Converting a six phase rectifier to three phase rectifier

[Ignoramus22091]

On Mon, 24 Apr 2006 19:25:02 GMT, Ignoramus21673



Mabey, I am not sure it would actually work out to be a problem, just
something to check.

OK, I will keep this in mind. My phase converter is 17.5 HP idler
capacity. When I loaded it with full 200 amps of welder output
(shorted leads), the third leg sagged to about 190 volts.

Sounds like a good plan to me, I trust you'll let us know how it goes.

I already started, the first thing that I am working on is mounting
the SCR modules.

Alas, I know next to nothing about plasma cutting. Never even tried
one. They weren't all that common when I was welding a lot in the
early 70's. I think it is a constant current process with the arc
confined to the torch and not transferred to the workpiece as in
plasma welding, but that is just a foggy memory of something I may
have read 20 years ago. Perhaps you can pick a particular torch and
find out what it needs.

I see. Well... If I connect my secondaries with each pair in series,
wye, it should give me about 208 volts AC to play with. I think that
it would result in about 300 volts OCV and 200 volts welding power.

But first things first, I want to make it work as a welder first to
get the proof of concept.

i

 

[joseph2k]

Not the easiest, but, definitely, not terribly hard. A little bit of
cutting and maybe a little bit of drilling will "get me there".



Well, I have only one controller set.


Makes sense.


That (about the phase converter) is a great point indeed.


It is a f*&^ing brilliant idea. I will do exactly that! I will try
starting tonight.


Well, my current SCRs are wired with gate towards the common DC+ bus,
so there is no way for the PCTI controller to sense voltage.

I already have enough isolated dual SCRs, adding them to the octagonal
DC+ bar is very easy. Note that the DC+ bar will no longer be
energized, it will only be used for cooling.

If you do not understand what I mean in the above paragraph, it is my
fault, my main message here is that I have all parts for using half
windings.



I have to admit to it! I think that you know what you are talking
about.

My plan is as follows:

1) use one half of the secondaries as a Wye connected 3 phase source
2) add new dual SCR modules to the octagonal heatsink (that used to
double as DC+ bar)

3) wire everything together with the PCTI modules for some test runs.

If everything works and produces voltage and current, then I could
work on doing a few things such as:

1) reconnecting second half of secondaries
2) making a proper welding control panel.

Glen, do you know anything about plasma cutting? The reason for this
question is that if I put secondaries in series, I would get a power
source that can supply 190V OCV and about 135V continuous voltage.

I could make this transformer switchable from paralleled secondaries
to secondaries in series, with three contactors (not too
expensive). Would it be able to do any plasma cutting?

It would be good to decide on whether to do it, sooner rather than
later. But, first things first, I will definitely try your great idea
about using only hald of the windings "as is".

i

Don't claim to know much about plasma cutters, but my one time neighbors
unit seemed to require an inert gas supply like GMAW and GTAW welders.

 

[Glen Walpert]

Glen, I put together the most basic configuration, and voila, the
controller did produce the voltage that I expected. No current tests
yet, but that the voltage was correct, makes me think that everything
is alright. Now that I know that it is working, I will actually wire
it up properly, that will take quite a while to get all the copper
wiring, contactors etc, right.

I am now going to ask about making a plasma power supply.

i

Sounds good so far. Do you have a MIG torch for testing under load
yet?

An alternitive to transformer reconfiguration with a bunch of large
contactors would be a patch panel with the transformer windings and
load connections brought out to studs on an insulating panel, with
copper bussbar links and a few jumper cables able to switch to any of
the 5 configurations (6-phase, and 3-phase wye and delta both with
series or parallel winding connections). Compare the time difference
in assembly with the likely time spent changing configurations, and of
course parts on hand ...

When connecting transformer windings in parallel it is always a good
idea to connect one end first, power up and check the voltages between
the unconnected ends, which will not only let you know if you have a
winding backwards without letting any smoke out, it will show any
winding imbalance with the correct connection, which should be very
small. Do the same thing with the delta connection, first connecting
the 3 windings in a U configuration and checking voltages between the
open ends before making the final delta connection. If there is a
significant voltage present (more than a few percent of phase voltage)
then check for circulating current at no-load with an amprobe or shunt
when you make the connection. Since it is not an always-on power dist
transformer you could live with some circulating current; eg 10% of
rated current circulating still leaves 90% available for welding and
my hunch is it will not be that high, what there is will tend to
equalize phase voltages, and it is just part of the cost of using
unbalanced power from your rotary converter.

(BTW I think it is considered poor netiquette to stick a persons name
in a message subject line, even though it is done all the time. Glen
or GW would be better than the full name in caps if you want my
attention, but I tend to keep an eye out for your project so you could
put that in the body and I would still probably notice.)

 

[Ignoramus5749]

A little update, I took out the old rectifier from my Cyber Tig, which
freed up a lot of space, for a nice installation of isolated case dual
SCRs.

I will hopefully install the heatsink and SCRs in a couple of days. It
will need some brackets to fit into the space available.

Sounds good so far. Do you have a MIG torch for testing under load
yet?

No, all I have is TIG equipment...

An alternative to transformer reconfiguration with a bunch of large
contactors would be a patch panel with the transformer windings and
load connections brought out to studs on an insulating panel, with
copper bussbar links and a few jumper cables able to switch to any of
the 5 configurations (6-phase, and 3-phase wye and delta both with
series or parallel winding connections). Compare the time difference
in assembly with the likely time spent changing configurations, and of
course parts on hand ...

