Three Phase Current Values in a 3-Phase System

Division of Three Phase Current Values in a 3-Phase System

The following points may be noted for understanding the division of Three Phase Current in a 3-Phase System

1. The arrow symbols shown alongside three phase current (I_R, I_Y and I_B) in fig 1 and 2 does not indicate the currents directions in a particular moment but shows the current directions when we assume them positive. It may be noted that there is no moment in which the direction of all three phase currents are same, i.e., there is not a possible moment where all the three phase currents are going or leaving to the common point at once.

The arrow symbols show that the first current is leaving from R Phase, then after 120° phase time, this current leaving Y phase and for next 120°, it’s leaving B phase.

2. In any one or two conductors, the leaving current is same to the entering current in that conductor (or conductors). In other words, each conductor provides a return path to the currents of the other conductors. This way, current division is continuously changing in three lines. Hence, the algebraic sum of three currents is Zero (0) at any instant.

Explanation of Three Phase Currents in Poly-phase System:

Three Phase Currents are shown in the above fig having the same peak value of 8A but displaced from each other by 120°.

In the fig above, at point “a”, the values of currents in R and B phases are +4A and the direction of these currents is outside, while the value of Y phase is -8A. Its mean the Y phase current provides the returning path to the R and B phases currents.

Similarly,

• At point “b” = I_R =+6A, I_Y = +2A, I_B =-8A …. Now B provides returning path for Y and R currents
• At Point “C” = I_Y =+6A, I_B = +2A, I_R =-8A …. Now R provides returning path for Y and B currents
• At point “d” = I_R =0A, I_B = +6.9A, I_Y =-6.9A …. I.e. B current is outgoing and Y returning from the Y path

Thus, it may be noted that although the current distribution in three phases is continuously changing, but at any instant, their algebraic sum of the instantaneous values is Zero (0) i.e.

I_R + I_Y + I_B = 0   ……………. Algebraically

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