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Alternators and Generators
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So what is this article doing here? Well, Ari Terblanche (sound engineer for Boulevard Blues) and I are involved in ongoing discussions about renewable energy, becoming independent of the National Power Grid, and various other technical matters. This discussion of Alternators vs Generators being used to recharge car batteries used as backup power resulted in this article.
Thanks to Albert Stemmett for his valuable input



Q: What is the difference between a generator and an alternator?

Here the discussion is related to the common Alternators and Generators used in motor vehicles.

Background basics: A voltage is generated when a fixed magnetic field is disturbed.
The Current available depends on the thickness of the copper coil "disturbing' that magnetic field.

Generators generate electricity by spinning an armature inside field coils that are energized by the battery it charges. (The magnetic field generated by the field coils are "disturbed")

Current and voltage output is directly proportional to the speed at which the Generator armature spins.
The field coils are fixed. They provide the Magnetic force inside which the armature spins.
A new generator armature or overhauled armature always needs to run as a "motor" for a few seconds to magnetize the field core a little to the correct polarity, to start the generation process. Output is then regulated by the voltage regulator.

Big drawbacks are of course that as the Armature carries the output current. The output is limited to the size of copper on the armature windings. Heavy currents also take their toll on carbon brush and commutator wear. The energy produced by the Armature output is DC (Direct Current)

The bigger the difference between the battery voltage and output voltage or the generator, the higher the current output (within limits)
With a motor vehicle typically running off 12 volts, the normal generated output is 14 volts.
Normal generator output is designed to be a few volts higher than the fully charged voltage of the battery.

The workings of an Alternator is a slightly different story...
Here the Stator, that supplies the output, is in a Fixed Position. (stator = stationary)
(With generators, the spinning armature supplies the output.)

Alternator ratings are therefore classified in current output.
A motor vehicle for example will have a 40 amp Alternator where a fishing boat or large truck might have a 500 watt alternator, depending on the devices it needs to supply.

Alternators usually have 3 stator coils. And the speed sets the Frequency of the supply, the current output.
For DC applications frequency of course is immaterial as the AC output is converted to DC with a bridge rectifier.

The advantages of alternators over generators are obvious. Much higher output currents are possible as the Stator (that supplies the output) is stationary, and can therefore take far heavier gauge copper wire for higher currents. The only real loss is the Copper wire resistance, rectifier loss, and bearing friction.

This is why you can have a constant voltage output irrespective of the speed. Higher rotational speed of the rotor only changes the AC frequency and not the output voltage. Another huge benefit is that as the alternator output is taken from the stationary field coils and not the Armature. Carbon brushes are no longer required meaning less wear and tear, friction and maintenance

Generator output is DC (Direct Current)
Alternator output is AC (Alternating Current)