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).

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    \$\begingroup\$ These are two different question: 1. "What is the advantage of AC generators in cars", and 2. "Is there an efficiency advantage?" Since 1. is the more general question, I restructured your question. \$\endgroup\$ Commented Mar 3, 2017 at 10:52
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    \$\begingroup\$ Because if the brush commutator. The AC dynamo is brushless, while it still has the slip rings for supplying the rotor excitation current. The slip rings are uniform, they don't have segments. \$\endgroup\$ Commented Mar 3, 2017 at 11:09
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    \$\begingroup\$ The "diode" bridge in your car may not actually have any diodes at all. You can replace all of the schottky diodes with low loss fets and a low power controller \$\endgroup\$
    – Steve Cox
    Commented Mar 3, 2017 at 14:39

4 Answers 4


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.

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    \$\begingroup\$ Just to reinforce the efficiency point, alternators take something like 30-40W to generate the magnetic field ( according to Hugh Piggott's work on small wind turbines) so a permanent magnet AC generator (PMSM or BLDC motor) would be more efficient. But 30-40W loss is currently regarded as acceptable when driven from a 30-40kw (or more) engine. And permanent magnets cost money... \$\endgroup\$
    – user16324
    Commented Mar 3, 2017 at 11:09
  • \$\begingroup\$ Well, commutators with low contact resistance can be built - one is used in the starter motor - a machine which must handle very high rotor current in order to produce enough torque to turn over a cold internal combustion engine against the opposition of thick oil, cold-reduced clearances and, with diesels, high compression. High commutator losses would leave you stranded:( \$\endgroup\$ Commented Mar 3, 2017 at 12:59
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    \$\begingroup\$ That's true @MartinJames, but then most starter motors run for only seconds a day. Even then, they are replaced almost if not more often than alternators. \$\endgroup\$
    – Trevor_G
    Commented Mar 3, 2017 at 14:02
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    \$\begingroup\$ @Trevor yes, the low duty-cycle means even more economies in construction. Since most engines, (with their computerized injectors, ignition etc), start first time these days, I suspect that starters are now even more lossy and likely to fail than in previous decades:( \$\endgroup\$ Commented Mar 3, 2017 at 14:21
  • \$\begingroup\$ For that matter, permanent magnets can be used to activate an exciter winding which can then feed power into the generator field. \$\endgroup\$ Commented Mar 4, 2017 at 2:28

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. enter image description here

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.

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    \$\begingroup\$ You have my +1 for the inclusion of the field regulator, one of the best reasons not to use permanent magnets in a generator. \$\endgroup\$
    – Kaithar
    Commented Mar 3, 2017 at 18:06
  • \$\begingroup\$ just to be sure this is understood correctly: the sentence "... electro-magnetism generated by the stator coil is regulated..." should have been "rotor coil" - and the component labeled "voltage regulator" in the schematic is in fact the field regulator? \$\endgroup\$
    – dlatikay
    Commented Mar 3, 2017 at 19:33
  • \$\begingroup\$ @Kaithar well, if you're going to use permanent magnets, then you need to do something more sophisticated on output control, e.g. MPPT with dump would certainly suffice. \$\endgroup\$ Commented Mar 4, 2017 at 3:52

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.

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    \$\begingroup\$ Before the invention of silicon power diodes, car alternators indeed were DC machines, with the field controlled by the stator winding and a small mechanical regulator. +1 anyway. \$\endgroup\$
    – Janka
    Commented Mar 3, 2017 at 13:20
  • \$\begingroup\$ @Janka yes. A system guaranteed to evetually leave you stuck with either a flat or boiled-dry battery. My wife's Mk1 Escort had such a contraption - we replaced it with an alternator ASAP :) \$\endgroup\$ Commented Mar 3, 2017 at 13:26

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.


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