Types of Starter Drives
Starting Motor Construction
A starting motor uses a gear reduction to transmit
A sectional view of a heavy-duty starting motor is
its cranking power to the engine. The method most
shown in figure 6-66. This starter employs a magnetic
commonly used to obtain this gear reduction is a large
switch to control the starter, a reduction gear driving
diameter ring gear on the engine flywheel and a pinion
head, and a Bendix drive. Except for the drive end and
gear of much smaller diameter on the starting motor
control mechanism, the general construction charac-
armature shaft. In operation, the pinion engages and
teristics of the starting motor are similar to the dc
drives the ring gear, which turns the engine. The ratio
between the pinion and ring gear is different on engines
The connections and arrangements of the armature
of different sizes and horsepower. It is usually between
coils are such that current is flowing in all the coils at
10:1 and 16:1. The starting motor armature revolves 10
the same time (series wound). This allows each coil to
to 16 times for each revolution of the engine flywheel.
add its turning effort to the other coils. They work
When the pinion is engaged with the ring gear, the
together to turn the motor armature.
starting motor turns 2,000 to 3,000 rpm. This turns the
The field windings are used to increase the strength
engine at speeds up to 300 rpm. During operation, the
of the magnetic field, and thereby increase the power of
engine may reach speeds of 3,000 to 4,000 rpm. If the
the motor. The field windings are connected in series
pinion should fail to disengage from the ring gear after
with the brushes and armature windings. The current
the engine is started, the starting motor could be rotated
that flows through the field windings also flows
by the engine at speeds up to 64,000 rpm. This high a
through the armature windings. A heavy conductor is
speed throws the windings from the slots in the
used in the field and the armature so the resistance of
armature and the segments from the commutator. To
the motor windings is very low. This permits an
prevent this, various methods of engaging and
extremely large current flow, and the motor develops
disengaging the pinion with the ring gear have been
high torque.
devised.
MAGNETIC SWITCH
TERMINAL
THRU BOLT
CONNECTOR
OILER
GREASE PLUG
COVER
BAND
GEAR HOUSING
OILER
REDUCTION
BRONZE
BEARING
OILER
GEAR
BRUSH
COMMUTATOR
ARMATURE
BENDIX DRIVE
FIELD COIL
DRIVE
ASf06066
HOUSING
COMMUTATOR
END FRAME
Figure 6-66.--Sectional view of a heavy-duty starter.
6-54
