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App-18
IM 760301-01E
• Wiring Three-Phase AC Power and Load
Generally, the power lines are connected using star wiring or delta wiring on three-
phase power supplies and loads.
R
S T
R
S
T
Star wiring Delta wiring
• Vector Display of Three-Phase AC Power or Load
Generally the voltage of each phase of a three-phase AC power or load is offset by
120 ° . Below is an illustration of a three-phase AC power 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 the power 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. In addition, the line voltage is sometimes measured when measuring a
three-phase AC power using two single-phase wattmeters (called the two-wattmeter
method). If the magnitude of the phase voltage of each phase is equal and each
phase is offset by 120 ° , the line voltage is √ 3 times the phase voltage, and the phase
is offset by 30
°
.
Below is a vector representation of the relationship between the phase voltage and
line current of a three-phase AC voltage or load whose current phase is lagging the
voltage phase by φ ° .
φ
U
1
I
1
φ
φ
I
3
I
2
U
3
U
2
Appendix 4 Power Basics (Power, Harmonics, and RLC of AC Circuits)