How Do Car Generators Work?

by Paul Favors


A car generator is a dynamo. It is able to produce electricity through the rotation of tightly wound fine wires in a magnetic field. This magnetic field is maintained by a fixed arrangement of magnets or electromagnets surrounding the spinning windings of wire. The current and voltage produced depends on the speed at which the wires spin and the strength of the magnetic field.


The spinning and winding of wires in a car generator is called the armature. This component rotates through an arrangement of belts and pulleys that are connected to the car's engine. The speed of the rotation is thus controlled by the speed of the engine. A car generator can put out its maximum power only at a minimum engine speed or above it.


Attached to the armature is the commutator, and the main purpose of this device is to convert the alternating current (AC) generated by the spinning armature to direct current (DC). This step is necessary because the car's other electrical devices, such as the headlights and horn, operate only in direct current. Within the assembled structure of the commutator and armature are two spring-loaded brushes that slide along the surface of the commutator. As the assembly spins, these brushes touch various contact points on the commutator, and it is through this mechanical action that the current is converted.


A car generator has three connections, namely the field, the armature and the ground. Like all electrical systems, the ground connection is there to protect the whole system from excessive voltage. In a car's electrical system, and particularly in the car generator, this is a given, as most components are made of metal. The armature connection that is labeled "A" is the main link and carries the electrical output of the generator. The field connection that is labeled "F" is a smaller link, and this supplies power to the electromagnets surrounding the armature and controls the strength of the magnetic field. All three connections are joined to a car's regulator.


The regulator is commonly located in the car's inner fender, and its function is to regulate the electrical output of the generator so that a proper voltage is constantly maintained. It does this by controlling the strength of the magnetic field in the generator. Older units function mechanically by rapidly closing and opening a series of relays until the correct voltage is achieved.


The alternator is an improvement over the car generator. Modern automobile models no longer actually use generators. The structure is reversed in an alternator. Instead of spinning wound wires within a fixed magnetic field, the magnets are spun within the winding. This design no longer needs a commutator but rather uses diodes. These are solid-state devices that allow electricity to move in only one direction, successfully achieving the conversion of alternating current to direct current. Alternators are more efficient because they can output more power even in low engine speeds, which means that they can charge the car battery even when the car is idling, a function that a car generator is not capable of performing.

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