Does a car alternator produce AC or DC current?

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The clue is in the word “alternator”. It produces alternating current. A rotating magnet induces a current in the windings of the stator on the outside. The output has to be rectified. Alternators only became useful for cars when semiconductor technology made small efficient rectifiers possible. (In a power station, a small DC generator supplies power to the rotor to create the rotating magnetic field. In a car the rotating field is also an electromagnet. I previously thought they used a permanent magnet. The advantage of an electromagnet is that the can control it to keep the output voltage constant.)

Before then cars had dynamos (DC generators) which were large and heavy. The armature generated the current by rotating in the magnetic field of an electromagnet that got its energy from the current generated by the rotor. Positive feedback—clever, eh! Initially there is a small amount of residual magnetism to start it off, but at low revs the generator was cut out to prevent it from acting as a motor. At higher revs the power to the field magnet was switched on and off to control the voltage.

The inside-out arrangement of an alternator made it possible to have larger windings than for a dynamo to create more power for less materials.

So the advantages of an alternator are that they are smaller and lighter and that they can charge your battery at reasonably low revs.

I read in other answers that car alternators are 3 phase (like in power stations). I didn’t know that.

2 thoughts on “Does a car alternator produce AC or DC current?”

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