The document discusses armature reaction and commutation in DC machines. It describes how armature reaction demagnetizes and distorts the main magnetic field, requiring brush shift. Commutation involves the reversal of current in armature coils as they pass between poles. Sparking can occur due to reactance voltage impeding quick current reversal. Methods to improve commutation include resistance commutation using carbon brushes and EMF commutation using interpoles to neutralize reactance voltage.

1. DC Machines
CHAPTER 2CHAPTER 2

2. ARMATURE REACTION AND
COMMUTATION

3. Armature reaction
• Armature reaction: The effect of magnetic field set up by armature
current on the distribution of flux under main poles of a dc machine.
The armature magnetic field has two effects:
• (i) It demagnetizes or weakens the main flux and
• (ii) It cross-magnetizes or distorts it

4. Polar axis and MNA
Polar axis: The flux is distributed
symmetrically with respect to polar axis,
which is the line joining the centers of NS
poles.
Magnetic neutral axis:
The magnetic neutral axis(MNA) or plane
coincides with the geometrical neutral axis or
plane. It is the axis along which no emf is
induced in the armature conductors because
they then move in parallel with the lines of
flux.
Therefore the brushes are placed along
MNA. Hence, MNA is called the axis of
commutation because of the reversal of
current in armature conductors takes place
across this axis. M.N.A. is the axis which is
perpendicular to the flux passing through the
armature
Fig1.Main magnetic field
Fig2.Armature magnetic
field

5.  As seen from Fig 1, Vector OFm
represents, both magnitude and
direction, of the mmf producing the
main flux.
 Fig 2 shows the field (or flux) set up
by the armature conductors alone
when carrying current, the field coils
being unexcited.
 The current direction is downwards in
conductors under N-pole and
upwards in those under S-pole.
 The armature mmf (depending on the
strength of the armature current) is
shown separately both in magnitude
and direction by the vector OFA.
Fig1.Main magnetic field
Fig2.Armature magnetic
field

6. Armature reaction
 Under actual load conditions,
the two mmf exist
simultaneously in the
generator as shown in Fig. 3.
 It is seen that the flux through
the armature is no longer
uniform and symmetrical
about the pole axis, rather it
has been distorted.
 The flux is seen to be
crowded at the trailing pole
tips but weakened or thinned
out at the leading pole tips
(the pole tip which is first met
during rotation by armature
conductors is known as the
leading pole tip and the other
as trailing pole tip).
 The strengthening and
weakening of flux is
separately shown for a four-
pole machine in Fig. 4.
Fig 3 Fig 4

7. Armature reaction
 In Fig.3 is shown the resultant mmf
OF which is found by vectorially
combining OFm and OFA. And the
new position of M.N.A which is always
perpendicular to the resultant mmf
vector OF, is also shown in the figure.
With the shift of M.N.A. So, through
an angle θ brushes are also shifted so
as to lie along the new position of
M.N.A.
 Due to this brush shift , the armature
conductors and hence armature
current is redistributed. All conductors
to the left of new position of M.N.A.
but between the two brushes, carry
current downwards and those to the
right carry current upwards. The
armature mmf is found to lie in the
direction of the new position of M.N.A.
(or brush axis).

8. Armature reaction
 The armature mmf is now represented by
the vector OFA. OFA can now be resolved
into two rectangular components, OFd
parallel to polar axis and OFC perpendicular
to this axis. We find that:
 (i) Component OFC is at right angles to the
vector OFm representing the main mmf
 It produces distortion in the main field and
is hence called the cross-magnetizing or
distorting component of the armature
reaction.
 (ii) The component OFd is in direct
opposition of OFm which represents the
main mmf
 It exerts a demagnetizing influence on the
main pole flux. Hence, it is called the
demagnetising or weakening component of
the armature reaction.
 The distorting and demagnetizing effects
will increase with increase in the armature
current.

9. Demagnetizing and
Cross-magnetizing
Conductors
All conductors lying within angles
AOC = BOD = 2θ at the top and bottom
of the armature, are carrying current in
such a direction as to send the flux
through the armature from right to left. It
is these conductors which act in direct
opposition to the main field and are
hence called the demagnetizing
armature conductors.
Now consider the remaining armature
conductors lying between angles AOD
and COB. These conductors carry
current in such a direction as to produce
a flux at right angles to the main flux.
This results in distortion of the main
field. Hence, these conductors are
known as cross-magnetizing conductors
and constitute distorting ampere-
conductors.

10. Solution for Cross-
magnetizing and
Demagnetizing effect
The armature demagnetizing
ampere-turns are neutralized
by adding extra ampere-turns
to the main field winding.
Compensating windings are
used for nullifying the cross
magnetization effect. These
windings are embedded in
slots in the pole shoes and are
connected in series with
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.

