detects imbalance and shuts down.

I measured input voltage from the phase converter, it was as perfect
as it could get, 248, 250 and 251 volts.

There is no electronics in the rewiring circuit, only contactors.

Contactors do engage, visibly, and there is no voltage difference
across any pair of contacts, so I think that they close properly.

This issue defies all explanations.

Very weird. Our neighbors will take our son along with their son to a
festival tonight, so I hope that I will have time to look at it.

i

 

[John - KD5YI]

You can consider this post to be venting, as such, possibly I do not
give enough information to solve my problem, and I am not trying to
avoid spending time with a multimeter.

So, I have this welder, with three phase transformer, where there are
two secondaries on every leg.

I have five contactors that switch from paralleling two secondaries on
every leg, and delta connecting them, for welding, to wiring them in
series Wye connected.

Put in the microcontroller, made some fixes regarding voltage feedback
and could run some stick electrode beads.

And then, after a short while, this thing stopped working because the
SCR firing board detected an error. As it turned out, there is
markedly lower voltage on one of the legs leading to the SCRs.

Voltage between legs 1 and 2: 68 volts, 1-3: 25 volts, 2-3: 25 volts.

Lower voltage on *one* leg? From the above, it appears that you have
*higher* voltage on one leg, or lower voltage on *two* legs.

So the SCR firing board detects imbalance and shuts down.

I measured input voltage from the phase converter, it was as perfect
as it could get, 248, 250 and 251 volts.

There is no electronics in the rewiring circuit, only contactors.

Contactors do engage, visibly, and there is no voltage difference
across any pair of contacts, so I think that they close properly.

This issue defies all explanations.

Very weird. Our neighbors will take our son along with their son to a
festival tonight, so I hope that I will have time to look at it.

i

Draw some vector diagrams of your connections. Then play around with
swapping their polarities and how they are summed on the diagram. You will
see that your combination of voltages is possible if the windings are
mis-connected, I think. For example, for a Y connection, add the negative of
phase a to phase b and consider the magnitude of the resultant with respect
to the positive of phase a. If each phase has a magnitude of 1, then the
resultant has a magnitude of 2.65 which is 25V*2.65=66V. Pretty close to
your measurement.

I'm not saying this is your problem. It's just a clue.

Good luck.

John

 

[Ignoramus2330]

Lower voltage on *one* leg? From the above, it appears that you have
*higher* voltage on one leg, or lower voltage on *two* legs.

I agree, that's a better way of putting it.

Draw some vector diagrams of your connections. Then play around with
swapping their polarities and how they are summed on the diagram. You will
see that your combination of voltages is possible if the windings are
mis-connected, I think. For example, for a Y connection, add the negative of
phase a to phase b and consider the magnitude of the resultant with respect
to the positive of phase a. If each phase has a magnitude of 1, then the
resultant has a magnitude of 2.65 which is 25V*2.65=66V. Pretty close to
your measurement.

I'm not saying this is your problem. It's just a clue.

Thanks. I will save your post and double check. Here's the weird part,
it was all working great up until some point!

My own feeling was that it is due to some contactor not closing. OTOH,
I checked voltages between all pairs of contacts that should close,
all such voltages were 0. I will doublt and triple check everything.

I am sure that it is something simple, there ain't much there, just
contactors.

i

 

[John - KD5YI]

I agree, that's a better way of putting it.




Thanks. I will save your post and double check. Here's the weird part,
it was all working great up until some point!

My own feeling was that it is due to some contactor not closing. OTOH,
I checked voltages between all pairs of contacts that should close,
all such voltages were 0. I will doublt and triple check everything.

I am sure that it is something simple, there ain't much there, just
contactors.

i

Make sure that there is some current through the contacts when you make your
measurements. That is, you may need a load on the output to see if there is
a drop across the contacts.

I agree that it must be simple. It is just elusive because our thinking is
not in the right groove.

Cheers,
John

 

[Ignoramus2330]

Thanks John. I think that I figured it out. One of the big wire nuts
almost "lost" one of the wires.

i

 

[Paul Hovnanian P.E.]

Thanks John. I think that I figured it out. One of the big wire nuts

[snip]

Hey! Watch it! We prefer the title 'eccentric'.

 

[John - KD5YI]

Thanks John. I think that I figured it out. One of the big wire nuts
almost "lost" one of the wires.

i

That's good news, i. So many problems are caused by simple things.

I admire your tenacity.


Cheers,
John

 

[Ignoramus28437]

That's good news, i. So many problems are caused by simple things.

I admire your tenacity.

Thank you.

John, did you see my message that I have now a fully functioning tig
and arc welder? I did some final wirings, then tested last night and
things seem to work fine.

i

 

[John - KD5YI]

Thank you.

John, did you see my message that I have now a fully functioning tig
and arc welder? I did some final wirings, then tested last night and
things seem to work fine.

i

Yes. I have been following pretty much all your posts. That post and your
earlier posts are what prompted my comment about your tenacity.

j

 

[Ignoramus28437]

Yes. I have been following pretty much all your posts. That post and your
earlier posts are what prompted my comment about your tenacity.

Oh, I see, thank you for the compliment. My spouse does not always
appreciate such tenacity.

Like I said, I have a few bugs to work out: high voltage mode for some
reason has unstable voltage, I need to put in 500V analog voltmeter,
get pulsed tig to work (should be just a matter of enabling it), etc.

i