Why don't we use 3 Phase, 6 Wire?

[JDBrown]

Okay, so this is one of those cases where (to paraphrase an old saying), rather than remaining silent and being thought a fool, I'm going to open my mouth and remove all doubt. But that's how I learn.

Is there a good reason why we can't take a 3 Phase Delta-Delta transformer with 240V secondary, center-tap all three windings, ground all the center-taps, and use it to feed three 120/240V single phase panelboards? We were discussing this at work this morning, and it seems to me that there's something wrong with the idea, but I can't seem to pinpoint what it is. Thoughts?

 

[iceworm]

Okay, so this is one of those cases where (to paraphrase an old saying), rather than remaining silent and being thought a fool, I'm going to open my mouth and remove all doubt. But that's how I learn.

Is there a good reason why we can't take a 3 Phase Delta-Delta transformer with 240V secondary, center-tap all three windings, ground all the center-taps, and use it to feed three 120/240V single phase panelboards? We were discussing this at work this morning, and it seems to me that there's something wrong with the idea, but I can't seem to pinpoint what it is. Thoughts?
View attachment 9880

Well, if you draw out the vectors, there is 120V between NA and NB, NB and NC, NC and NA. So you can't common them up with out having a majors short circuit.
this part is truth

this part is just goofing off
So you could do it if you didn't ground the center-taps. Just have three ungrounded single phase panels.

But the NEC wouldn't like that. So go with 480/240 delta. Then you could have three single phase panels all with 240/480 three wire un-grounded

ice

 

[JDBrown]

Thanks, Ice. My gut told me there was something off about tying all the center-taps together, but I couldn't place exactly what it was. Should've sketched out the vectors, I guess. :slaphead:

 

[Besoeker]

Thanks, Ice. My gut told me there was something off about tying all the center-taps together, but I couldn't place exactly what it was. Should've sketched out the vectors, I guess. :slaphead:

Slightly different to your topic.
Six-phase or hexaphase is used for some applications.
High current, low voltage rectifiers are one such.



One such beast:

 

[GoldDigger]

Notice that in Besoeker's diagram you can identify two different three point wyes in the output, and you could connect a suitable voltage delta load to either of them, but you cannot connect any of the non-neutral endpoints together.

 

[dereckbc]

Actually it is used in special applications under Article 647 Sensitive Electronic Equipment. It is 120/60 volt where no neutral is used. AKA Balanced Power systems used in recording studios.

 

[GoldDigger]

Actually it is used in special applications under Article 647 Sensitive Electronic Equipment. It is 120/60 volt where no neutral is used. AKA Balanced Power systems used in recording studios.

Three phase balanced power, now there is something I had not thought about.
But if you use only the end to end connections to feed into rectifiers, you still get only three pulse charging instead of six.

 

[busman]

The last ship I worked on had 24-phase power (48-pulse). If you are going to rectify it and use it for DC, the more phases the better. But, really bad for generating harmonics.

Mark

 

[kwired]

Thanks, Ice. My gut told me there was something off about tying all the center-taps together, but I couldn't place exactly what it was. Should've sketched out the vectors, I guess. :slaphead:

You don't really need to understand the vectors all that much, just look at what you did post, and take out the bonding between the "center taps". Then if we assume each phase is 120/240 volts, wouldn't it make sense that if there is 240 volts from each corner to another corner, then there should be 240 volts between each center tap? If you connect a zero/near zero resistance between two or more points that have voltage between them you will have a short circuit.

 

[GoldDigger]

There is actually 120 volts between the center taps, not 240. Look at similar triangles.

Tapatalk!

 

[kwired]

There is actually 120 volts between the center taps, not 240. Look at similar triangles.

Tapatalk!

You are correct, and that did cross my mind that it wouldn't be 240, but then somehow that thought left me while composing the earlier post.

I guess part of it was I was thinking if you shorted all the center taps the total KVA of the fault should be the same as the KVA if you shorted all three corners instead, shouldn't it?

 

[iceworm]

You are correct, and that did cross my mind that it wouldn't be 240, but then somehow that thought left me while composing the earlier post.

I guess part of it was I was thinking if you shorted all the center taps the total KVA of the fault should be the same as the KVA if you shorted all three corners instead, shouldn't it?

You don't really need to understand the vectors all that much,

Well, if you draw out the vectors, there is 120V between NA and NB, NB and NC, NC and NA.

JDB -
It's okay to look at the vectors :roll:

ice

 

[kwired]

JDB -
It's okay to look at the vectors :roll:

ice

Still have part of my foot stuck in my mouth, but I'm working on it.

 

[GoldDigger]

Depends on the impedance of the transformer.
I would look at it as the difference between shorting the primary or the secondary of an auto transformer fed from the 240 delta secondary.
If the fault current is limited by the actual primary circuit, then both methods of shorting the secondary should be equivalent.

Tapatalk!

 

[iceworm]

Still have part of my foot stuck in my mouth, but I'm working on it.

I was giving it my best to poke gently at you and not draw blood. Sounds like I succeeded and you did take it that way.

ice

 

[Fulthrotl]

what always amazes me here....

is not that a bunch of folks will chime in why something won't work,
this is da 'net, and that happens everywhere...

but the range of experience of stuff people have worked on, and
with, here... there were examples given of where something like
this DOES exist....