11. Commutation
A,B,C represent the three coils of
armature. a,b,c are the
Commutator segments.
The brush width is equal to width
of one Commutator segments and
one mica insulation.
In the figure it can be seen that
Commutator is moving in such a
way that the brush first touches
Commutator segment b and then
segment a.
From figure(a) it can be seen that
each coil is carrying 20 A. Brush is
connected to segment b. Thus
brush current is 40 A.
Prior to short-circuit the coil B
belongs to the group of coils lying
to the left of brush.
As seen from figure(b) coil B has
entered the short-circuit period and
has reached 1/3 of period.

12. Commutation
So the current through coil B has
been reduced from 20 A to 10 A, the
other 10 A is flowing from segment a.
In figure(c) it can be seen that the
coil is short-circuited and current
through it is zero.
Figure(d) shows that the coil has
become a part of the coils lying to the
right of brush. It can be seen that the
brush contact area with segment a is
increased as compared to that with
segment b.
Coil B now carries 10 A but in
reverse direction.
Figure(e) shows that the brush is not
touching segment b and coil B carries
15 A, while 5A directly jumps through
air to segment a causing sparking.

13. Reactance voltage
 So, it can be concluded that sparking at the brushes, which results in poor
commutation is due to the inability of the current in the short-circuited coil to reverse
completely by the end of short-circuit period (which is usually of the order of 1/500
second).
 The main cause which retards or delays this quick reversal is the production of self-
induced emf in the coil undergoing commutation.
 It may be pointed out that the coil possesses appreciable amount of self inductance
because it lies embedded in a material of high magnetic permeability. This self-
induced emf is known as reactance voltage.
 The value of reactance voltage= co-efficient of self-inductance × rate of change of
current
 Time of short circuit or commutation is the time required for the Commutator to move
a distance equal to circumferential thickness of brush(Wb) minus the thickness of one
insulating plate of strip of mica(Wm).
 Tc = (Wb-Wm)/v
 v=peripheral velocity of Commutator segments in cm/second
 If I is the current through one conductor than 2I is current through conductor during
short circuit period.
 Therefore reactance voltage = L × (2I/ Tc)(for linear commuation)
 reactance voltage =1.11L × (2I/ Tc)(for sinusoidal commuation)

14. Methods of Improving Commutation
There are two practical ways of improving commutation i.e. of
making current reversal in the short-circuited coil as spark less as
possible. These methods are known as
 (i) resistance commutation and
 (ii) emf. Commutation
Resistance Commutation:
This method of improving commutation consists of replacing
low-resistance Cu brushes by comparatively high-resistance
carbon brushes.
If copper brushes are used then since they have low resistance
some current passes through bar b and more current passes
through bar a.
But if high resistance carbon brushes are used, then the current
will prefer going through bar a, as (i) the resistance r1 of the first
path will increase due to diminishing area of contact of bar ‘b’ with
brush and because
(ii) Resistance r2 of second path will decrease due to rapidly
increasing contact area of bar ‘a’ with the brush.
Disadvantage of carbon brush:
Due to their high contact resistance (which is beneficial to
sparkless commutation) a loss of approximately 2 volt is caused.

15. E.M.F commutation
In this method, arrangement is made to
neutralize the reactance voltage by
producing a reversing emf in the short-
circuited coil under commutation.
This reversing emf, as the name shows,
is an emf in opposition to the reactance
voltage and if its value is made equal to
the latter, it will completely wipe it off,
thereby producing quick reversal of
current in the short-circuited coil which
will result in spark less commutation. The
reversing emf may be produced in two
ways:
 (i) either by giving the brushes a
forward lead sufficient enough to bring the
short-circuited coil under the influence of
next pole of opposite polarity
 (ii) by using interpoles.

16. Interpoles of Compoles
These are small poles fixed to the yoke and spaced in
between the main poles. They are wound with
comparatively few heavy gauge Cu wire turns and are
connected in series with the armature so that they carry full
armature current. Their polarity, in the case of a generator,
is the same as that of the main pole ahead in the direction
of rotation. The function of interpoles is two-fold:
(i) As their polarity is the same as that of the main pole
ahead, they induce an emf in the coil (under commutation)
which helps the reversal of current. The emf induced by the
compoles is known as commutating or reversing emf. The
commutating emf neutralizes the reactance emf thereby
making commutation spark less. With interpoles, spark less
commutation can be obtained up to 20 % to 30% overload
with fixed brush position.
(ii)Another function of the interpoles is to neutralize the
cross-magnetizing effect of armature reaction. Hence,
brushes are not to be shifted from the original position.