App-15
IM 760201-01E
Appendix
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App
Index
Three-Phase AC Wiring
Generally three-phase AC power lines are connected using star wiring or delta wiring.
R
S
T
R
S
T
Star wiring
Delta wiring
Vector Display of Three-Phase Alternating Current
In typical three-phase AC power, the voltage of each phase is offset by 120°. The figure
below expresses this offset using vectors. The voltage of each phase is called the phase
voltage, and the voltage between each phase is called the line voltage.
R
S
T
Line voltage
Phase voltage
If a power supply or load is connected in a delta wiring configuration and no neutral
line is present, the phase voltage cannot be measured. In this case, the line voltage
is measured. Sometimes the line voltage is also measured when measuring three-
phase AC power using two single-phase wattmeters (the two-wattmeter method). If the
magnitude of each phase voltage is equal and each phase is offset by 120°, the line
voltage is √3 times the phase voltage, and the line voltage phase is offset by 30°.
Below is a vector representation of the relationship between the phase voltages and line
currents of a three-phase AC voltage when the current lags the voltage by
f
°.
F
U
1
I
1
F
F
I
3
I
2
U
3
U
2
Appendix 3 Power Basics (Power, harmonics, and AC RLC circuits)
