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Posted by David Sherman on Tuesday, December 30, 2008 at 12:58:18 :
In Reply to: 3 phase? posted by David on Tuesday, December 30, 2008 at 04:04:13 :
We'll start with the familiar DC generator. The old-fashioned generator has a stationary field attached to the housing, and a rotating armature inside of it. The armature has a dozen or two separate little windings each connected to a pair of copper bars on opposite sides of the shaft. Brushes mounted on the housing make contact with one pair of bars, and hence one winding, at a time as the armature spins. The parts are mechanically arranged so that the voltage on any given winding is at its peak and pointing in the right direction when it comes around for its turn at the brushes. The brushes carry the full output current of the generator, they transfer current from the rotating part to the stationary part, and they "rectify" the current, that is, they make sure it's going in the right direction to keep the DC battery charged.
An alternator still has brushes, but it's quite a different arrangement. In an alternator the armature (output windings) are stationary and mounted on the frame, so the heavy output current doesn't have to go through any brushes. The field winding is on the rotor and spins. Brushes still get the current to the rotor, but they're only carrying field current, which is no more than 5 or 10% of the output current. Furthermore, because all they need to do is convey DC current to the field, rather than rectify AC coming out of the armature, they can be a pair of solid slip rings mounted side by side and connected to one simple winding rather than a large number of individual windings connected to a slotted commutator. This is simpler to make, can use smaller brushes, and doesn't wear them out as fast. So, you can see why alternators have replaced DC generators.
The catch is that the current coming out of that stationary armature is now alternating current. The rotating DC field makes the current in the stationary armature windings go past it first one way and then the other every time a pair of poles passes it. To turn the AC into DC, diodes are needed. A diode is the same as a rectifier. Conventionally, "rectifier" is used for higher-power units and "diode" for lower-power ones, but electrically, they both do the same thing -- let electricity go only one way. The reason the armature is wound as 3-phase rather than single-phase is a bit more complicated, but essentially it's the most economical way to get the most power in the smallest package. On bigger units, it's also important that a 3-phase unit evens out what would otherwise be torque pulsations in the drive shaft.
It takes 6 rectifiers (or diodes) to properly rectify a 3-phase output. One way that alternators gradually fail is for a diode to open up. When this happens, the other two windings still provide enough output to keep things going. Then if another diode fails, the remaining winding tries to carry the full load. If one diode fails short, it will probably cause another one to fail open. The lesson here is check all the diodes when rebuilding an alternator. The fact that it works (somewhat) and puts out current (somewhat) doesn't mean that it's working right.
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