rattus said:It certainly is confusing isn't it? Now, when computing average power, current must be LEAVING the source in question. We do that on paper just by reversing the arrow and shifting the phase by 180 degrees. The average power is positive as it has to be.
All in the way you look at it.
rattus said:Are we ready to tackle something really hard? Like 120/208 Wye?
Wye? Why? :-? grinrattus said:Are we ready to tackle something really hard? Like 120/208 Wye?
With the voltages as defined, there will be no current. Write the loop equation:Rick Christopherson said:Jim, does this graphic help you out? If not, just tell me what needs to be modified. (I didn't feel like making a whole new drawing, so this is just photoshoped from the text book scan I took earlier. That's why the names are different.)
jghrist said:With the voltages as defined, there will be no current. Write the loop equation:
jim dungar said:Only your method is confusing.
1. If current must be LEAVING the source then you should not define the sources as you have.
2. Please show how the current in the two wire circuit can LEAVE Van and also LEAVE Vbn.
3. Please show how simply adding a center tap connection changes the two wire source currents.
4. I agreed that the average power must be positive however it is your choice of voltage sources that makes it look negative. Again, you said that Vbn = -Van, so therefore it looks like P=-IVan
jghrist said:With the voltages as defined, there will be no current. Write the loop equation:
115V@180? + 115V@0? = I2?100ohm
115V@180? = -115V@0?
-115V@0? + 115V@0? = I2?100ohm
0V = I2?100ohm
I2 = 0A
jim dungar said:You are forgetting that the method Rattus uses is to subtract the voltages, he said this is tricky. There are hundreds and hundreds of posts describing his reasoning.
Just because you can, doesn't mean you should.
rattus said:1. If we use the neutral as a common reference, we have no choice. One voltage is the inverse of the other.
2. They cannot, the current has to ENTER the second source.
3. They do not change.
4. The fact that the current is entering one source allows us to slip in a negative sign to make the product positive.
Slip In: verb, Insert casually; stealth; interject without notice; "She slipped in a negative sign without her boss noticing."rattus said:4. The fact that the current is entering one source allows us to slip in a negative sign to make the product positive.
Then you need to reverse the + and - on the lower source symbol.rattus said:Remember these voltages are referenced to the common neutral. To obtain the voltage between them, we must SUBTRACT not add one to the other, e.g,
Vab = 115 @ 0 - Vab @ 180 = 115 @ 0 + 115 @ 0 = 230V @ 0. This is a critical point which can be demonstrated with a phasor diagram.
You so funny:grin:Rick Christopherson said:Slip In: verb, Insert casually; stealth; interject without notice; "She slipped in a negative sign without her boss noticing."
mivey said:Is it really "wrong?
BTW, I'm not being argumentative, this is a real question.mivey said:Is having a negative the whole problem? Is it really "wrong?
No argument from me.jim dungar said:I have been trying to point out the advantages and disadvantages of different presentations.
Making an adjustment (i.e. so that the voltages seem correct) can lead to having to make other adjustments later.
You say that as if it's a universal requirement.rattus said:Remember these voltages are referenced to the common neutral.
I slipped in a bathtub once; does that count?Rick Christopherson said:Slip In: verb, Insert casually; stealth; interject without notice; "She slipped in a negative sign without her boss noticing."