Car Parts and Accessories

Current from the auto battery rotates the engine

The job of the starter motor is to turn the engine until it fires and can continue to run under its own power.

Most petrol engines are rotated at 250-300 rpm while starting. This needs considerable electric power, particularly in winter when the engine is cold and the oil is thick. Anyone who has started a car with a cranking handle will know how much effort is needed to turn the engine.

The starter motor imposes the biggest drain of any electrical component on the car battery. The instant it operates, it can draw as much as 300 to 400 amp. Because of this high current, the starter motor needs a heavy-duty switch to connect it directly, via the live battery cable, to the battery. This switch—the solenoid is remotely operated from the ignition switch.

At the same time as it turns the starter motor, the battery must supply current to the ignition coil so that sparks to fire the plugs are produced. This can lead to difficulty in starting if the battery is failing. It may drain so much current from the battery that the ignition system cannot work at peak efficiency, and fails to generate a high enough voltage to cross the spark-plug points.

The starter motor turns the engine’s crankshaft through a pair of gear wheels. One, the pinion, is mounted on the starter motor shaft. It engages with the other, a toothed ring fitted around the edge of the flywheel, which rotates the crankshaft.

The gear ratio between these two is generally about 15:1. The starter motor’s pinion must disengage from the flywheel gear when the engine fires, or the engine would drive the motor and spread the windings on the armature—so destroying the starter motor. The most common mechanism used is the Bendix drive.

How the auto starter motor works

The starter motor operates on the same principle as any other electric motor—it makes use of movement between magnets. If two magnets are held close together, the two like poles (two north or two south poles) push each other apart, and the unlike poles (a north and a south) attract each other.

An electric motor contains electromagnets—coils of wire wound on soft iron cores. Electricity flowing through each coil magnetizes the core, setting up a magnetic field which has north and south poles. A starter motor has a fixed set of coils, generally four, spaced round the inside of the motor body. These are the field coils. Free to rotate inside them is the armature, which consists of a series of coils, each joined to a pair of the insulated copper segments that make up the armature’s commutator. When current flows through an armature coil, this coil also behaves as a magnet.

Current is fed by stationary brushes (which make contact with the commutator) to an armature coil. Attraction and repulsion between the magnetic fields of the field coils and the armature coil cause the armature to rotate.

As soon as the commutator begins to rotate, the brushes make contact with the next pair of copper segments, which are connected to another armature coil. This results in further rotation. The action is repeated in sequence as each pair of commutator segments makes contact with the brushes. In this way, the armature con, tinues to spin as the brushes feed current to each armature coil.

The starter motor needs no control unit : the same connection to it supplies both the armature and the field windings and is wired in such a way that it draws from the battery as much current as it requires to turn the engine, and no more.

Bendix auto starter drive

ONCE the engine is running, the starter- motor pinion must be disengaged from the flywheel which it has set in motion. This is achieved by having a pinion which fits loosely on a threaded shaft and is free to move along the thread.

When the shaft starts to rotate, the inertia of the pinion (its resistance to being moved) makes it rotate more slowly than the shaft. As a result, the pinion travels along the threaded shaft and engages with the teeth on the flywheel. Once engaged, it rotates the flywheel which, since it is bolted to the crankshaft, turns the engine.

When the engine starts to run under its own power, the flywheel gear starts to drive the pinion instead of being driven by it. Once the driven speed of the pinion exceeds that of the starter-motor shaft, the pinion screws itself back along the shaft, out of engagement. It is flung out of mesh as soon as the engine fires.

Auto Starter switch

Since the starter motor uses high current, the switch that operates it must also be able to pass high current. To do this it needs heavy-duty contacts.

On old cars, the heavy-duty switch was generally mounted on the starter motor and worked directly by a cable pulled by the driver. On modern cars, the switch is worked by a relay or solenoid, in which a strong electromagnet pulls the heavy-duty contacts together. The starter solenoid, which needs only a low current, is in turn operated by another switch. This smaller switch is incorporated within the ignition switch.

The electrical leads from the battery to the solenoid and from the solenoid to the starter motor must be thick and heavy and securely fastened to carry the high current involved. On the other hand, the lead to the driver’s switch, which passes only low current, can be thin and light.

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