Question about DC/AC generators

[TheEquineFencer]

I was sitting here wondering about the way a brush type PM generator works in relation to a "normal" AC generator. In the larger commercial gas/diesel powered units they put DC into an exciter stator coil, it's picked up by the exciter stator rotor as AC, then rectified to DC and sent to the main rotor that's turning on the same shaft, the AC stator mounted around the main rotor picks it up as AC and is sent out for power. The voltage is regulated by the amount of DC put into the exciter stator coil.

Now if we were to do the reverse, use a PM generator turning, take the DC from the rotor and put it into a coil at the end of the rotor with a stator mounted around the end, it would pick it up as AC. Keep in mind I'm just thinking out loud. This is just about like taking a car alternator and putting the PM on the rotor, just reversed right? This would give a single phase output. Would this be better than the car alternator way of doing it? Is this the way PM brushless servo motors work but turned around? It would not have brushes to wear out. Please feel free to enlighten me your thoughts.

[TomW]

Fencer;

First distinction should be:

If the device puts Direct Current [DC] out on its terminals, it is a generator.

If the device puts out Alternating Current [AC] then it is an alternator.

By extension a "brushless" motor like an "ECM" will put out AC.

A "Brushed" motor will put out DC.

Lots of misuse of these terms here and elsewhere. Many folks call the dual rotor alternators as seen built here "wind gennies" or "wind generators" But they are actually alternators

Just a couple points I hope help in clearing it up some.

Good luck figuring it out, it is not that hard but can be confusing due to the common misuse of terms.

Tom

[Flux]

You correctly describe a brushless alternator at the start except for getting exciter rotor and stator confused.

From then on I loose the plot completely.

By pm generator I assume you are referring to a machine with permanent magnets on the stator. There is no dc on the rotor. If it is a brush machine then the ac on the rotor is rectified to dc on the brushes by the commutator. The windings are ac just as in the exciter of the brushless machine that you describe at the start.

You can put a permanent magnet on the rotor of a car alternator in place of the wound rotor and it becomes brushless but you loose ability to regulate it.

A pm brushless servo motor is nothing less than a permanent magnet 3 phase alternator.

If you fit a 3 phase rectifier it becomes a dc generator. If you fit an active controlled rectifier that acts as the equivalent of the mechanical commutator then it will run as a motor.  Mosfets or similar commutate the current to the windings in a sequence dictated by hall sensors so the thing basically runs as a synchronous motor whose supply frequency and phase is dictated by the magnetic sensors.

There are several ways of making brushless alternators with controlled dc excitation. Any pm alternator is brushless anyway. The next generation of car alternator will probably be brushless but it comes at considerable increase in complexity. The scheme that you describe for power station alternators is too costly for small vehicle alternators.

Flux