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Electricity is important in keeping a vehicle moving

By unknown | Apr 11, 2007 | COMMENTS [ 0 ]

It is easy to forget what an important role electricity plays in the workings of a motor vehicle.

It is easy to forget what an important role electricity plays in the workings of a motor vehicle.

Electricity is a quiet, unobtrusive power we take for granted - never paying a second thought to what happens behind the scenes ... until something goes wrong.

But that does not happen often on a modern car because it means an expensive visit to an auto-electrician. Owners of older cars experience this problem because their electrical systems are not reliable.

But the faults are usually cheaper to repair. The high demand for ignitions, fuel injections and electric fuel pumps prove the older the car the more demand for the parts.

Then there are the prolonged loads like headlamps, tail lamps, radio and heater fan. And the brief demands like the starter motor - a massive load, hopefully very brief - heated rear window, wipers, hooter, indicators, electric radiator fan, electric window motors, reversing lamp and diesel glow plugs.

The battery, the car's storage device for electrical energy, cannot cope with these demands for long, unless it is regularly recharged.

For this purpose, a generator is installed in the engine, driven by a belt from a pulley at the end of the crankshaft.

It is a small power station, operating on the same principles as Eskom's gigantic power plants, but getting its input energy from the fuel in the tank, not from coal or nuclear reactions.

The underlying principle is that whenever a conducting wire finds itself in a changing magnetic field, the electrons in the wire will feel an urge to flow along the wire, and if the wire is part of a closed circuit, an electric current will be induced in the circuit. We are back to Faraday's principle of electromagnetic induction, which must surely be one of the most far-reaching discoveries.

Two generator designs have been used on cars. The first was the direct-current (DC) generator, often still called "generator". It had limitations. It did not charge the battery.

As traffic congestion increased and vehicles were forced to spend more time idling, the stage was reached when it had to make way for a superior design - the alternating-current generator, aka alternator.

The alternator has a higher electromagnetic efficiency than the DC generator, which means it can maintain the battery at full charge even during prolonged winter stop-start driving with headlights on and heater fan working.

It is also lighter, more compact, quieter and more reliable than its predecessor. The three-phase alternating current produced by an alternator has to be changed to direct current before it can be supplied to the battery and the car's electrical accessories.

For that, a rectifier unit, consisting of robust diodes, is built into the alternator.

The output voltage, on both a generator and an alternator, rises with increasing engine speed.

That is why the voltage must be regulated to avoid the battery and other components being destroyed by overvoltage.

On both designs, this is achieved by controlling the current through the windings creating the magnetic field inside the unit, the so-called "field windings". You find these inside the rotating magnetic pole pieces in the alternator.

The electronic circuitry of the voltage regulator continuously modulates the field current to peg the output voltage at close to 14-volts on a 12-volt system.

One has to tip one's hat to a qualified auto-electrician. In the old days it was enough to figure out how volts, ohms and amps interact. Nowadays you still need to know all that, but you also need to understand a fair amount of semiconductor theory to fathom the mysteries of zener diodes and integrated circuits.

Next week we look at the preventive maintenance that will extend an alternator's life, the symptoms of a faulty alternator and the latest advances in alternator design.


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