Given a three phase suppy at 63Amps per phase, standard Star 5 wire configuration with all three phases fully loaded. How much current flows in the neutral?
A. 63 Amps
B. 189 Amps
C. Zero Amps
.... well there are three phases L1 L2 L3 and they all share the same neutral wire.... hmmmm
And the answer is ....
C. Zero amps
Assuming the phases are balanced in terms of current load and the power factor per phase is the same. The relative phase offsets of 120 degrees per phase means that if you sum the voltages at any point in time over the three phases you end up with a total voltage of zero volts, so zero current flows.
This is what it looks like:
Another question follows: If the loading is hopelessly unbalanced... what is the worst case current in the neutral?
Here is a clue... how much bigger is the neutral wire compared to the three live wires?????
Care to comment?? ;-)
4 comments:
I'm thinking of the vector diagram and the what goes in must come out rule, and the comment about the unbalanced phase(s) pulling the neutral point, and I'm conluding the neutral can't be worse than single phase current.
Maybe I'm wrong...
I'm thinking of the vector diagram and the what goes in must come out rule, and the comment about the unbalanced phase(s) pulling the neutral point, and I'm conluding the neutral can't be worse than single phase current.
Maybe I'm wrong...
Neutral current can be aout twice the phase current, with the right sort of imbalance and harmonic loads.
Well all of this depends on the power factor of the load. However in response to the neutral current being "about twice" the phase current. I dont think so... care to expand? Remember in our world we are looking at dimmers, balasted striking lamp loads and audio amplifiers. Mark
Post a Comment