ece 4411 introduction to three-phase power. ece 4412 typical transformer yard
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TRANSCRIPT
ECE 441 4
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 6
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
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Phasor (Vector) Form for abc
Va=Vm/0°
Vb=Vm/-120°
Vc=Vm/+120°
Note that KVL applies .... Va+Vb+Vc=0
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Three-Phase Generator
• 2-pole (North-South) rotor turned by a “prime mover”
• Sinusoidal voltages are induced in each stator winding
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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
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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
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-VBN
Using the Tip-to-Tail Method
VAB = VAN – VBN = √3VΦ
VΦ = Line-to-Neutral, or Phase Voltage
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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°.