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Starter Motors - How they Work

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1. Back to basics

The starter motor engages with a gear on the flywheel of the engine, to crank the engine, in order to make it fire.  

2. Why it doesn't stay in mesh

To explain further - the gear on the starter has for instance, ten teeth. The flywheel gear has 100. The gear ratio is therefore ten turns of starter to one of the engine. 

If the gear on the starter itself turns at 2,500 rpm, this will relate to an engine speed of 250 rpm. If the starter stayed engaged once the engine had fired, the internals of the starter would not be able to cope with the centrifugal forces generated by the engine now turning the starter! 

The average engine will routinely rev to 4,000 rpm and many are capable of well in excess of this. The net result of a starter staying in mesh is that, with a gearing of 10:1 it would rev at 40,000 per minute (well in excess of the designed speed) and almost certainly cause a complete failure.

So the first requirement of the starter (apart from spinning the engine) are that it disengages. 

3. 'Inertia' starter motors

Until the mid 1970's most British vehicles used an ‘Inertia’ Starter Motor. This uses the sudden acceleration of the motor when energised, over the inertia of a gear to throw it into the flywheel ring gear. 

The problem was that the gear would hit the flywheel,  spinning at high speed and literally force it’s way into mesh, causing damage to both gears. When the engine fired it went faster than the starter motor and threw it back out of mesh. 

This crude system is still used on various small engines e.g. lawn mowers, outboards etc, but on the whole has been superseded by the pre-engaged starter.

4. 'Pre-engaged' starter motors

The pre-engaged starter does what it says. It engages the drive gear into the ring gear prior to the motor itself spinning up. On the whole this prevents wear to either of the gears.

Solenoids

To achieve this a solenoid is mounted above the starter. Some purists call this bit an actuator, but no one in the motor trade does!

When the starter is switched the solenoid pulls a plunger into itself. The plunger is connected to an engaging lever that in turn is connected to the starter gear (known in the trade as ‘the drive’). So switching the starter pulls a plunger in which pushes the drive out into the ring gear.

At the top of the solenoid are a set of 3 contacts. One comes from the battery, one goes to the starter motor itself and between them is a moving contact. The plunger (which I talked about in the previous paragraph) comes back and pushes the moving contact onto the others, thereby connecting the battery to the starter motor.

So the motor now turns, however as the drive is now engaged in the ring gear before the motor spins, it doesn’t wear out the gears.

On pre-engaged starters the drive has an internal one-way clutch. This slips in one direction and locks up in the other. This is again designed to prevent the starter running with the engine as it helps to stop over-speeding, because when the ring gear is going faster the drive it slips.

5. The motor and gearbox

Behind the starter motor is the motor itself - sometimes directly connected to the drive but more frequently these days through a gearbox.

The aim of the gearbox is to reduce the size of the starter motor required. A smaller, higher revving motor can now be used with the added benefits of lower weight and current requirements. The downside is that the armature is now spinning very fast indeed and the gearboxes have been known to fail. 

The gearboxes themselves are relatively simple to repair and on the whole can easily be tacked by any competent D.I.Y mechanic.

6. The armature

This sits in a field housing (or body – yoke). There are two types of field housing; one with permanent magnets, the other with field coils which are wound around ‘pole shoes’. These pole shoes only become magnetised when a current flows around them.

Most field coil windings can be replaced - although they are often hard to remove because the pole shoe screws are normally very tight indeed. The permanent magnet version suffers more from un-stuck magnets (which can be stuck back) - but don't turn them round. Broken magnets are normally caused by being tapped with a hammer.

7. The commutator

Usually at the back of the armature is the commutator. The brushes bear directly on this to transfer the electrical current to and from the armature.

A Note Of Caution

Some vehicles are now coming onto the market with combined starting and charging systems, and also quick start mechanisms. In  these vehicles the engine stops when the vehicle comes to rest and restarts instantly when the accelerator is pressed reducing fuel consumption and emissions. If you have a vehicle fitted with this type of system most of what has been spoken about in this section will not be relevant.

 

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