Is this calc right?

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Ingenieur

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Smart $ said:
The empirical value(s) get the sign change where appropriate. Not the vectorial representatives. K's Law says all current entering and leaving a node sum to zero. Making one or more variables a negative is making the result something other than the sum. Entering and leaving is merely stated as a required event for the occurrence of the anomaly.
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sounds technical with big words and all, but
incorrect
a sign convention must be observed
 
mivey

mivey

Senior Member
david luchini said:
OK, In=Ia+Ib+Ic. The angle is now incorrect...nothing to to with signs.
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Ingenieur said:
nope
Ia, Ib, Ic are entering the node
In leaving
opposite signs (by convention and power flow)

Ia + Ib + Ic -In =0
Ia + Ib + Ic = In
not In = -(I a + Ib + Ic)
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You are free to pick either convention.

The spreadsheet used the convention that at any point on the feeder the sum of currents is zero. This is a convention used in engineering analysis.

The other is also used in engineering analysis. Nothing wrong with either convention.

What is wrong is saying one is more valid than the other and calling the answer wrong without recognizing the convention used.

The spreadsheet result was correct.
 
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Ingenieur

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mivey said:
You are free to pick either convention.

The spreadsheet used the convention that at any point on the feeder the sum of currents is zero. This is a convention used in engineering analysis.

The other is also used in engineering analysis. Nothing wrong with either convention.

What is wrong is saying one is more valid than the other and calling the answer wrong without recognizing the convention used.

The spreadsheet result was correct.
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I'll go with the textbook definition
and no it isn't
 
mivey

mivey

Senior Member
Smart $ said:
That's not a proper application of Kirchoff's Current Law (and just outright wrong :D).
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Well the sum to zero convention is a better match to Kirchhoff, strictly speaking, but doesn't really make sum of ins equaling sums of outs wrong.
 
mivey

mivey

Senior Member
Ingenieur said:
what is the answer?
I know it also
are they the same when converted to rectang coordinates?
simple question
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1&4 are the same, 2&3 are the same. 1&4 are opposite to 2&3 (opposite sign, opposite vector direction: your choice).
 
I

Ingenieur

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mivey said:
Well the sum to zero convention is a better match to Kirchhoff, strictly speaking, but doesn't really make sum of ins equaling sums of outs wrong.
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actually based on conservation of charge
what goes must equal what goes out
This law is also called Kirchhoff's first law, Kirchhoff's point rule, or Kirchhoff's junction rule (or nodal rule).
The principle of conservation of electric charge implies that:
At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node

Kirchhoff's Current Law
This fundamental law results from the conservation of charge. It applies to a junction or node in a circuit -- a point in the circuit where charge has several possible paths to travel.
In Figure 1, we see that I[SIZE=-2]A[/SIZE] is the only current flowing into the node. However, there are three paths for current to leave the node, and these current are represented by I[SIZE=-2]B[/SIZE], I[SIZE=-2]C[/SIZE], and I[SIZE=-2]D[/SIZE].
Once charge has entered into the node, it has no place to go except to leave (this is known as conservation of charge). The total charge flowing into a node must be the same as the the total charge flowing out of the node. So,
  • I[SIZE=-2]B[/SIZE] + I[SIZE=-2]C[/SIZE] + I[SIZE=-2]D[/SIZE] = I[SIZE=-2]A[/SIZE]
Bringing everything to the left side of the above equation, we get
  • (I[SIZE=-2]B[/SIZE] + I[SIZE=-2]C[/SIZE] + I[SIZE=-2]D[/SIZE]) - I[SIZE=-2]A[/SIZE] = 0
 
mivey

mivey

Senior Member
Ingenieur said:
I'll go with the textbook definition
and no it isn't
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Which textbook chief?:) What makes the one you picked the world standard? I have a library full of engineering and electrical books and could show you both conventions.

The simple fact is that there is no standard. I'm pretty sure I have that statement in multiple texts as well.
 
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Ingenieur

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mivey said:
Which textbook chief?:) What makes the one you picked the world standard? I have a library full of engineering and electrical books and could show you both conventions.

The simple fact is that there is no standard. I'm pretty sure I have that statement in multiple texts as well.
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EVERY textbook skippy
 
I

Ingenieur

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mivey said:
no doubt. Again, pick whatever convention you want. That doesn't make another's choice wrong.
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it sure does if one states for a wye node In = -(Ia + Ib +Ic)

what is the rectang coord for the previous question?
are they all the same?
even though the polar coords have 2 distinct representations?
isn't that a conflict?

Re = signed mag x cos (signed ang)
Img = signed mag x sin (signed ang)
 
mivey

mivey

Senior Member
Ingenieur said:
EVERY textbook skippy
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You have not read every textbook.

To prove your point, you would have to read every textbook (a monumental challenge in itself) and find that all use In = Ia + Ib + Ic (an impossibility).

To prove my point, I only have to find at least one of each convention, which I already know I have.
 
I

Ingenieur

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mivey said:
You have not read every textbook.

To prove your point, you would have to read every textbook (a monumental challenge in itself) and find that all use In = Ia + Ib + Ic (an impossibility).

To prove my point, I only have to find at least one of each convention, which I already know I have.
Click to expand...

I have read every textbook on electrical engineering re: circuit theory
767 of them (and counting)
cover to cover
all In = Ia + Ib + Ic
:happyyes:
 
I

Ingenieur

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mivey said:
1&4 are the same, 2&3 are the same. 1&4 are in diagonally opposite quadrants to 2&3.
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nope
not if converted to rectang

try it:
Re = signed mag x cos (signed ang)
Img = signed mag x sin (signed ang)

that is an issue
probably signing/convention related
 
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