Figure 1-2.The elementary generator.
The pole pieces (marked N and S) provide the magnetic field. The pole pieces are shaped and
positioned as shown to concentrate the magnetic field as close as possible to the wire loop. The loop of
wire that rotates through the field is called the ARMATURE. The ends of the armature loop are connected
to rings called SLIP RINGS. They rotate with the armature. The brushes, usually made of carbon, with
wires attached to them, ride against the rings. The generated voltage appears across these brushes.
The elementary generator produces a voltage in the following manner (fig. 1-3). The armature loop is
rotated in a clockwise direction. The initial or starting point is shown at position A. (This will be
considered the zero-degree position.) At 0º
the armature loop is perpendicular to the magnetic field. The
black and white conductors of the loop are moving parallel to the field. The instant the conductors are
moving parallel to the magnetic field, they do not cut any lines of flux. Therefore, no emf is induced in
the conductors, and the meter at position A indicates zero. This position is called the NEUTRAL PLANE.
As the armature loop rotates from position A (0º) to position B (90º), the conductors cut through more and
more lines of flux, at a continually increasing angle. At 90º
they are cutting through a maximum number
of lines of flux and at maximum angle. The result is that between 0º
, the induced emf in the
conductors builds up from zero to a maximum value. Observe that from 0º
, the black conductor
cuts DOWN through the field. At the same time the white conductor cuts UP through the field. The
induced emfs in the conductors are series-adding. This means the resultant voltage across the brushes (the
terminal voltage) is the sum of the two induced voltages. The meter at position B reads maximum value.
As the armature loop continues rotating from 90º
(position B) to 180º
(position C), the conductors which
were cutting through a maximum number of lines of flux at position B now cut through fewer lines. They
are again moving parallel to the magnetic field at position C. They no longer cut through any lines of flux.
As the armature rotates from 90º
, the induced voltage will decrease to zero in the same manner
that it increased during the rotation from 0º
. The meter again reads zero. From 0º
conductors of the armature loop have been moving in the same direction through the magnetic field.
Therefore, the polarity of the induced voltage has remained the same. This is shown by points A through
C on the graph. As the loop rotates beyond 180º
(position C), through 270º
(position D), and back to the
initial or starting point (position A), the direction of the cutting action of the conductors through the
magnetic field reverses. Now the black conductor cuts UP through the field while the white conductor
cuts DOWN through the field. As a result, the polarity of the induced voltage reverses. Following the
sequence shown by graph points C, D, and back to A, the voltage will be in the direction opposite to that