Part of a lecture series delivered by me on DC machines to BE Third Year Students, Z. H. College of Engg. & Technology, AMU, Aligarh, 2012-13. Please comment and feel free to ask anything related. Thanks!

1. B. E. Electrical III Year, 2012-13
Instructor: Mohd. Umar Rehman
EEE-311
Electrical Machines-II
Notes on
Armature Reaction
1
Definition & Scope
If the magnetic field windings of a dc machine are connected to a power supply and the rotor of
the machine is turned by an external source of mechanical power, then a voltage will be induced
in the conductors of the rotor. This voltage will be rectified into DC output by the action of the
machine’s commutator.
However, a DC generator is intended to supply the output voltage (current) to some load
connected across its terminals. When the DC generator is supplying load current, then this
current will be flowing through the armature conductors. This current will produce its own
magnetic flux/field which is an unavoidable and undesirable phenomenon. The armature flux
interacts with the main field flux & has the following effects:
1. The armature flux distorts the main flux (cross-magnetization) and/or,
2. The armature flux weakens the main flux (demagnetization).
This interaction of the armature flux and the field flux is termed as Armature Reaction (AR).
Problems associated with AR
1. Shifting of magnetic neutral plane1
In a DC machine magnetic neutral plane (MNP) is the plane where the velocity of the armature
conductors is exactly parallel to the magnetic flux lines, so that induced EMF in the conductors
in the plane is exactly zero. The direction of MNP is perpendicular to the resultant flux in the
machine. Commutator brushes are placed along this plane for successful and sparkless
commutation which is described as follows.
Whenever a brush makes contact with two commutator segments having a coil connected
between them, the brush short circuits the coil and carries the full load current. However, when
the coil passes the MNP the voltage induced (& hence current) in it is zero momentarily.
Therefore, the brushes are always placed along the MNP to prevent damage from excessively
large currents.
Figure 1 shows the flux distribution and associated MMFs in a two pole DC machine under
different conditions. In an unloaded machine when only field winding is excited and armature
conductors are not carrying any current, the only flux present is the main field flux. The MNP in
this situation is exactly vertical2
and coincides with the GNP and the associated MMF is
perpendicular to it. When only armature current is present and field is not excited, the flux will
be the armature flux and the associated MMF will be along the MNP. However, in a practical
DC machine both the windings will be excited simultaneously and hence there will be fluxes
present in the machine simultaneously, the main field flux and the armature flux. These two
fluxes will interact with each other to give rise to a resultant flux whose direction is as shown.
Now the direction of the MNP will change (say by angle θ) according to the direction of
resultant flux which will be proportional to the load current (Why?)
1
Neutral Plane is more appropriate term than neutral axis.
2
Note that armature flux is entirely cross-magnetizing in nature & is sometimes called cross-flux

2. B. E. Electrical III Year, 2012-13
Instructor: Mohd. Umar Rehman
EEE-311
Electrical Machines-II
Notes on
Armature Reaction
2
(i)
(ii)
(iii)
Fig. 1 Flux Distribution & Resultant MMF in a two pole DC Machine
(i) Machine on no load, only field excited, MNP coincides with GNP
(ii) Only armature flux and resultant MMF
(iii) Both fluxes present under load conditions, MNP shifts in the direction of
rotation (for generator)
MNP (Shifted)
θ
FM
GNP & MNP GNP & MNP
FMO
GNP
FA
O
N
S
GNP
O
FA
FR

3. B. E. Electrical III Year, 2012-13
Instructor: Mohd. Umar Rehman
EEE-311
Electrical Machines-II
Notes on
Armature Reaction
3
Now, the position of the brushes needs to be changed in accordance with that of MNP for
sparkless commutation. If the brushes are kept at the original position (GNP), then EMF induced
in the conductors will not be zero and a large SC current will flow through the brushes causing
sparking and consequent damage. Again if the brushes are moved at the new MNP position then
load fluctuations will cause repeated change in the MNP position. Thus, moving of brushes to
the new MNP position is impractical. In small rating machines, brushes are placed at an
intermediate position to reduce the sparking. But for large rating machines the solution to AR
problems is to use compensating windings &/or commutating poles (interpoles).
Compensating winding is placed on the pole faces of the machine and connected in series with
the armature winding in such a way that the current in compensating winding conductors is
equal & opposite to that in the armature winding conductors. This is the best but most expensive
method to neutralize the effects of AR.
Compensating winding cannot nullify the armature flux completely. Additional small poles
called interpoles are provided in between the main poles in large machines to get rid of the
commutation problem arising out of armature reaction. The basic idea here is that if the voltage
in the wires undergoing commutation can be made zero, then there will be no sparking at the
brushes. These commutating poles are located directly over the conductors being commutated
and their polarity is same as the main pole in the direction of rotation. By providing a flux from
the commutating poles, the voltage in the coils undergoing commutation can be exactly
cancelled. If the cancellation is exact, then there will be no sparking at the brushes. Both the
above methods are used to nullify the cross-magnetizing effect of AR.
(a) (b)
Fig. 2 (a) Compensating Winding & (b) Interpoles in a DC machine

4. B. E. Electrical III Year, 2012-13
Instructor: Mohd. Umar Rehman
EEE-311
Electrical Machines-II
Notes on
Armature Reaction
4
2. Field Weakening
Most machines operate at flux densities near the saturation point. Therefore, at locations on the
pole surfaces where the rotor MMF adds to the pole MMF, only a small increase in flux occurs.
But at locations on the pole surfaces where the rotor MMF subtracts from the pole MMF, there
is a larger decrease in flux. The net result is that the total average flux under the entire pole face
is decreased.
The flux weakening causes problems in both generators and motors. In generators, the effect of
flux weakening is simply to reduce the voltage supplied by the generator for any given load. In
motors, the effect can be more serious. When the flux in a motor is decreased, its speed
increases. But increasing the speed of a motor can increase its load, resulting in more flux
weakening. Field excitation should be increased if the flux is to be kept constant at its no-load
value.
Fig. 3 A typical magnetization curve shows the effects of pole saturation where armature and
pole MMFs add/subtract

5. B. E. Electrical III Year, 2012-13
Instructor: Mohd. Umar Rehman
EEE-311
Electrical Machines-II
Notes on
Armature Reaction
5
Effect of Brush Shift and Demagnetizing & Cross Magnetizing Ampere Turns
As stated earlier, MNP shifts in the direction of rotation due to AR in a DC generator. To
achieve sparkless commutation brushes can be shifted to the new MNP. The armature flux/MMF
should be along the brush axis. Now armature MMF can be decomposed into two components:
one that is opposing the main flux and produces demagnetizing effect, and second that is
perpendicular to the main flux & is responsible for cross-magnetizing effect. Observation of the
Fig. 4 reveals that the conductors responsible for demagnetizing effect are under the range of 2θ
(on top & bottom) produce flux that is directly opposite to the main flux, and the rest conductors
under 180 2θ°− (on left & right) are responsible for cross-magnetizing effect.
(i)
(ii)
Fig. 4
(i) Effect of brush shift on the resultant flux in a DC machine
(ii) Demagnetizing & cross-magnetizing ampere turns
Total AT = Demagnetizing AT + cross-magnetizing AT
ATT = ATD + ATC
1
(per pole) , (per pole)
360 2 360
D CAT ZI AT ZI
p
θ θ 
= × = × − 
 
3
3
Z: total number of armature conductors, I: current in each armature conductor, p: no. of poles