Three-phase systems generate and transmit electricity using three coils rotated 120 degrees apart which produces three independent and equal voltage supplies. Electricity is sent across high voltage transmission lines and then stepped down through substations before reaching homes and offices at utilization voltages. Motors are often wired in a delta configuration for efficiency but use a star-delta starter to prevent high starting currents. Total three-phase power can be calculated using the line voltage and current multiplied by the power factor.

1. Three Phase Systems – Supplies
Generation, transmission and distribution of electricity is achieved via the
National Grid system using three-phase alternating current.
A three-phase generator system uses three sets of coils placed 120˚ apart
and rotated in a uniform magnetic field, resulting in three independent
supplies of equal amplitude.
Rotation of three coils in a magnetic field 120° apart.

2. Star connected balanced load
IL = IP VL = 3 x VP
Three Phase Systems – Star Connection
The advantage of star connection is that two voltages become available
Line voltage
Phase Voltage
Line voltage is sent across the National Grid system
Phase Voltage is the voltage used in domestic supplies (homes, offices etc)

3. Three Phase Systems – Delta Connection
To obtain greater power efficiency most three phase motors are operated in
delta.
To prevent large starting current these motors are initially connected in star
then once up to speed they are connected in delta (star/delta starter).
VL = VP IL = 3 x IP
Delta connected balanced load
VL
VL
IL
IL
IL
VL

4. Three Phase Systems – Power
Line voltage is sent across the National Grid system
Phase Voltage is the voltage used in domestic supplies (homes, offices etc)
and is the line voltage with respect to neutral.
Power in a single phase system is given by;
Power = VI cos θ (W)
In a balanced three-phase system total power is;
Total three phase Power = 3VP IP cos θ (W)
For a star or delta connected system;
Total three-phase power = 3 VLIL cos θ (W)

5. Three identical loads each having an impedance of 22 ohms are connected
first in star and then in delta across a 415V 50Hz supply. Calculate the line
and phase current in each case.
Three Phase Systems – Power Distribution
For Star connection
.9.10
22
240
240
3
415
3
AII
Z
V
II
V
V
V
PL
L
PL
L
P
==
===
===
For Delta connection
.67.3286.183
3
.86.18
22
415
415
A
II
A
Z
V
I
VVV
PL
P
P
PL
=×=
=
===
==

6. Three Phase Systems – Power Distribution
Electricity
Boards
National
Grid
400,000 V 400,000/132,000 V 132,000/33,000V 33,000/11,000 V 11,000/415/240 V
Transformer Grid System Transformer Transformer Transformer
Generation, transmission and Distribution of Electricity
Generating
Companies
National Power
Power Gen
Nuclear Electric
French Electric
Scottish Electric

7. Three Phase Systems
Activity
1. A balanced star-connected three-phase load of 10 ohms per phase operates
from a 400V supply at a power factor of 0.9. Calculate a) phase voltage, b)
line current and c) total power consumed.
2. A balanced load having an inductive reactance of 15Ω and resistance 5Ω is
connected first in star then in delta to a 415V three-phase supply. Calculate
the line and phase current in each case.
3. A 15kW, 415V, balanced three-phase star connected load has a power factor
of 0.86 lagging. Determine the line current.

8. Three Phase Systems – Summary
For a Star connected system,
IL = IP VL = 3 x VP
• For a Delta connected system
VL = VP IL = 3 x IP
• For a star or delta connected system;
Total three-phase power, P = 3 VL IL cosθ (W)