Three Phase Theory
Eng. Thilini Kumarasinghe
NDT, B. Tech. Engineering (Hons)
Visiting Lecturer
Division of Electrical, Electronic and
Telecommunication Engineering Technology,
Institute of Technology,
University of Moratuwa, Sri Lanka
 Three Phase Electricity
Three-phase power is a three-wire AC power circuit with each phase AC
signal 120 electrical degrees apart.
• Residential homes are usually served by a single-phase power supply
• Commercial and industrial facilities usually use a three-phase supply.
Difference between single
phase and three phase
power
Here are the important differences between a single-phase and three-phase
connection. In a single-phase connection, the flow of electricity is through a
single conductor.
A three-phase connection, on the other hand, consists of three separate
conductors that are needed for transmitting electricity.
Three-phase electricity
generation
Three-phase power is generated by spinning a magnet inside three separate
independent coils of wire. Each phase wire of a three-phase distribution line
is connected to one of the coils. The three separate independent coils of wire
produce three separate independent voltages with different timing, as shown
here.
Benefits of 3 phase power
• A three-phase circuit provides greater power density than a one-phase
circuit at the same amperage, keeping wiring size and costs lower.
• Three-phase power makes it easier to balance loads while minimizing
harmonic.
• Optimizes utilization of electrical capacity for increased power
efficiency.
• Do not require any starters to three-phase motors used in big
industries, since it has sufficient phase difference to supply initial
torque for the motor to start.
Three Phase system
P
Equal Magnitude/ frequency
Phase difference -120o
2
Phasor Diagram
Star -Delta Configuration
A balanced three phase supply can be
connected either in star or delta
4
Star - Delta Covertion
Star and Delta
Configuration Cont…
A balanced star connected load can be
converted to an equivalent delta and the other
way round too.
Z D =3Z s
In star connected supply :I L =Ip & VL =√3Vp
In delta connected supply : I L =√3ID
5
Phase and Line Voltages
Phase Voltage - V p
The voltage between any phase and the neutral
(The phase-to-neutral voltage)
Line Voltage - VL
The voltage between any two lines
(The line-to-line voltage)
If, R- phase voltage VR =Vp <0o ;
VY =Vp <(- −2𝜋 )
3
VB =Vp <(- −4𝜋 )
3
3
Unbalanced Three Phase
Systems
It may be caused by the supply being
unbalanced, the load being unbalanced or both.
The currents and voltages in one phase does not
tell us the current or voltages in the other phases.
Thus all phase quantities must be independently
determined.
11
Star Connected supply
feeding a star connected
load
12
Delta Connected supply
feeding a star connected
load
13
Delta Connected supply
feeding a Delta
connected Load
14
Star Connected supply
feeding a Delta
connected Load
15
19
Thank you