WYE Voltages question
- Thread starter Designer69
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- Placerville, CA, USA
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- Retired PV System Designer
To get a unique answer from among the choices listed requires at least three assumptions:
1. The supply is 480Y/277
2. The voltages are measured to ground.
3.There is no load connected on the load side of the fuses.
1 and 2 are reasonable.
Add 3 and the correct answer would be C.
If there is a load connected, the possible results for the voltage on B range from zero to 277 or higher if there is a motor load or roughly 239 if there are resistive loads on A-B and B-C only.
It is a poorly designed question or your recollection of it is incomplete.
JFletcher
Senior Member
- Location
- Williamsburg, VA
Not very well written as there is no reference to what the 480V is measured from; line to line or line to neutral. With a wye, I'd assume line-neutral as I dont know of any 480V to neutral systems here, and the answers arent given as L-L voltages (AB, AC, BC, but A, B and C). I'd answer C as loss of one (B) leg in a 480/277V wye system would result in 0V on B phase when measured to neutral and A and C phases shouldnt change voltage. 277 - 0 - 277.
I assumed that the OP meant 480V line to line.
JFletcher
Senior Member
- Location
- Williamsburg, VA
As did I but it is still an assumption (pretty valid as 480V to neutral with a wye would be 831V L-L, which afaik doesnt exist at all). It was a typo on my part.
- Location
- New Jersey
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- Journeyman Electrician
The OP states that the system is 480Y/277. If the load is disconnected as Golddigger suggests then I agree the answer from phase to ground is C.
Designer69
Senior Member
Damn I selected d) 480 0 480. I'm pretty sure it asked for line-line voltages, so I didn't think the line-line of 480 could drop to 277 (line-neutral).
I'm gonna fail.
I'm gonna fail.
- Location
- New Jersey
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- Journeyman Electrician
Damn I selected d) 480 0 480. I'm pretty sure it asked for line-line voltages, so I didn't think the line-line of 480 could drop to 277 (line-neutral).
If they wanted line to line then it should have given answers that stated which line to line, i.e.: A-B, B-C, A-C. If you just say A, B, or C then the assumption would be a voltage reading to ground or possibly the neutral.
Designer69
Senior Member
Thank you, I appreciate the help.
I do not believe the question asked for resulting line-ground or line-neutral voltages because if it did I would automatically have known that it's 277 and would have chosen C as everyone in this post suggests.
I wish I could remember more exact details.
Thanks again!
with one fuse blown you now have single phase(at the load)
they do not show the neut going to the load
how would you get (measured at load)
A 480
B 0
C 480
???
AB ?
AC ?
BC ?
Electric-Light
Senior Member
Poorly written. The neutral doesn't go to load.
If it's a motor, it will keep going and derive the third phase unless shut off by protective device and you'll have still have pretty close to the voltage you had before.
If it's a motor, it will keep going and derive the third phase unless shut off by protective device and you'll have still have pretty close to the voltage you had before.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
In my opinion these are the voltages:
AC=480V
AN=AC/2=480/2=240 V
CN=-480/2=-240V
BN=0
AB=AN=240V
BC=-CN=240V
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
I agree; it's a poorly written question. Voltages relative to what? Measured on which side of the fuses? Whoever wrote this question should be taking a test, not writing one.
kwired
Electron manager
- Location
- NE Nebraska
OP supposedly has a load connected between A B C, but not to N.
kwired
Electron manager
- Location
- NE Nebraska
If you have a three phase load with no load connected to neutral and equal resistance/impedances on all three legs, the open B section becomes the midpoint of 480 volts between A and C, you will read 240 volts from A-B and from B-C. B to N could be anywhere between zero and 277.
If connected loads are line to neutral loads you measure zero volts from B to N, 277 volts A-B and B-C.
As mentioned if the load were a three phase motor and the fuse is lost while it is running, you may have near "normal" looking voltages.
If connected loads are line to neutral loads you measure zero volts from B to N, 277 volts A-B and B-C.
As mentioned if the load were a three phase motor and the fuse is lost while it is running, you may have near "normal" looking voltages.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
:?
kwired
Electron manager
- Location
- NE Nebraska
Voltages you marked across the windings make sense if 480 volts is applied to all three corners but "B" has open circuit or blown fuse.
Not sure what you are showing with the other image, if measuring voltages across the fuses like it looks like you are showing, the only one that would have a reading other then zero is the blown fuse, presuming there is load connected between all three phase conductors.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
I apologize if you bother insisting. If we compare open delta transformer with 2 phases supplied load then:
1) If it is a transformer the EMF will be:
EAB=EXY*nlv/nhv
AB secondary low voltage winding XY primary high voltage winding.
nlv=secondary winding number of turns
nhv =primary winding number of turns
The same way:
EAC=EXZ*nlv/nhv
VBC=EAB-EAC
If there are only impedances without EMF then only VAC exists and supply parallel impedances ZCA||ZAB+ZBC.
The total impedance seen between A and C will be:
Ztot=ZCA*(ZAB+ZBC)/(ZAB+ZBC+ZCA)
IA=-IC=VAC/Ztot
IAB=IBC=VAC/(ZAB+ZBC)
VAB=IAB*ZAB
VBC=IBC*ZBC
VAB=VAC*ZAB/(ZAB+ZBC)
VBC=VAC*ZBC/(ZAB+ZBC)
IF ZAB=ZBC then VAB=VBC=VAC/2
JFletcher
Senior Member
- Location
- Williamsburg, VA
^ too much math and thinking.
Wye grounded = neutral. No mention of a motor or load type is made; you'd have to assume it disconnected here or the answers change. If you lose the B phase fuse, and measure voltage from phase to ground or phase to neutral (which may or may not be at the load), you will get 277 0 277.
GoldDigger nailed it in post #2.
Wye grounded = neutral. No mention of a motor or load type is made; you'd have to assume it disconnected here or the answers change. If you lose the B phase fuse, and measure voltage from phase to ground or phase to neutral (which may or may not be at the load), you will get 277 0 277.
GoldDigger nailed it in post #2.
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