Most cars use an AC generator then convert its voltage to DC via a bridge diode rectifier to charge the 12v battery.
Why wouldn't a DC dynamo be used instead?
Is it because AC dynamo gives better efficiency?
(Even bike and wind energy use AC dynamo/turbine).
No, it's not for efficiency reasons.
DC generators typically have commutators, i.e. contacts with brushes that reverse the polarity of the voltage at the generator clamps every half rotation. In essence, DC generators are just AC generators that have a "mechanical" rectifier.
You can build generators without any electrical contacts between moving parts, but you cannot build commutators without those.
Since such contacts are very likely to fail under constant use, in dirty and vibrating environments, it's very desirable not to use them in cars. I'd also go as far as to say that unless you build a very expensive one, the contact resistance might be higher than what you lose over a bridge rectifier.
The alternator HAS to generate A LOT of amps at a very wide range of rotation speeds. Further the electrical load can and will change drastically from moment to moment.
A fixed magnet dynamo would require an extremely beefy voltage regulation system to generate the required voltages and demands.
A much simpler, and in my mind, more elegent solution is the humble alternator.
Despite what people think, cars DO NOT have a voltage regulator. They actually have a "Field Regulator".
The alternator has a rotating rotor coil which generates the MAGNET part. This coil is made up of many turns of relatively thin wire. Around that rotor is the stator which contains the generation coils which are made of MUCH heavier wires capable of carrying the AMPS you need to operate your car.
In order to maintain a steady maximum voltage at the output of the alternator, the amount of electro-magnetism generated by the rotor coil is regulated by the field regulator. Since that coil has a high resistance, it does not use much current to excite it. In fact, when the engine is not started, it is excited from the battery through the charge light.
The beauty of this is that if you are not using any power the alternator produces very little load on the engine. When you need power, it is available almost instantly.
The parts are simple, cheap, reliable, and efficient.
BTW: Alternator is a bit of a misnomer. Your car alternator produces DC not AC, all be it with a significant ripple. All the above is contained within most alternators. Generator would have been a better name.
So why can't you do something similar with a dynamo? That is replace the permanent magnets with electro-magnets. Well, the truth is you could. However, you have to realize that a dynamo is simply an alternator with a mechanical rectifier. A dynamo requires a commutation ring and brushes to switch in and out the coils at the appropriate time in the voltage cycle.
That adds way more expense than a few diodes, has issues with wear and reliability, and is VERY electrically noisy. Further while commutating there are losses involved when coils are temporarily shorted by the brush or are left open loop and not giving you any power.
So if you were designing such a beast, you would think, hmmm.. how do I get rid of this mechanical rectifier... OOO.. Lets use these new fangled things called diodes... before you know it.. you are back at what we now call the alternator.
That is probably how the thing was invented in the first place.
Another point is that alternators with a fixed armature and field on the rotor are more easily, and cheaply, regulated. Only DC field power needs to be fed to the rotor, so reducing the rating of the slip-rings compared to a DC machine commutator. Since the lead-acid batteries used in cars have a very low internal resistance and so narrow range of working PD, any charging system driven by a vehicle internal combustion engine, (with its wide range of rpm), requires controling/regulating. A DC 'dynamo' with a PM stator would require the full output of the machine to be controlled, An alternator can be regulated by controling the field current to the rotor with a relatively low-power electronic regulator, the stator armature output being connected directly to a rectifier block and battery with no moving contact.
Alternators have a rotor that can turn with much more speed without breaking apart due to centrifugal forces. So, it can generate enough voltage even at the lower iddle speeds, with the proper transmission ratio of the belt. This might be the main reason for using them in cars.
