Randall Jesup and I had a discussion at lunch today where we were trying
to figure out how alternators work. The alternator in my Spit isn't charging
the battery, and I took it apart last weekend to figure out what is wrong.
Here's what I found:
1. The diode stack has three nodes on it. Each node is hooked up to the
alternator coils through a three phase full wave rectifier circuit.
Two of the nodes are plugs on the outside of the case marked "+" and
"-". The third seems to go to the plug that ends up hooking to the
alternator light on the dash.
2. There are brushes in this thing. I knew this, but it didn't really occur
to me that an AC generator shouldn't need brushes until I took mine
apart. The brushes go to solid (not segmented) contacts on the
part of the alternator that spins (armature?).
3. The car has a wire only going to the "-" plug on the diode stack.
OK, enough for what we knew. Here's our theory of how it all works:
1. The "+" and "-" outputs on the diode stack are for positive and
negative ground applications.
2. The brushes are to get current to the wire on the rotating part of the
alternator (I'll call it the armature after this; apologies if that's
not the right term). This is necessary because you need a magnetic
field on the armature to generate current, and this current provides
the magnetic field.
3. The light on the dash of the car is hooked up to the car battery on one
end, and to one side of the armature on the other. The other side
of the armature is tied to ground. When the alternator isn't spinning,
the current through the light and armature provide the current needed
to have a magnetic field. Once the alternator starts spinning, the
mysterious 3rd node on the diode stack provides this current, and the
light bulb goes out because there is no longer any current going
through it.
4. (This one just occured to me): The regulator regulates the current through
the armature coil, which regulates the current at the output. Perhaps
the regulator doesn't even start working until the 3rd node of the
diode stack puts out current.
So, okay alternator gurus, how did we do? Please post to the list, as I'm
sure this is of general interest.
Joe
P.S.: I forgot to mention that my Spit's diode stack has one shorted diode,
so I know what's wrong with my alternator; I'm just asking because
I'm curious as to how these things work.
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