Armature reaction

Notes of Armature Reaction

1. GujaratPowerEngineering
&ResearchInstitute
By-
-Karan Thawani(58)
Contents:-
-Armature Reaction
-Compensating winding
-Interpoles

2. Armature Reaction
-Armature Reaction is the effect of “Armature
field” on the “Main Field”.
-Armature field is the field which is produced by the
armature conductors due to current flowing through
them.
-Main field is the field which is produced by the
poles which is necessary for the operation.

3. (1) De-Magnetisation
-It weakens /reduces the main flux.
(2) Cross-Magnetisation
-It distorts the main flux.
Effects Of Armature Reaction

4. Terminology Related To
Armature Reaction
-MNA(Magnetically Neutral Axis)
It is the axis along which no E.M.F is produced, hence
brushes are kept on this axis.
-GNA(Geometrically Neutral Axis)
It is the axis which divides the armature core in two
equal parts.
-Polar Axis
It is the imaginary line which joins the
center of NS poles.

5. oConsider a two pole D.C Generator.
oFor the sake of simplicity, the brushes are shown
directly touching the armature conductors, but
practically they touch commutatorsegments.
oAssume that there is no current flowing through
armature conductors, Hence..
(1)The flux is distributed symmetrically with respect
to polar axis.
(2) The M.N.A coincides the G.NA.

6. N
S
Pole
Armature
Brush
Main Field
Armature conductors
MNA/GNA

7. oNow assume that there is current flowing through
the armature conductors and the direction of rotation
is clock-wise.
oAccording to Fleming’s right hand rule, the direction
of current is inwards in conductors which are influence
of N pole and outwards which are influence of S pole.
oInward flow of current is represented by “ ”
whereas the outward flow is represented
by “ ” .

8. oNow due to the current flowing through the armature
conductors, magnetic flux is produced around the
armature conductors.
oAccording to Fleming’s right hand rule, the direction
of this flux is clock-wise in conductors which are
influence of N pole and Anti-clock wise in conductors
which are influence of s pole.
oNow, the main flux is no more longer symmetrical
about the pole axis.

9. oHence the main field is distorted.
oNow, the resultant flux is the vector sum of the main
field and the armature field.
oHence the position of M.N.A changes(which is always
perpendicular to the resultant flux).
oDue to change in position of M.N.A, the position of
brush also changes which in known as “Brush Lead”.
oBrush lead is in the direction of rotation in the case
of D.C. Generator, while in the opposite direction of
rotation in case of D.C. Motor.

10. N
S
Direction of rotation
Current going inside
Current coming outside
Main Field
Armature Field

11. oNow due to brush shift, the armature conductors and
hence armature current is redistributed.
oSome armature conductors which were earlier under
the influence of N-pole come under the influence of S-
pole and vice-versa.
oAccording to Fleming’s right hand thumb rule,
armature conductors in 2(where = brush lead)
produces De-magnetizing effect and remaining
produces cross-magnetizing effect.

12. N
S
Main FieldDe-magnetizing
Cross-magnetizing
New MNAGNA
=Brush Lead

13. oMagnetic flux density increases over one half of the
core and decreases over the other half.
oThe flux wave is distorted and there is a shift in the
position of M.N.A.
oIt causes the commutation problem.
Effects Of Armature
Reaction

14. Compensating winding
oCompensating windings are used to nullify the cross-
magnetizing effect.
oThese windings are kept in slots of pole faces.
oIt carries current in opposite direction to the
current of armature winding just below the pole faces.
oIt is connected in series with the armature winding.

15. N
S
Direction of rotation
Assuming this as
pole face
Main Field
Armature Field
Field by Comp. winding

16. Drawbacks:
oThere is only one disadvantage of compensating
winding and that is it is costly.
Advantages:
oIt is used in large machines heaving heavy
fluctuation.
oIt is used when same output is required at low
voltage.

17. oAs the compensating windings are too costly, inter-
poles are used to neutralize the Cross-magnetizing
effect of armature reaction.
oThese are small poles fixed to the yoke and spaced in
between the main poles.
oThey are wound with comparatively few heavy gauge
copper wire turns and are connected in series with the
armature so that they carry full armature current.
oTheir polarity is same as the “Leading pole” in the case
of D.C. Generator and “Trailing pole” in d case of D.C.
Motor considering the direction of rotation.
Inter-poles:

18. N
N
S S
D.C. Generator
Main Pole
Inter-Pole

19. N
N
S S
D.C. Motor
Main Pole
Inter-Pole

20. N
S
N
S
Main Field
Field by inter poles
Armature Field
Main Field
Field by inter poles

21. oThere are mainly two functions of inter-poles:-
1.As their polarity is the same as that of the leading
pole, they induce an e.m.fin the coil(under
commutation) which helps the reversal of current.
The e.m.finduced by the inter-poles is known as
reversing-commutating e.m.f. This e.m.fneutralizes
the reactance e.m.fand makes commutation
sparkless.
2.Another function of the inter-poles is to neutralize
the cross-magnetizing effect of armature reaction.
Hence, brushes are not be shifted from the original
position.