Starter Motor: Description and Operation

Construction

Exploded view:

Starter Motor:

The main parts of the starter motor are:
a solenoid and shift lever,
field coils with pole shoes,
an armature with drive, a commutator and carbon brushes.
Connections
16 - to ballast resistance, ignition coil
30 - from battery's (+) terminal
50 - from ignition switch, connection 50

Principle





The construction of a starter motor is based on the principle that a current carrying conductor in a magnetic field is influenced by a force which tries to push it away from the magnetic field.

Series motor





The illustration shows the principle behind a two pole series motor. The armature consists of a coil which rotates because of the force in effect between it and the electromagnet's magnetic field. The armature and field coils are connected in series and consequently the same current flow through both of them.


Function





The link between the flywheel ring gear and the starter motor occurs via the moveable drive (pinion) on the armature shaft. The drive is moved by a shift lever connected to a solenoid. The starter motor drive is equipped with a free running clutch which prevents the starter motor from being driven when the engine starts and the starter motor is kept engaged.


Current flow and magnetic flux





The magnitude of the force exerted on a current carrying conductor in a magnetic field is proportional to the strength of the magnetic field and the current. The force reaches a maximum when the magnetic field and the current are at right angles to each other. The magnetic flux in the field coils is shown in the illustration.

Output and torque curves





The starter motor produces its highest torque at low revs, with maximum current flowing. It should never be kept engaged for more than 30 seconds since longer periods may cause overheating and damage.

Solenoid





The solenoid is in principle a strong relay which with the help of a relatively small current can switch on or off larger currents and at the same time-slide the drive (pinion) in to mesh with the flywheel gear.

Free running clutch





When the starter motor turns, the spring loaded rollers lock the drive (pinion). The car's engine cannot drive the starter motor because the drive (pinion) spins against the rollers. The free running clutch is connected to the armature shaft by spiral splines which facilitate the engagement/disengagement of the flywheel gear.

Armature brake





It must be possible to brake the armature quickly after each attempt to start the vehicle so that new restarts can be tried quickly. The braking force is obtained from springs attached to carbon brushes. Worn springs and carbon brushes adversely affect the brakes.

Meshing of drive (pinion)





The solenoid's pull-in and holding windings receive current when the ignition switch is turned on. (The pull-in winding is connected through the field coils, armature and carbon brushes.) As a result of this the starter motor's drive (pinion) meshes with the flywheel gear.

Starting the engine





When the ignition is switched on and the starter is closed, current flows through the solenoid (producing a magnetic field) and the plunger is drawn in. The drive (pinion) meshes with the flywheel gear. The holding winding holds the drive (pinion) in position and the armature turns the engine. The current now flows through the twin connected field coils to the positive carbon brushes and through the armature. The current is grounded via the negative brushes. When the ignition switch is released the current to the holding winding is cut off and a return spring withdraws the drive (pinion).