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Starter (motor car)
Assignment
It should be as light-weight as possible and not require too much current while bringing the combustion engine up to the starting
speed of over 300 RPMs (with a
warm engine). As soon as the engine has started, it should be mechanically disengaged immediately. In motor cars, only the thrust-pinion-starter is used.
| The faster it turns, the lower the current input is. |
Function
Modern starters are electric motors, whose field-coil can be replaced by a permannt magnet. For a short time, during the starting procedure, it's gear is meshed into that of the engine's flywheel by a solenoid switch. Because of the high RPMs of the electric motor and the necessary torque, a high gear-ratio (10 - 20:1) is necessary. This is achieved by having a small
gear on the starter-shaft and the large gear of he flywheel. Both are straight-cut to enable easy meshing. Through a planetary-gear on the armature shaft, this rotates even quicker, with a constant torque output. Thus,
the complete system, made up of batter y, generator and starter, can be built to weigh 1/3 less.
The starter itself consists mainly of an electric motor. Due to the advancements made in the development of permanent magnetics and because of the low torque, one can dispense with the stator-coil. The armature-
coil is activated through a commutator. Two, or four spring-loaded carbon brushes are pressed against the rotating commutator, this creates a conducting connection from the casing to the armature coil. The surface
of the commutator is divided into segments. As is usual in a DC-electric motor, the armature has half as many windings as the commutator has segments. Thereby, the ends of each winding are connected with two
opposing segments. Because of the special demands made on the torque and on the current flow, the cross-section between the carbon brushes and the segments is especially wide. When four brushes exist (see
picture 4), two windings can be effective at the same time.
Important
At the beginning of the starting procedure, the electric motor can take on more than 100 Amp. of current just before the engine springs to life. It has no cooling vanes and may therefore only operated for a short time. If
the battery is short circuited for a stress-test, this may only take place for a few seconds, otherwise the heat build-up could cause serious damage. 09/10
| Did you know, that if the combustion engine is defective, one can use the starter to pull oneself out of a critical situation (e.g., a level-crossing)? Simply let the starter run
with a gear engaged and without pressing the clutch down. |
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