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

kwired

Electron manager
Location
NE Nebraska
Not 208. A-g= 120, B-g= 240, c-g=120
@aDave, thank u sir, that registered. So, regardless of B @ 240v any order of the phases still gets me at 240v, correct? Obviously not considering rotation, I know to switch out a phase/rotate..
Not possible to have 240 to ground on B if you have 120 on A and C, the phase angles work out to 208 volts from B to ground. If you have a wye system with 120 to ground it is not possible to have anything but 208 phase to phase. Now if you have closer to 125 volts (common around here) then that high leg will run close to 216.

If you had a 240 volt phase to phase wye system then the voltage to neutral would be ~139 volts.

If you were to wire a delta system that was 208 phase to phase then the A and C to neutral would be 104 and the high leg would be 180 volts to neutral.

You can't cheat the math for the phase angles, you can change some of the base voltages to different levels but all other voltages will change in the same proportions.
 

TNGuy81

Member
Location
Nashville TN
...

...

Ggun USA, TN, 3 phase DELTA 240 ON B. 100% SURE.. 120v A and C..
@jag, its delta, the wye was a example, though both can be terminated on motor, just wired different. The jumpers can be switched, I was just unsure of the how. Apparently its wired similar to single phase with 240v delta vrs wye say 480 L1-A, L2-B, L3-C, 4-7, 5-8, 6-9.. The jumper configuration changes. Why would you need a neutral on a 3 phase... Its three hots and a ground. Now single phase a neutral would be needed.
 

kwired

Electron manager
Location
NE Nebraska
Ggun USA, TN, 3 phase DELTA 240 ON B. 100% SURE.. 120v A and C..
@jag, its delta, the wye was a example, though both can be terminated on motor, just wired different. The jumpers can be switched, I was just unsure of the how. Apparently its wired similar to single phase with 240v delta vrs wye say 480 L1-A, L2-B, L3-C, 4-7, 5-8, 6-9.. The jumper configuration changes. Why would you need a neutral on a 3 phase... Its three hots and a ground. Now single phase a neutral would be needed.
Your typical 9 lead wye wound motor is rated 480 volts (high voltage connection diagram) and 240 volts (low voltage connection diagram) If you are connecting to a 208 volt system you still connect per low voltage diagram but motor will draw approximately 10% more current at full load then it will at 240 volts. Some motors are marked with 208-240/480 volts rating.

Neutral is not needed to run these motors - neutral is still available with the supply so you can run other 120 volt loads however. There are some cases where you don't have a center tap and only 240 volts is available - those will usually be a "grounded phase" system or can be an ungrounded system where permitted. In either of those cases if you need 120 volts for anything you need to separately derive it with an additional transformer.
 

TNGuy81

Member
Location
Nashville TN
.

.

Kwire. It exist! Its a wild leg on b phase 240v! Not 208.. I'm very familiar with 120/208Y, this is again DELTA , 3 PHASE 240VOLT highleg is 240v to ground. I even had my super Master electrician verify it. Its older wiring from poco that still exist. Its got me for a loop also, though I read the meter and that's what it is. Phase to phase is 240v, though B to earth is 240v, A and C 120v..
 

kwired

Electron manager
Location
NE Nebraska
Kwire. It exist! Its a wild leg on b phase 240v! Not 208.. I'm very familiar with 120/208Y, this is again DELTA , 3 PHASE 240VOLT highleg is 240v to ground. I even had my super Master electrician verify it. Its older wiring from poco that still exist. Its got me for a loop also, though I read the meter and that's what it is. Phase to phase is 240v, though B to earth is 240v, A and C 120v..
Then we need more details to find out why, as this is not possible unless something has gone wrong or is not connected right in the first place. But in either of those cases it is still likely you throw other measurements off of what is otherwise expected.


Are you measuring voltage at service equipment or first disconnecting means if a separately derived system? Is the system loaded or unloaded while taking measurements? What happens if you change from loaded to unloaded or vice versa?
 

electricalist

Senior Member
Location
dallas tx
Interesting thread,,because I have heard many times around here of a 240 on one line. At a city water well one of my supervisors said he had a line with no voltage to ground but 240 line to line. I wasnt there but he seemed genuinely concerned about it.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
No! Digi multimeter..
@Don , as In not center tapped delta. Like 277/480 ABC-G= 277, AB=480, BC=480V
I am asking about the internal wiring of the motor. Is that wye or delta? The motor does not care if a wye or delta system supplies, it as long as the voltage supplied to the motor is correct. The low voltage hook-up in the motor junction box is different between a wye connected motor and a delta connected motor.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Interesting thread,,because I have heard many times around here of a 240 on one line. At a city water well one of my supervisors said he had a line with no voltage to ground but 240 line to line. I wasnt there but he seemed genuinely concerned about it.
That sounds like a corner grounded system. You will have 240 line to line, two legs that are 240 to ground, and the third leg is zero volts to ground.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Kwire. It exist! Its a wild leg on b phase 240v! Not 208.. I'm very familiar with 120/208Y, this is again DELTA , 3 PHASE 240VOLT highleg is 240v to ground.
You don't seem to understand what kwired is trying to tell you.

