Unbalanced resistive loading of three phase system
Unbalanced resistive loading of three phase system
(OP)
Can anyone prescribe the method of calculating the Line currents in a three phase Mesh connected system with equal resistive loads connected between Line 1,2 and line 2,3 with Line 2 as the common connection. Wish to be able to calculate all line currents. I suspect Line 1 and Line 3 currents equal V/R but cannot fathom Line 2 by calculation or vector method.
Many thanks
david707
Many thanks
david707






RE: Unbalanced resistive loading of three phase system
RE: Unbalanced resistive loading of three phase system
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RE: Unbalanced resistive loading of three phase system
RE: Unbalanced resistive loading of three phase system
Line volts = 208v
Resistive load = 20 ohms
Loads connected between Line 1,2 and 2,3
Mesh/Delta source configuration
Readings obtained, Line 1 current = 10.4 amps = Line 3 current
Line 2 current measured at c18 amps.
RE: Unbalanced resistive loading of three phase system
Oops! Vector SUM<\b> of the two is in fact sqrt(3)*I1 - I managed to confuse myself and subtracted the vectors. This corresponds with the readings that you took (10.4*1.73 = 18A).
RE: Unbalanced resistive loading of three phase system
RE: Unbalanced resistive loading of three phase system
Many thanks again chaps,
David707
RE: Unbalanced resistive loading of three phase system
-Draw the equilateral voltage triangle 1-2-3
-As the load is resistive, the current vectors I12 and I32 are in phase with the respective voltages V12 and V32
-Add the vectors I12 and I32; the resultant is a vector with magnitude {sqrt(3)*I12}, at an angle leading V12 by 30 degrees (or lagging V32 by 30 degrees)
Hope this helps.
RE: Unbalanced resistive loading of three phase system
The only justification for changing the sign of the I3 vector is by being strict about the directions of the three line currents as seen on the circuit diagram. This has been at thye centre of my problem throughout notwithstanding the fact that the measured value of I2 is c18 amps.
Cheers
David707
RE: Unbalanced resistive loading of three phase system
The crux of the visualization problem is that you are drawing the wrong vectors. For your scenario, you need to draw the currents in phase with V12 and V32 respectively (60 degrees apart) - the way the loads are connected, NOT V1 and V3 (120 degrees apart). Having done that, I think that all will become clear.
RE: Unbalanced resistive loading of three phase system
Then the phase currents are,
Ia=Iab.
Ib=-Iab+Ibc.
Ic=-Ibc.
The corresponding values are:
Ia=(V/R)@120deg.
Ib=sqrt(3)(V/R)@-30deg.
Ic=(V/R)@180deg.
RE: Unbalanced resistive loading of three phase system
David707