Faculty of Engineering

Electrical Department
Semester 6
Tel. 0910169287
Electromechanical Energy
conversion
Lecture_ 3: Armature Reaction
Instructor : Dr. Eltaib Said Elmubarak
 Quick Recap
• Construction of the d.c. machine
• Armature winding arrangements
• Calculation of the e.m.f. induced in the
armature windings
• By the end of this lecture student would be
familiar with :-
• The effect of Armature reaction
• Armature & field windings connections
• DC machine performance
Armature reaction
• Armature reaction is the effect of armature
ampere-turns upon the value and the
distribution of the magnetic flux entering and
leaving the armature core
• Let us, for simplicity, consider a two-pole
machine having an armature with eight slots
and two conductors per slot, as shown in
figure
• The curved lines between the conductors and
the commutator segments represent the front
end connections of the armature winding and
those on the outside of the armature
represent the back end connections.
• The armature winding – like all modern d.c.
windings – is of the double-layer type, the end
connections of the outer layer being
represented by full lines and those of the
inner layer by dotted lines
• Brushes A and B are placed so that they are
making contact with conductors which are
moving midway between the poles and have
therefore no e.m.f. induced in them
• thus over the one halve of the pole faces the
cross flux is in opposition to the main flux,
thereby reducing the flux density, whereas
over the leading halves the two fluxes are in
the same direction, so that the flux density is
strengthened
• Hence, in a motor, the effect of armature
reaction is to distort the flux against the
direction of rotation
• One important consequence of this distortion
of the flux is that the magnetic neutral axis is
shifted through an angle θ from AB to CD; in
other words, with the machine on no load and
the flux distribution of Figure (a), conductors
are moving parallel to the magnetic flux and
therefore generating no e.m.f. when they are
passing axis AB
• When the machine is loaded as a motor
and the flux distorted as in Figure (c),
conductors are moving parallel to the flux and
generating no e.m.f. when they are passing
axis CD
• The effect of the armature ampere-turns upon
the distribution of the magnetic flux is
represented in following Figure
• The dotted lines is represent the distribution
of the magnetic flux in the air gap on no load
• The corresponding variation of the flux
density over the periphery of the armature is
represented by the ordinates of Figure(b)
• Figures (c) and (d) represent the cross flux due
to the armature ampere-turns alone
• the armature current being assumed in the
direction in which it flows when the machine
is loaded as a generator
• The effect of magnetic saturation would be to
reduce the flux density at the leading pole
tips, as indicated by the shaded areas P, and
thereby to reduce the total flux per pole
• It will also be seen from Figure(f) that the
points of zero flux density, and therefore of
zero generated e.m.f. in the armature
conductors, have been shifted through an
angle θ against the direction of rotation to
points C and D
Armature & field windings connections
• The general arrangement of the brush and
field connections of a four-pole machine is
shown in following figure.
• The positive brushes are connected to the
positive terminal A and the negative brushes
to the negative terminal A1
• The four exciting or field coils C are usually
joined in series and the ends are brought out
to terminals F and F1
• These coils must be so connected as to
produce N and S poles alternately
• The arrowheads in Figure indicate the
• direction of the field current when F is positive
• In general, we may divide the methods used
for connecting the field and armature
windings into the following groups:
• Separately excited machines – the field
winding being connected to a source of supply
other than the armature of its own machine.
• Self-excited machines, which may be
subdivided into:
(a) shunt-wound machines – the field winding
being connected across the armature
terminals;
(b) series-wound machines – the field winding
being connected in series with the armature
winding;
(c) compound-wound machines– a combination
of shunt and series windings.
A d.c. machine as generator or motor
• There is no difference of construction
between a d.c. motor and a d.c. generator
• In fact, the only difference is that in a motor
the generated e.m.f. is less than the terminal
voltage, whereas in a generator the generated
e.m.f. is greater than the terminal voltage.
• For instance, suppose a shunt generator D
shown in Figure to be driven by an engine and
connected through a centre-zero ammeter A
to a battery B.
• If the field regulator R is adjusted until the
reading on A is zero, the e.m.f., ED, generated
in D is then exactly equal to the e.m.f., EB, of
the battery.
• If R is now reduced, the e.m.f. generated
• in D exceeds that of B, and the excess e.m.f. is
available to circulate a current ID through the
resistance of the armature circuit, the battery
and the connecting conductors.
• Since ID is in the same direction as ED, machine
D is a generator of electrical energy.
• Next, suppose the supply of steam or oil to
the engine driving D to be cut off
• The speed of the set falls, and as ED decreases
• ID becomes less, until, when ED = EB, there is
no circulating current. But ED continues to
decrease and becomes less than EB, so that a
current Im flows in the reverse direction.
Hence B is now supplying electrical energy to
drive D as an electric motor.
• The speed of D continues to fall until the
difference between ED and EB is sufficient to
circulate the current necessary to maintain
the rotation of D.
• It will be noticed that the direction of the field
current If is the same whether D is running as
a generator or a motor.
• The relationship between the current, the
e.m.f., etc. for machine D may be expressed
thus. If E is the e.m.f.
• generated in armature, V the terminal voltage,
Ra the resistance of armature circuit and Ia the
armature current, then, when D is operating as a
generator,
E = V + IaRa
• When the machine is operating as a motor,
the e.m.f., E, is less than the applied voltage V,
and the direction of the current Ia is the
reverse of that when the machine is acting as
a generator; hence
E = V - IaRa
• Since the e.m.f. generated in the armature of a
motor is in opposition to the applied voltage,
it is sometimes referred to as a back e.m.f.