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Introduction to Three-Phase PowerPowerPoint Presentation

Introduction to Three-Phase Power

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Presentation Transcript

Typical Transformer Yard

ECE 441

Basic Three-Phase Circuit

ECE 441

What is Three-Phase Power?

- Three sinusoidal voltages of equal amplitude and frequency out of phase with each other by 120°. Known as “balanced”.
- Phases are labeled A, B, and C.
- Phases are sequenced as A, B, C (positive) or A, C, B (negative).

ECE 441

Three-Phase Power

ECE 441

Definitions

- 4 wires
- 3 “active” phases, A, B, C
- 1 “ground”, or “neutral”

- Color Code
- Phase A Red
- Phase B Black
- Phase C Blue
- Neutral White or Gray

ECE 441

Phasor (Vector) Form for abc

Vc=Vm/+120°

Va=Vm/0°

Vb=Vm/-120°

Note that KVL applies .... Va+Vb+Vc=0

ECE 441

Three-Phase Generator

- 2-pole (North-South) rotor turned by a “prime mover”
- Sinusoidal voltages are induced in each stator winding

ECE 441

How are the sources connected?

- (a) shows the sources (phases) connected in a wye (Y).
- Notice the fourth terminal, known as Neutral.

- (b) shows the sources (phases) connected in a delta (∆).
- Three terminals

ECE 441

Look at a Y-Y System

ECE 441

Definitions

- Zg represents the internal generator impedance per phase
- Zl represents the impedance of the line connecting the generator to the load
- ZA,B,C represents the load impedance per phase
- Zo represents the impedance of the neutral conductor

ECE 441

Using the Tip-to-Tail Method

-VBN

VΦ = Line-to-Neutral, or Phase Voltage

VAB = VAN – VBN = √3VΦ

ECE 441

Conclusions for the Y connection

- The amplitude of the line-to-line voltage is equal to √3 times the amplitude of the phase voltage.
- The line-to-line voltages form a balanced set of 3-phase voltages.
- The set of line-to-line voltages leads the set of line-to-neutral (phase) voltages by 30°.

ECE 441

Summary

ECE 441

Look at the Delta-Connected Load

ECE 441

Conclusions for the Delta Connection

- The amplitude of the line current is equal to √3 times the phase current.
- The set of line currents lags the phase currents by 30°.

ECE 441

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