1. THREE PHASE CIRCUIT ANALYSIS

2. ELE101/102 Dept of E&E,MIT Manipal 2
1. Higher Efficiency
2. Lower Cost
3. Size Reduction
4. Less amount of power-conductor material for transmission
5. Unidirectional torque production in 3 phase motor
6. 3 phase motors are self starting
7. Parallel operation of 3 phase generators
8. Better operating power factor of 3 phase motors
9. Lighter, efficient and cheaper 3 Phase transformer
10. Two different levels of voltages can be obtained
Why three phase supply?

3. ELE101/102 Dept of E&E,MIT Manipal 3
Generation of three phase voltages
N S
R
B
Y
R1
B1
Y1
rotatio
n
e
2π/3 π
R
R'
Y
Y'
B
B'

4. ELE101/102 Dept of E&E,MIT Manipal 4
At any instant,
eR + eY + eB = Em [Sin(ωt) + sin (ωt-120o) + sin (ωt-240o)] = 0
Sum of the instantaneous values of the voltages is zero
Note: emfs are assumed to be positive when acting from
R| to R,Y| to Y and B| to B respectively
eR = Em Sin (ωt) = E00
eY = Em Sin (ωt -120˚) = E-1200
eB = Em Sin(ωt-240˚) = E-2400

5. ELE101/102 Dept of E&E,MIT Manipal 5
Terminologies:
Phase sequence: Is the order in which three phase voltages (currents)
attain their maximum values
Ex: RYB, YBR, BRY are same
But, RYB and RBY are different
(change in direction of rotation)
Phase Voltage: voltage induced in each winding. Ex: VRR1
Line voltage: voltage between two lines. Ex: VRY
Balanced System: Load impedances are same / phase and line
currents are equal.
Unbalanced System: Load impedances are unequal / phase and
line currents are not equal.
Note: Even if the loads are unbalanced, the supply is balanced

6. ELE101/102 Dept of E&E,MIT Manipal 6
Types of 3 phase connections
The six treminals can be inter connected to form 2 types of
3 phase connections.
 DELTA connection
 STAR connection

7. ELE101/102 Dept of E&E,MIT Manipal 7
DELTA connected system
R' Y'
B'
R Y B
There is no neutral wire
Y
B
Y
R
R
Y
B
B
R
,
0
' 

 E
ERR
,
E
E '
YY



 120



 240
E ' E
BB
'
RR
E
'
YY
E
'
BB
E
120˚
12
0˚
Phase sequence: RYB

8. ELE101/102 Dept of E&E,MIT Manipal 8

N
R
R'
Y
Y'
B
B'
Vph
VL
R
Y
B
N
eR
eY
eB
R'
Y'
B'
STAR CONNECTION
Interconnection point of R︲ ,Y︲ and B︲
is known as the Neutral














240
,
120
,
0
E
E
E
E
E
E
BN
YN
RN
RN
E

YN
E

BN
E

120˚
120˚