Armature Reaction in dc machines...........

1. Presentation on
ARMATURE REACTION
by :-
Vishal Thakur

2. What is Armature Reaction ?
Armature Reaction in a d.c. machine is
basically the effect of armature produced
flux on the main flux or field flux .

3. Effects…
• The armature reaction produces the following
two undesirable effects:
1. It demagnetizes or weakens the main flux.
2. It cross-magnetizes or distorts the main flux.

4. Reduction in main flux per voltage reduces
the generated voltage and torque whereas
distortion of the main-field flux influences
the limits of successful commutation in d.c.
machines.

5. Explanation…
• To understand this process let us first assume a 2-
pole d.c. machine at no load . At that instant there is
no armature current . So the flux due to mmf
produced by field current in the machine at north
pole of the magnet will flow towards the south pole
of the magnet.

6. • The net / resultant flux of the system can be taken as
a straight horizontal line OA and can be shown in
phasor as :-
• Also at that instant the Magnetic Neutral Axis
(M.N.A) of the machine will concide with the
Geometrical Neutral Axis (G.N.A) of the machine as
the M.N.A is always perpendicular to the net flux.

7. • Now when the dc machine is loaded , current flows
in armature windings . This armature current set up
armature flux . With field windings unexcited , the
flux can be shown as vertical lines across armature
conductors .
• The conductors on the left side of the M.N.A will
have current flowing in inside direction whereas on
right side of MNA , the current will flow in outside
direction. The direction of the flux thus produced can
be determined by using Maxwell‘s Right hand Screw
rule.

8. • The resultant flux of the system is a straight vertical
line OB and can be shown in phasor as :-
• Note that the magnitude of OB will always be less
than OA since the cause of armature flux is field flux
and it is known to us that effect is always less than
cause. Here armature flux is the effect and field flux
is its cause.

9. • An examination to the above two phasors reveals
that the path of armature flux is perpendicular to the
main field flux.
• In other words, the path of the armature flux crosses
the path of the main field flux. This can be shown in
phasor as :-
• Thus the effect of armature flux on the main field
flux is entirely ‘cross-magnetizing’ and it is for this
reason that the flux produced by armature mmf is
also called as cross-flux.

10. • When the current flows in both the armature and
field windings, the resultant flux distribution is
obtained by superimposing theses two fluxes. i.e.

11. • It is observed that the armature flux aids the main
field flux at the lower end of the N-pole and at the
upper end of the S-pole, therefore at these two
poles, the armature flux strengthens the main field
flux.
• Likewise, the armature flux weakens the main field
flux at Upper end of the N-pole and at lower end of
the S-pole.

13. • Now, if there is no magnetic saturation, then the
amount of strengthening and weakening of the main
field flux are equal and the resultant flux per pole
remains unaltered from its no load value.
• Actually, the magnetic saturation does occurs and as
a consequence, the strengthening effect is less as
compared to the weakening effect and the resultant
flux is decreased from its no-load value. This is called
‘Demagnetizing effect of armature reaction’

14. • So when the machine is run loaded , M.N.A will shift
from G.N.A of the machine .
• The resultant shift is completely dependent on the
magnitude of armature current.
• Thus, greater the value of armature current , greater
is the shift of MNA from GNA .

15. • It may therefore be stated from the above that
net effect of armature flux on the main field flux
is:-
1. To distort the main field flux thereby causing
non-uniform distribution of flux under the main
poles.
2. To shift the MNA in the direction of the rotation
for a generator and against the direction of
rotation for a motor.
3. To reduce the main field flux from its no-load
value due to magnetic saturation.

16. Methods to reduce Armature Reaction
• There are various methods of reducing the armature
reaction, some of them are:-
1. Compensate Windings
2. Interpoles or copoles

17. Compensating Windings
• The compensating windings consist of a series
of coils embedded in slots in the pole faces.
• These coils are connected in series with the
armature in such a way that the current in
them flows in opposite direction to that
flowing in armature conductors directly below
the pole shoes.

18. • The series-connected compensating windings
produce a magnetic field, which varies directly
with armature current.
• As the compensating windings are wound to
produce a field that opposes the magnetic
field of the armature, they tend to cancel the
effects of the armature magnetic field.

20. Interpoles
• Another way to reduce the effects of armature
reaction is to place small auxiliary poles
called "interpoles" between the main field
poles.
• Interpoles have a few turns of large wire and
are connected in series with the armature.

21. • Interpoles are wound and placed so that each
interpole has the same magnetic polarity as the
main pole ahead of it, in the direction of rotation.
• The field generated by the interpoles produces
the same effect as the compensating winding.
This field, in effect, cancels the armature reaction
for all values of load current.