A 3 PHASE, 240 VOLT DELTA with a high leg will give you 208 Volts from the high leg to ground (assuming the neutral point is properly grounded,) not 240V. He is not suggesting you have a 120/208Y.

The system you are describing will have the following nominal voltages:

A-B: 240V
B-C: 240V
C-A: 240V

A-N: 120V
B-N: 208V
C-N: 120V

Take a close look at the diagram Roger posted in post #2.
 

kwired

Electron manager
Location
NE Nebraska
Interesting thread,,because I have heard many times around here of a 240 on one line. At a city water well one of my supervisors said he had a line with no voltage to ground but 240 line to line. I wasnt there but he seemed genuinely concerned about it.
Sounds like a corner grounded system to me as well.
 

kwired

Electron manager
Location
NE Nebraska
I thought he may have Said he was not using a digital tester. Is it possible a solenoid test reads at 240 but it 208
Post 13 he says he was using a digital meter, otherwise a solenoid tester could throw in some confusion depending on what threshold is between indicating 208 vs 240.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Here's a thought. Is it possible he has a transformer with a center tap between A and C but which is connected corner grounded, and when he thinks he is measuring A to ground and C to ground, he is measuring A and C to the floating center tap?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
I thought he may have Said he was not using a digital tester. Is it possible a solenoid test reads at 240 but it 208
It is also likely that he saw the correct measurement but remembered it wrong. His mind took a short cut, the value was a lot higher than 120V so his mind jumped to the next expected voltage which was 240V.

What is the quote? 'it says what it says, not what you think it says'
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Ggun USA, TN, 3 phase DELTA 240 ON B. 100% SURE.. 120v A and C..
@jag, its delta, the wye was a example, though both can be terminated on motor, just wired different. The jumpers can be switched, I was just unsure of the how. Apparently its wired similar to single phase with 240v delta vrs wye say 480 L1-A, L2-B, L3-C, 4-7, 5-8, 6-9.. The jumper configuration changes. Why would you need a neutral on a 3 phase... Its three hots and a ground. Now single phase a neutral would be needed.
I don't mean to sound condescending, but you said at the top of the thread that you do not understand three phase systems. I respectfully submit that if you did understand three phase power you would know that something is wrong with your measurements.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Kwire. It exist! Its a wild leg on b phase 240v! Not 208.. I'm very familiar with 120/208Y, this is again DELTA , 3 PHASE 240VOLT highleg is 240v to ground. I even had my super Master electrician verify it. Its older wiring from poco that still exist. Its got me for a loop also, though I read the meter and that's what it is. Phase to phase is 240v, though B to earth is 240v, A and C 120v..
Go back to the illustration in post #2 and look at the math, the high leg will be close to 208, not 240, this can not be changed in a 240V center grounded Delta.

You could have a custom built center grounded Delta with a different voltage on the high leg to ground but it wouldn't be a 240 V Delta and you wouldn't be reading 120 to ground on the center tapped winding.

Roger
 

electricalist

Senior Member
Location
dallas tx
It is also likely that he saw the correct measurement but remembered it wrong. His mind took a short cut, the value was a lot higher than 120V so his mind jumped to the next expected voltage which was 240V.

What is the quote? 'it says what it says, not what you think it says'
#13 he replied no ! Digital meter, with the question being digital or wiggy.
I mistook the answer
 

winnie

Senior Member
TNGuy81, you do have a puzzler there, and more information is needed in order to figure it out.

I'd suggest a more complete set of voltage measurements at both high and low impedance as a start. With a normal system you have things that you expect (eg ground to N voltage of zero, all L-N voltages equal) but once you have something strange then these expectations go out the door.

Remember that voltage is always measured between two points; a 'complete' set of voltage measurements means getting a voltage between all possible pair of system terminals. You can also have voltages caused by small leakage currents; if you use a high impedance digital meter you see one number, if you use a low impedance 'wiggy' you see a different number.

You have three phases (A, B, C) the 'neutral' N and ground. This gives you 10 pairs and 20 measurements (one digital, one 'wiggy'):
A->B
A->C
A->N
A->G
B->C
B->N
B->G
C->N
C->G
N->G

With those numbers we can tell you more.

WIth a _normal_ 'high leg' delta system, with a single operating frequency, the 'high leg' to neutral voltage should be 87% of the L-L voltage. But in your first post you reported "A to ground reads 120v, B - G reads 240volts, and C to ground is 120v." These voltage are not consistent with a normal 'high leg' delta.

These voltages are possible with some types of _two_ phase systems.

But you later say "Phase to phase is 240v, though B to earth is 240v, A and C 120v.." That throws a wrench into the 'two phase' theory. In a 'reasonable' system that gives your first set of readings, then the phase to phase voltages would be greater than 240V; the physics requires it.

So we are left wondering if perhaps you are seeing a number near to 240V reading it as 240V, or if there is something else going on. For example if there are enough harmonics present they could screw with measurements made given the assumption of pure 60Hz.

I could ramble on more here...but really I just want to see the numbers :)

-Jon
 
Top