Back to basics with starters

Pre-engaged starters are fitted to every modern vehicle, and provide a positive engagement with the ring gear as full power is not applied until the pinion is fully in mesh. The field windings on most car starter motors have been replaced with a permanent magnet, but the basic principle of interacting magnetic fields causing the motor to rotate is exactly the same. A typical car starter motor will have a power rating of about 1kW (for example, 100A at 10V), while larger vehicles can be in the region of 6kW.

Solenoid When the key switch is operated, a supply is made to terminal 50 on the solenoid. This causes two windings to be energised, the hold-on winding and the pull-in winding. The pull-in winding is low resistance so a high current flows. This winding is connected in series with the motor circuit, and the current flow causes the motor to rotate slowly to facilitate engagement.

Brushes and commutator To pass the current to the rotating armature, copper/carbon brushes and a commutator are required. As the armature (and therefore the commutator) rotate, current is passed to the different armature windings.

Armature The rotating armature is the main shaft of the motor. It consists of thick copper wires equally spaced around its circumference. The current through these windings produces high magnetism that interacts with the field magnetism to produce rotation and torque.

Field windings Starters use heavy-duty series windings or permanent magnets. If windings are used then the high current flow causes very strong magnetism and high torque. The permanent magnets weigh less and are smaller. Almost all cars use permanent magnet motors.

Engagement mechanism The magnetism created in the solenoid attracts the plunger and, via an operating lever, pushes the pinion into mesh with the flywheel ring gear. When the pinion is fully in mesh, the plunger at the end of its travel causes a heavy-duty set of copper contacts to close and supply full battery power to the main circuit of the starter motor. When the main contacts are closed, the pull-in winding is effectively switched off.

Intermediate epicyclic transmission The sun gear is on the armature shaft and the planet carrier drives the one-way clutch. The ring gear or annulus remains stationary and acts as an intermediate bearing. This gear arrangement gives a reduction ratio of about 5:1.

One-way clutch The torque developed by the starter is passed through a one-way clutch to the pinion. The purpose of this free-wheeling device is to prevent the starter being driven at excessively high speed if the pinion is held in mesh after the engine has started.

Pinion With a speed reduction (but torque increase) of around 10:1, the small pinion drives a large ring gear, which is usually located on the engine flywheel.