Chapter 30 Engine Starting Systems 505
causes the winding to move toward a vertical posi-
tion. A commutator and brushes are used to keep
the electric motor spinning. The commutator serves
as a sliding electrical connection between the arma-
ture winding and the brushes. It is divided into seg-
ments, which are insulated from each other. The
brushes slide on the commutator to carry battery
current to the spinning winding.
As the winding rotates away from the pole shoes,
the commutator reverses the electrical connection
between the brushes and the winding. This reverses
the magnetic fi eld around the winding. Then the
winding is pulled around and passes the other pole
shoe. The constantly changing electrical connection
keeps the motor spinning. A push-pull action is set
up as the winding moves around inside the pole
shoes. See Figure 30-3.
Increasing Motor Power
Several windings (loops of wire) and a commuta-
tor with many segments are used to increase motor
power and smoothness. Each winding is connected
to its own segment on the commutator. This provides
current fl ow through each winding as the brushes
contact each segment. As the motor spins, many
windings contribute to the motion. This produces a
constant and smooth turning force. See Figure 30-4.
12-Volt Starting Motor Construction
The construction of all 12-volt starting motors is very
similar. There are slight variations in design, however.
A typical starting motor is shown in Figure 30-5. Refer
to this fi gure as you study the main parts of a starting
motor covered in the following sections.
Armature
A starting motor must produce very high torque
(turning force) and relatively high speed. Therefore,
a system to support the windings and increase the
strength of each winding’s magnetic fi eld is needed.
A starter armature consists of the armature shaft,
armature core, commutator, and armature windings.
The armature motor shaft supports the armature
as it spins inside the starter housing. The commu-
tator is mounted on one end of the armature shaft.
The armature core holds the windings in place. The
armature core is made of iron to increase the strength
of the magnetic fi eld produced by the windings.
Field Windings
Field windings are stationary insulated wires
wrapped in a circular shape around iron pole shoes
located inside the starting motor housing, Figure 30-6.
When current fl ows through the fi eld windings,
the magnetic fi eld between the pole shoes becomes
very large. It can be 5–10 times that of a permanent
magnet. As the magnetic fi eld between the pole
shoes acts against the fi eld developed by the arma-
ture, the motor spins with extra power and torque.
Field Magnets
Some starting motors use special high-strength
magnets instead of conventional fi eld windings. The
magnets produce a strong magnetic fi eld capable of
rotating the armature with enough torque to crank
the engine. See Figure 30-7. Starting motors that use
fi eld magnets are referred to as permanent-magnet
starters.
S
N
Wire loop or armature
Stationary
magnet
Stationary
magnet
Current from
battery
Current to
battery
Commutator
Brush
Magnetic
field
S
N
Commutator
segments
Armature windings
Stationary
magnet
Stationary
magnet
Brush
Magnetic
field
Figure 30-3. A simple electric motor consists of a winding (loop
of wire), located inside stationary pole shoes. The magnetic fi eld
around the winding and the magnetic fi eld between the pole
shoes act upon each other to make the winding rotate.
Figure 30-4. A starting motor would not have enough power
with only a single winding. Instead, many windings are used
to increase power and smoothness. Note that each winding is
connected to a separate commutator segment.