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

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108 Chapter Six
Figure 6-36.
The motor principle.
Figure 6-34. When the Ford starter relay is
energized, the plunger contact disk moves against the
battery and starter terminals to complete the circuit.
Most simple automotive buzzers are sealed
units and simply plug into their circuits. Some
buzzers are combined in a single assembly with a
relay for another circuit (Figure 6-35), such as a
horn relay. This application is used on some
General Motors cars. While mechanical buzzers
are still in use, they are comparatively heavy and
draw a relatively high current compared to the
lighter solid-state chimes and buzzers provided
by electronic technology and tone generators.
Figure 6-35. Typical horn relay and buzzer circuits.
(Delphi Corporation)
When the switch is closed, current flow through
the buzzer coil reaches ground through the normally
closed contacts. However, current flow also magnetizes
the buzzer core to move the armature and open
the contacts. This breaks the circuit, and current flow
stops. Armature spring tension then closes the contacts,
making the circuit again (Figure 6-35). This
action is repeated several hundred times a second,
and the vibrating armature creates a buzzing sound.
Motors
The typical automotive electrical system includes
a number of motors that perform various jobs.
The most common is the starter motor (also called
a cranking motor), which rotates the automobile’s
crankshaft until the engine starts and can run by
itself. Smaller motors run windshield wipers,
power windows, and other accessories. Whatever
job they do, all electric motors operate on the
same principles of electromagnetism.
We explained the motor principle in terms of
magnetic field interaction in Chapter 4. When a
current-carrying conductor is placed in an external
magnetic field, it tends to move out of a
strong field area and into a weak field area
(Figure 6-36). This motion can be used to rotate
an armature. Now we will see how automotive
electrical motors are constructed and used.
A simple picture of electric motor operation
(Figure 6-37) looks much like the operation of a
simple generator. Instead of rotating the looped
conductor to induce a voltage, however, we are
applying a current to force the conductor to rotate.
As soon as the conductor has made a half-revolution,
the field interaction would tend to force it
back in the opposite direction. To keep the conductor
rotating in one direction, the current flow
through the conductor must be reversed.