# 10. THREE PHASE POWER CIRCUITS by Ulaby & Maharbiz All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press.

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10. THREE PHASE POWER CIRCUITS by Ulaby & Maharbiz All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press

A 4800-V rms single-phase connected to residential user through a 20 : 1 step-down transformer  All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press

Balanced Three Phase Common practice is to adopt a positive (123) phase sequence wherein the phase of V2 follows behind that of V1 by 120 ◦ and the phase of V3 follows behind that of V2 by 120 ◦. Hence, the phases of V1, V2, and V3 are 0, − 120 ◦, and − 240 All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press

Y & Delta For a balanced source, V1 + V2 + V3 = 0. All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press

Why 3-Phase?  Transmission of three-phase power from the generator to loads is more efficient than three separate, single-phase transmissions. A three-phase -source configuration uses 3 transmission wires and a Y-source configuration uses 3 or 4 wires (the neutral wire is not always used), whereas 6 wires are required to support three separate single- phase transmissions. Y-Delta configuration is the most common. All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press

For a balanced source, V1 + V2 + V3 = 0. Hence, V N = 0 (balanced network). All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press

Power in Balanced Networks Line-to-line voltage V L is related to the rms phase voltages by V L =√3 V YL. Also, I L =I YL. Hence All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press

Wattmeters Single Phase: Three Phase : Uses 2 Wattmeters All rights reserved. Do not copy or distribute. © 2013 National Technology and Science Press