1. Home
  2. Electronics Design
  3. Weird stuff with 3 phase transformer

Weird stuff with 3 phase transformer

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
Reply to
Ignoramus2330
Loading thread data ...

Lower voltage on *one* leg? From the above, it appears that you have

*higher* voltage on one leg, or lower voltage on *two* legs.

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

Reply to
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
Reply to
Ignoramus2330

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

Reply to
John - KD5YI

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

i
Reply to
Ignoramus2330

[snip]

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

-- Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com

------------------------------------------------------------------ I have a very firm grasp on reality. I can reach out and strangle it any time!

Reply to
Paul Hovnanian P.E.

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

I admire your tenacity.

Cheers, John

Reply to
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
Reply to
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.

j
Reply to
John - KD5YI

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
Reply to
Ignoramus28437

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.