Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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62 Chapter Four
Figure 4-23. Direct current (DC) voltage generated
in a rotating loop conductor.
When the wire loop makes a half-turn, the
energy generated rises to a maximum level, then
drops to zero, as shown in parts B and C of
Figure 4-23. As the wire loop completes a full rotation
the induced voltage would reverse itself and
the current would flow in the opposite direction
(AC current) after the initial half-turn. To provide
for an output having a single polarity (DC current),
a split-ring commutator is used. Thus, for the
second half-turn, the carbon brushes engage commutator
segments opposite to those over which
they slid for the first half-turn, keeping the current
in the same direction. The output waveform is not
a steady-level DC, but rises and falls to form a pattern
referred to as pulsating DC. Thus, for a
complete 360-degree turn of the wire loop, two
waveforms are produced, as shown in Figure 4-23.
The motion of a conductor may induce DC voltage
across a stationary magnetic field. This is the
principle used to change mechanical energy to electrical
energy in a generator. A looped conductor is
mechanically moved within the magnetic field created
by stationary magnets, as in Figure 4-23.
In Figure 4-23A, the voltage is zero because the
conductor motion is parallel to the flux lines. As the
conductor moves from A to B, the voltage increases
because it is cutting across the flux lines. At B, the
voltage is at a maximum because the conductor is
moving at right angles to the flux lines, breaking the
maximum number.
From position B to C, the voltage decreases to
zero again because fewer lines are broken. At C, the
conductor is again parallel to the flux lines. As the
conductor rotates from C to D, voltage increases.
However, the induced voltage is in the opposite
direction because the conductor is cutting the flux
lines in the opposite direction. From position D to
E, the cycle begins to repeat. Figure 4-23 shows
how voltage is induced in a loop conductor through
one complete revolution in a magnetic field. The
induced voltage is called alternating current (AC)
voltage because it reverses direction every half
cycle, as shown on the graph at the bottom of
the figure. Because automotive battery voltage is
always in one direction, the current it produces
always flows in one direction. This is called direct
current (DC). Alternating current cannot be used
to charge the battery, so the AC must be changed
(rectified) to DC. This is done in a generator by
the commutator. In a simple, single-loop generator,
the commutator would be a split ring of conductive
material connected to the ends of the conductor.
Brushes of conductive material ride on the surface
of the two commutator segments.