1. Electrical Machines-II
6th Semester, EE and EEE
By
Dr. Binod Kumar Sahu
Associate Professor, Electrical Engg.
Siksha ‘O’ Anusandhan, Deemed to be University,
Bhubaneswar, Odisha, India
Lecture-8
2. 2
Learning Outcomes: - (Previous Lecture_07)
To solve some numerical related to induced emf containing harmonics with short
pitched and distributed winding.
3. 3
Learning Outcomes: - (Today’s Lecture_08)
To analyse and understand the concept of armature reaction.
4. 4
Armature Reaction: -
The main field flux is produced by the DC field excitation i.e. the field current.
When the alternator is connected to an electrical load, current through the
armature coil also produces a flux called armature flux.
Armature reaction is the effect of armature flux on the main field flux.
f
N
S
X
.
X
. .
.
X
.
X
Field flux
or main flux
N S
I
6. 6
Nature of the armature reaction depends on the nature of the load, i.e. whether
it is resistive, inductive, capacitive, or combination of R-L or R-C.
Armature reaction with pure resistive load: -
For pure resistive load, induced emf and armature current will have no phase
difference.
If we take the vertical position of the
rotor as reference, i.e. t = 0.
Expression of three phase currents at t = 0
are as follows:
0
0
cos( )
cos( 120 )
cos( 240 )
r m
y m
b m
i I t
i I t
i I t
7. 06-05-2022 7
So, instantaneous expression of the corresponding fluxes are:
So, at t = 0
0
0
cos( )
cos( 120 )
cos( 240 )
r m
y m
b m
t
t
t
0
0
cos( )
cos( 120 ) 0.5
cos( 240 ) 0.5
r m m
y m m
b m m
t
t
t
9. 06-05-2022 9
f
r m
0.5
y m
0.5
b m
'
0.5
R m
1.5
a m
Phasor diagram at, t = 0, ωt = 00.
0
' 120
2 (0.5 )cos
2
0.5
R m
m
11. 06-05-2022 11
From the phasor diagram at t = 0 or ωt = 00, it is seen that the resultant
armature flux is leading the main flux by 900.
At t = T/6 (T is the time period), ωt = 600, the resultant armature flux as well
as the rotor, change their position by 600 in the direction of rotation of
armature. So at any time the angle between the main flux and the armature flux
is 900.
So, instantaneous values of the fluxes at ωt = 600
are:
So at unity power factor, the nature of
armature reaction is cross magnetizing or
distorting in nature.
0
0
cos( ) 0.5
cos( 120 ) 0.5
cos( 240 )
r m m
y m m
b m m
t
t
t
f
N
S
X
.
.
X
.
Na
X
Sa
R1
R2
Y1
Y2
B1
B2
12. 06-05-2022 12
b m
0.5
y m
0.5
r m
y
'
0.5
R m
1.5
a m
f
Phasor diagram
at, t = T/6, ωt = 600.
0
' 120
2 (0.5 )cos
2
0.5
R m
m
13. 06-05-2022 13
Armature reaction with pure inductive load: -
For pure inductive load, armature current lags the induced emf by 900 i.e. the
position of the rotor will advance by 900 at t = 0 as compared to its position at t
= 0 sec for pure resistive load.
Pure Resistive Load
Pure Inductive Load
f
N
S
X
.
X
. .
.
X
.
X
Na
Sa
R1
R2
B2
Y1
B1
Y2
f
N
S
X
.
X
. .
.
X
.
X
Na
Sa
a
15. 06-05-2022 15
From the phasor diagram it is clear that for pure inductive load the armature
flux acts in opposite direction to the main flux. So, the nature of armature
reaction is demagnetizing i.e. it reduces the air gap flux.
So, once the alternator is connected to an inductive load, the air-gap flux is
reduced which results in reduced emf and terminal voltage across the load.
16. 06-05-2022 16
Armature reaction with pure capacitive load: -
For pure capacitive, armature current leads the induced emf by 900.
So, we will have a similar situation 900 before the rotor comes to its reference
position i.e. at t = 0 sec for resistive load.
Pure Resistive Load
Pure Capacitive Load
f
N
S
X
.
X
. .
.
X
.
X
Na
Sa
R1
R2
B2
Y1
B1
Y2
N S
X
.
X
. .
.
X
.
X
Na
Sa
a
f
17. 06-05-2022 17
f
r m
0.5
y m
0.5
b m
'
0.5
R m
1.5
a m
18. 06-05-2022 18
From the phasor diagram it is clear that for pure capacitive load, the armature
flux acts in the same direction as that of the main flux. So, the nature of
armature reaction is magnetizing i.e. there is an increase in the air gap flux.
So, once the alternator is connected to capacitive load, the air-gap flux
increases, which results in increased emf and terminal voltage across the load.
19. 06-05-2022 19
Nature of load Power factor Nature of Armature Reaction
Resistive (R) Unity Cross-magnetizing
Pure Inductive (L) Zero lagging Demagnetizing
Pure Capacitive
(C)
Zero leading Magnetizing
R-L Load Lagging Partly cross-magnetizing and partly
demagnetizing
R-C Load Leading Partly cross-magnetizing and Partly
magnetizing.