Well, I already bought five 50a contactors for $9.99 each. I drew some
schematics, they should be enough to switch between parallel delta,
vs. series wye (for plasma).

I am going to start with wiring for just welding (parallel delta), and
make the welder part work fully. I want to move in smaller steps.

After that, I will add contactors to switch between parallel delta,
vs. series wye.

When connecting transformer windings in parallel it is always a good
idea to connect one end first, power up and check the voltages between
the unconnected ends, which will not only let you know if you have a
winding backwards without letting any smoke out, it will show any
winding imbalance with the correct connection, which should be very
small.

Great point, I will do just that.

Do the same thing with the delta connection, first connecting
the 3 windings in a U configuration and checking voltages between the
open ends before making the final delta connection.

This is a very nice idea, I was quite afraid of this delta connection,
but this is a safe way to go.

If there is a significant voltage present (more than a few percent
of phase voltage) then check for circulating current at no-load with
an amprobe or shunt when you make the connection. Since it is not
an always-on power dist transformer you could live with some
circulating current; eg 10% of rated current circulating still
leaves 90% available for welding and my hunch is it will not be that
high, what there is will tend to equalize phase voltages, and it is
just part of the cost of using unbalanced power from your rotary
converter.

Yes. Also, I want to finally make the transformer switch on when I
need to actually weld. It should not be on all the time, like it used
to be, it is a waste of electrons.

(BTW I think it is considered poor netiquette to stick a persons name
in a message subject line, even though it is done all the time. Glen
or GW would be better than the full name in caps if you want my
attention, but I tend to keep an eye out for your project so you could
put that in the body and I would still probably notice.)

Sorry about that Glen, I changed the Subject of this post.

i

 

[Glen Walpert]

A little update, I took out the old rectifier from my Cyber Tig, which
freed up a lot of space, for a nice installation of isolated case dual
SCRs.

I will hopefully install the heatsink and SCRs in a couple of days. It
will need some brackets to fit into the space available.


No, all I have is TIG equipment...

So I assume you are scanning ebay for a deal?

Well, I already bought five 50a contactors for $9.99 each. I drew some
schematics, they should be enough to switch between parallel delta,
vs. series wye (for plasma).

50 A is in the ballpark but seems a bit light for your transformer;
assuming a nominal rating of 200 A, 100 A per 3-phase section, 100 /
sqrt(3) = 58 A per winding. Probably close enough.

Your original rectifier did not make very effective use of the
transformer secondary windings, as each winding was used in one
direction only. With your full wave bridge each winding is used in
both directions, reducing copper losses by 1/sqrt(2) IIRC, so you
could use about 82 A per phase from one of the two windings for 82 *
sqrt(3) = 140 A RMS total welding current. (If you did that for both
secondary windings at the same time you would probably overload the
transformer primary, which was designed for the half wave rectifier
configuration). So you could do reasonable load testing with your
existing single winding configuration. Also you get the same VA with
either delta or wye, so the delta config of one winding would be good
for 140 * sqrt(3) = 245 A. With both windings paralleled the delta
connection will provide 200 * sqrt(3) = 346 A, limited by the
transformer priimary.

Sorry about that Glen, I changed the Subject of this post.

Thanks!

 

[Ignoramus32263]

So I assume you are scanning ebay for a deal?

Yes. I would like, I think, to buy some old industrial wirefeed unit.

50 A is in the ballpark but seems a bit light for your transformer;
assuming a nominal rating of 200 A, 100 A per 3-phase section, 100 /
sqrt(3) = 58 A per winding. Probably close enough.

Yes.

First, the contactors will not open or close under load.

Second, I believe that for most instances, none of the contactors
would carry full "two winding" load, they would carry one winding
load.

For the high voltage wye configuration (plasma), they would carry full
load in the wye point, but that would be much less than welding
current.

Your original rectifier did not make very effective use of the
transformer secondary windings, as each winding was used in one
direction only. With your full wave bridge each winding is used in
both directions, reducing copper losses by 1/sqrt(2) IIRC, so you
could use about 82 A per phase from one of the two windings for 82 *
sqrt(3) = 140 A RMS total welding current. (If you did that for both
secondary windings at the same time you would probably overload the
transformer primary, which was designed for the half wave rectifier
configuration).

(Glen, also note that this Cyber TIG seems to be the same model as the
300A 60% duty cycle unit, but with a controller only capable of 200A,
with the benefit of 100% duty cycle rating).

So you could do reasonable load testing with your
existing single winding configuration. Also you get the same VA with
either delta or wye, so the delta config of one winding would be good
for 140 * sqrt(3) = 245 A. With both windings paralleled the delta
connection will provide 200 * sqrt(3) = 346 A, limited by the
transformer priimary.

This is great. I will wire it up so that all wiring and semiconductors
would be capable of the ultimate 300A rating. If it works in 200A
configuration (using a 100 mV, 200A current shunt) I may switch to a
300 A shunt.

I am working right now on mounting a forced air cooled heatsink (with
a nice heatsink fan) in my welder. This heatsink will carry SCRs. I
hope to finish this today and install the heatsink. It will be mounted
on the main fan frame assembly. I had to make some mounting brackets.

The heatsink will have its own fan, and will be mounted next to the
big 0.11 HP fan also. The reason why the heatsink will have its own
fan is that the fins would be perpendicular to the flow of air
inside the welder, there is no other way to mount it. It's a nice
heatsink with big fins glued in and a shroud around the fan to direct
ait flow.

i