PPT: Three Phase Circuits (Star-Delta Transformations) | Network Theory (Electric Circuits) - Electrical Engineering (EE) PDF Download

 Page 1


Three-Phase AC Circuits
Page 2


Three-Phase AC Circuits
Significant Features of Three-Phase AC Circuits 
? Almost all ac power generation and transmission is in the form of three-
phase ac circuits
? AC power systems have a great advantage over DC systems in that their 
voltage levels can be changed with transformers to reduce transmission 
losses.
? Three-phase (3?) ac power system consists of
? 3? ac generators
? 3? transmission lines
? 3? loads
? Advantages of having 3? power systems over 1? ones:
? More power per pound of metal of electrical machines of 3?.
? Power delivered to a 3? load is constant, instead of pulsating as it does 
in a 1? system.
Page 3


Three-Phase AC Circuits
Significant Features of Three-Phase AC Circuits 
? Almost all ac power generation and transmission is in the form of three-
phase ac circuits
? AC power systems have a great advantage over DC systems in that their 
voltage levels can be changed with transformers to reduce transmission 
losses.
? Three-phase (3?) ac power system consists of
? 3? ac generators
? 3? transmission lines
? 3? loads
? Advantages of having 3? power systems over 1? ones:
? More power per pound of metal of electrical machines of 3?.
? Power delivered to a 3? load is constant, instead of pulsating as it does 
in a 1? system.
Generation of 3? Voltages and Currents
A 3? generator consists of three 1? 
generators:
- voltage of all phases are equal in 
amagnitude
- differing in phase angle from each 
aother by 120
o
.
Page 4


Three-Phase AC Circuits
Significant Features of Three-Phase AC Circuits 
? Almost all ac power generation and transmission is in the form of three-
phase ac circuits
? AC power systems have a great advantage over DC systems in that their 
voltage levels can be changed with transformers to reduce transmission 
losses.
? Three-phase (3?) ac power system consists of
? 3? ac generators
? 3? transmission lines
? 3? loads
? Advantages of having 3? power systems over 1? ones:
? More power per pound of metal of electrical machines of 3?.
? Power delivered to a 3? load is constant, instead of pulsating as it does 
in a 1? system.
Generation of 3? Voltages and Currents
A 3? generator consists of three 1? 
generators:
- voltage of all phases are equal in 
amagnitude
- differing in phase angle from each 
aother by 120
o
.
Three-Phases of the Generator Connected to Three 
identical Loads
V
A
V
B
V
C
Phasor diagram showing the 
voltages in each phase
Page 5


Three-Phase AC Circuits
Significant Features of Three-Phase AC Circuits 
? Almost all ac power generation and transmission is in the form of three-
phase ac circuits
? AC power systems have a great advantage over DC systems in that their 
voltage levels can be changed with transformers to reduce transmission 
losses.
? Three-phase (3?) ac power system consists of
? 3? ac generators
? 3? transmission lines
? 3? loads
? Advantages of having 3? power systems over 1? ones:
? More power per pound of metal of electrical machines of 3?.
? Power delivered to a 3? load is constant, instead of pulsating as it does 
in a 1? system.
Generation of 3? Voltages and Currents
A 3? generator consists of three 1? 
generators:
- voltage of all phases are equal in 
amagnitude
- differing in phase angle from each 
aother by 120
o
.
Three-Phases of the Generator Connected to Three 
identical Loads
V
A
V
B
V
C
Phasor diagram showing the 
voltages in each phase
Currents in the Three Phases and the Neutral
? - - ? =
? ?
- ?
=
? - - ? =
? ?
- ?
=
? - ? =
? ?
?
=
0
0
0
0
0
240
240
120
120
0
I
Z
V
I
I
Z
V
I
I
Z
V
I
c
b
a
( ) ( )
( ) ( ) ? ? ( ) ( ) ? ?
( ) ( ) ? ?
0
240 240
120 120
240 120
0 0
0 0
0 0
=
?
?
?
?
?
?
?
?
? - - + ? - - +
? - - + ? - - + ? - + ? -
=
? - - ? + ? - - ? + ? - ? =
+ + =
sin j cos
sin j cos sin j cos
I
I I I
I I I I
c b a N
Currents flowing in the three phases
It is possible to connect the negative ends of these three single phase 
generators and the loads together, so that they share a common return 
line, called neutral.
As long as the three loads are equal, the return current in the neutral is zero.