Help!! Delta... 3 phase 240v!!

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Trained investigators know that eye witness accounts are often mistaken.

"...the mere fault of being human results in distorted memory ..."
http://agora.stanford.edu/sjls/Issue One/fisher&tversky.htm
OK, but if...

...his system has a high leg transformer which is corner grounded at A or C instead of at the center tap between A and C...
...and if his measurement of the B phase was to ground instead of the center tap...
...and if his measurements of A and C were to the center tap...

...wouldn't he get the numbers he has stated?

EDIT: Here's a question: Is a high leg 240 transformer ever intentionally left ungrounded? If so, what if that's what he has plus a ground fault on A or C? He didn't answer my question of what the voltages to ground were at A, B, C, and "N"; that would tell a tale.
 
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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
OK, but if...

...his system has a high leg transformer which is corner grounded at A or C instead of at the center tap between A and C...
...and if his measurement of the B phase was to ground instead of the center tap...
...and if his measurements of A and C were to the center tap...

...wouldn't he get the numbers he has stated?
Probably.
But, his OP clearly states that his measurements were line to ground. Somewhere the story is wrong.
Things might be clearer if all 10 possible voltage readings were presented.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Probably.
But, his OP clearly states that his measurements were line to ground. Somewhere the story is wrong.
Things might be clearer if all 10 possible voltage readings were presented.
Later, in post #41, he says these measurements are to neutral. The man is somewhat confused, it seems to me. I mean no offense to the OP; I myself have been confused many, many times, not the least of which was when I first tried to wrap my brain around three phase power. The fact that for his first experience he has encountered what appears to be a... shall we say, non standard wiring situation can only make things more mysterious than they might otherwise be.
 

kwired

Electron manager
Location
NE Nebraska
Later, in post #41, he says these measurements are to neutral. The man is somewhat confused, it seems to me. I mean no offense to the OP; I myself have been confused many, many times, not the least of which was when I first tried to wrap my brain around three phase power. The fact that for his first experience he has encountered what appears to be a... shall we say, non standard wiring situation can only make things more mysterious than they might otherwise be.
OP does need to clarify if all measurements to "ground" and/or "neutral" are all to the same point when taken. If A or C was grounded he should get a questionable reading someplace else - if the same reference point is used for all "to ground" measurements. And the voltage from "B" to "neutral" is still going to be 208 no matter what point is grounded, if all phase to phase voltages are 240.

Here's a question: Is a high leg 240 transformer ever intentionally left ungrounded? If so, what if that's what he has plus a ground fault on A or C? He didn't answer my question of what the voltages to ground were at A, B, C, and "N"; that would tell a tale.
If you have a system configured from of two or three single phase 120/240 transformers - which is typical when POCO supplies this system - you have two other center taps that are also 208 volts to the opposite phase conductor - but you can only ground one point of the system, it can be one of the center taps or one of the corners. I guess what I am saying is every delta system with center tapped windings as part of the construction is high leg system, but unless you extend one or all of those taps to the supplied equipment you don't really have a usable high leg. You should always extend any point intentionally grounded to the supplied equipment though.
 

electricalist

Senior Member
Location
dallas tx
I didn't get to read the whole thread but I did see #42 .
There are some amazing electrician s on here with top notch morals and character. They will keep restating what we sometimes are mis understanding.
To all who help that don't have to,,,you guys are truly the best in this business.
 

meternerd

Senior Member
Location
Athol, ID
I didn't read all of the posts, so if I'm saying what's already said, shoot me. Why is everyone saying "voltage to ground". If the center tap is not grounded, any readings to ground would be useless. Alas, I have seen more than one 120/240 4W than has the ground disconnected from the center tap. Usually due to a connection that has the ground wire loose, disconnected or corroded. Causes all kinds of weird voltage readings as load changes on the 120 legs if measuring to ground. Especially with a high impedance meter.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I didn't read all of the posts, so if I'm saying what's already said, shoot me. Why is everyone saying "voltage to ground". If the center tap is not grounded, any readings to ground would be useless. Alas, I have seen more than one 120/240 4W than has the ground disconnected from the center tap. Usually due to a connection that has the ground wire loose, disconnected or corroded. Causes all kinds of weird voltage readings as load changes on the 120 legs if measuring to ground. Especially with a high impedance meter.
I have been working on the assumption that something is grounded, either the center tap or a corner. If the center tap is grounded, then his B phase to neutral/ground cannot be 240V.
 

wirenut1980

Senior Member
Location
Plainfield, IN
Interesting thread. Center tapped delta with a buck boost transformer on the wild leg and trying to put load on it? Raising the voltage from 208 V to 240 V?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Interesting thread. Center tapped delta with a buck boost transformer on the wild leg and trying to put load on it? Raising the voltage from 208 V to 240 V?
Bzzzzt! Thanks for playing ...:D

He said the voltages were 240V from B to A and B to C, and 240V B to G and/or N (sometimes he said ground and sometimes he said neutral).
 

kwired

Electron manager
Location
NE Nebraska
Interesting thread. Center tapped delta with a buck boost transformer on the wild leg and trying to put load on it? Raising the voltage from 208 V to 240 V?
That would raise voltage to neutral, but would also raise voltage from high leg to other two phases, which it was said those voltages were still 240.

Would not effect 120 volts from other two phases to neutral though.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
Quality of Meter

Quality of Meter

I have personally measured "365" volts to G on a
480Y/277V system. We failed to realize that
just up the street, on the same transformer, there
was a significant impact on our measurements made
by a relatively large VFD for the size of the shared
transformer. So I can believe this, but when our
scope and better meters were brought out the wave
was very distorted, thus rendering the error. The
expensive equipment showed us we had a little over
280V to ground. And notified the pump operator for
their own sake too b/c they had 277V loads that had
this distortion in their building as well. They installed
something ahead of the VFD and the line to ground
was running around 280V, where it should be when
we closed out the case.
Moral of the story: use GOOD measuring devices and
know their capabilities and limitations.

JR
 
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GoldDigger

Moderator
Staff member
Even the best true RMS meters will still give you confusing numbers when there are high levels of triplen harmonics on a three phase system.
Those harmonics will affect all three line to neutral voltages but will cancel out when you measure line to line. Peak meters will also give confusing results, while averaging meters will give what appear to be better results but will only mask the underlying problem.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
I agree

I agree

Gold, you are of course right about the
limitations on all methods of metering,
but it led to further investigation. The
meters are only tools, the staff are the
ones who have to resort to further
investigation or abstract thinking to
ultimately solve the problem.
And that is what makes problem
solving interesting.

Peace

JR
 
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