Skip to content



Why You Should Understand How dynamo works for car battery charging system

The dynamo has its rotating armature mounted between two stationary coils of wire, known as the field windings, wound on soft iron shoes.

When current is passed through the field windings the shoes become the poles of a magnet, with its lines of force passing between them through the armature.

Each armature winding terminates at a copper strip which is part of the commutator. Each coil is joined to the next, producing, in effect, one very large coil which has a connection to a copper strip on the commutator every few turns.

As the armature rotates, electricity is generated in these windings, most of it in the windings directly adjacent to the two magnetic shoes.

Each winding becomes negative as it passes one shoe and then positive as it passes the other. By having a graphite brush rubbing against the commutator in line with each shoe, current can be taken from the armature.

In this way, one brush is always negative and the other positive, so the output is direct current (DC), suitable for charging the car battery and operating the car’s electrical equipment.

How dynamo works for car battery charging system
How dynamo works for car battery charging system

How its control box works

If left without regulation, a dynamo would produce too high an output voltage, resulting in damaged light bulbs and other components. To correct its tendency to overproduce, a control box is used.

The control box contains a regulator which limits the output voltage. It also has a ‘cut-out’ that connects the dynamo to the battery when its output is sufficiently high and disconnects it when the engine is stopped, so preventing the battery from discharging through the dynamo.

How the regulator works

Some of the dynamo output passes through a set of contacts in the regulator to the field windings, increasing the magnetic field and the dynamo output.

The output current also passes through the voltage regulator winding, and when this output reaches a pre-set value, the magnetic field it produces is sufficiently strong to pull open the regulator points against spring pressure.

This begins a chain reaction, and once the contacts separate, the magnetic field in the dynamo reduces and so its output falls.

This means that the magnetic field produced by the regulator winding is also reduced and when it drops to a certain value, the spring-loaded regulator points close again, increasing the output.

This cycle of operation occurs hundreds of times a minute and controls the output voltage of the dynamo irrespective of the speed of the engine.

How dynamo works for car battery charging system
How dynamo works for car battery charging system

How the ‘cut-out’ principle works

The output current is also passed through the cut-out winding which has a pair of contacts normally spring-loaded in the open position.

When the dynamo output exceeds that of the battery, the magnetic field produced by the winding closes the cut-out points and the dynamo starts to charge the battery and provide current for the electrical system.

When the engine is switched off and the dynamo stops generating, the magnetic field produced by the cut-out winding reduces, allowing the cut-out pointe to open, automatically disconnecting the battery.

Most control boxes also have a third relay that controls the output current. This relay is connected in series with the voltage regulator and operates excessive current from being produced, which would damage the generator.

Although control boxes have a provision for adjustment, they cannot be adjusted correctly without expensive instruments and detailed manufacturer’s data. Work of this kind should be entrusted to an auto-electrician.

How dynamo works for car battery charging system
How dynamo works for car battery charging system

To prevent the dynamo from overloading itself, its output is adjusted by a control box which limits the voltage output by controlling the strength of the magnetic field in the dynamo field windings.

Current from the output terminal is fed to the control box which contains two relays. When dynamo output exceeds battery voltage, a proportion of it flowing through the cut-out winding generates a magnetic field strong enough to close the cut-out points and feed the bulk of the output to the car’s electrical system.

Also, as the voltage increases, a proportion of it flowing through the regulator winding generates sufficient magnetism to open the regulator points.

This shuts off power to the field windings and the magnetic field within the dynamo diminishes, reducing the output.

The lower output reduces the magnetism of the regulator winding, allowing the points to close and the process to begin again. The regulator points open and closes several hundred times per minute.

Leave a Reply

Your email address will not be published. Required fields are marked *

error: Content is protected !!