51 From Kirchoff voltage law we have ( ) VRY =VR −VY =VR + −VY ( ) VYB =VY −VB =VY + −VB ( ) VBR =VB −VR =VB + −VR In phasor diagram For balanced load VR , VY and VB are equaled but out of phase: ( ) P o VRY = 2VR cos30 = 3 V therefore VRY = VR30; VYB = VY-90; VBR = VB150; ( ) P o VYB = 2VY cos30 = 3 V ( ) P o VBR = 2VB cos30 = 3 V ( ) VL VP = 3 L P I = I Three similar resistors are connected in star across 400V, 3 phase lines. The line current is 5A. Calculate the value of each resistor. To what value should the line voltage be changed to obtain the same line current with the resistors are in delta connected?
52 Power in 3-Phase System • DC power system : W = • AC power system : W = IV cos / phase • Power for 3-phase system (refer to phase element) • Power for 3-phase system (refer to line element) P = 3VP IP cos P = 3VL I L cos Summary of 3-Phase System
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