Honolulu star-bulletin., 1917-03-17
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ACTION BRUSH
DETAIL MOTORISTS
CBy Special Permission from Motor

tBy Special Permission from Motor
Age.1 ......-
In order 10 understand the operation
of the third brush machine II will be
necessary to make a brief study OF
the magnetizing action OF the current
In the armature winding OF a geneT-
ator in combination with the magne-
tizing action OF the current in the field
windings.
A cross section through the arma,
ture and poles OF two-pole generator
is shown in Fig /. The wires on the
surface OF the armature are represent
ed by 20 small circles spaced equal
distances apart around the outside OF
the large circle which is supposed TO
represent the armature core. As a
matter OF fact, there are more than
20 wires on the surface Of the arna.
ture, but this number has been used
to simplify the diagram, the results
being exactly the same. The polarity
OF the poles is indicated by N and s.;
the polarity OF the armature core by
N and s, and the direction Of the mag.
netic field by the small arrows.
IL the armature be revolved in the
y lckwise direction, as indicated by
tho large curved arrow at the top OF
the figure, an electrical pressure
whose direction is away from the OBJ.
server will be induced in the wires on
the left hand side of vertical line
through the armature, and an elec.
trical pressure whose direction Is to-
ward the observer will be induced IN
the wires on the right hand side of
the armature An electrical pressure,
or current, toward the surface Of the
paper is represented by plus sign
(+) and one away from the observer
by a minus sign Il. II you will
think of the and - signs as being
respectively the feathered end and the
flattened point Of an arrow pointing
along the wire, the diagram becomes
plain. The direction OF the current al.
ways can be found by the right hand
rule mentioned previously.
Wires lnterconnected
The wires on the surface OF the
commutator segments, and current
will be produced in them when the
brushes resting on the commutator arc
connected to a closed electrical cir.
cult. This current IN the armature
VIII produce o .magnetixing action lust
the same 90 the. current IN the field
vindlngs. The magnetic. effect Of the
current in the. armature winding OF
the generator may be Investigated by
sending a current through the arma-
ture from an outside source, such as
battery, In the same direction as the
current the generator itself would pro.
duce, with the arrnature standing still
and no current in the field windings.
The general form of the magnetic field
produced by the armature current
would correspond to the dotted lines
shown in Fig 2, and its direction
through the armature would be from
the lower toward the upper side. The
polarity of the armature is indicated
by N and s In the figure. As matter
o2fact, this magnetic field can never
exist alone, but the magnetizing ef.
feet OF the armature current combines
with the magnetizing effect Of. the
field current to form resultant field
whose general form will correspond
to the one shown In Fig. 3.
The magnetizing effects OF the arm.
ature and field currents may be con,
sidered just the same as two mechani-
cal forces which are acting on an. OBJ
ject at right angles to each other
Thus the direction of the rnagnetizing
force OF the field current may be rep
resented by the lire marked F In Fig.
4, and its direction corresponds to the
direction in which the arrowhead
points. Likewise, the magnetizing ef
feet of the armature current may bt
represented by the line A. and its di-
rection corresponds to the direction In
which the arrowhead points. The two
forces combine to form the result
ant force r, which produces the mag.
netic field whose direction CORE
spends to the arrowhead. The angle
the result r makes with the horizon.
tai will depend upon the relation be.
tween the two forces, and F. the
larger the value of the A the greater
the angle. This magnetizing action
OF the armature current is called arm
ature reaction.
IL the brushes be placed on the com.
nluI6tOT III sum pOctIiOu that lucs
rest on segments that are connected to
conductors on the surface OF the arm.
ature in which there is no induced
electrical pressure. q maximum volt
age for given field strength and
speed win exist between them. The
brushes A and B in Fig. l are shown
in a position for maximum voltage. II
a third brush, c be placed on the com
murator midway between the brushes
A.and B, the voltage between A and c
I'm be exactly the same as the volt
age between { and b, because the
same amount OF magnetic flux is cut
by the conductors IN moving from
10 A rs is cut in moving from 3 to c,
When the magnetic field is distorted
due to armature reaction. as shown in
Fig. 3, the voltage between B and c
will be less than the voltage between
C and A, since there is greater
amount of magnetic flux cut by the
conductors In moving from c to A
than is cut by them in moving from 3
TO 9.
Position OF Brushes
The position of the brushes shown