YOKOGAWA WT1800 User Manual page 120

Appendix 2 Power Basics (Power, harmonics, and AC RLC circuits)
Three-Phase AC Wiring
Generally three-phase AC power lines are connected in star wiring configurations or delta wiring
configurations.
Star wiring
T
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.
Phase voltage
T
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 magnitude of the line voltage is
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 Φ °.
I
3
U
3
App-14
R
S
R
Line voltage
U
1
I
1
Φ
Φ
Φ
I
2
Delta wiring
T
S
times the magnitude of the phase voltage, and the line
3
U
2
R
S
IM WT1801-03EN