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Publication Title | Basic Electric- Alternators

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Basic Electric- Alternators

Electricity and Magnetism

To understand how an alternator works, let's review some electrical fundamentals. When you pass an electric current through a conductor, such as a copper wire, concentric circles of magnetism are created around the wire. As we increase the current in the wire, this "magnetic field" grows in strength or intensity. Unfortunately, no matter how much current we pass thru a straight conductor, the field around it is too weak to be of value for most applications. If we take this straight conductor, however, and wind it in a series of loops to form a coil, the magnetic field intensifies greatly and "poles" are produced at each end of the coil. These poles are called North and South. The magnetic lines of force leave the coil at the North pole and re-enter the coil at the South. If we take an iron core and place it inside this coil, the magnetic field produced by current passing thru our conductor is intensified further still, since iron offers a much easier path for magnetism to pass through than air, the magnetic lines squeeze down, become more concentrated, and stronger. Now we've got something to work with!

Yer Basic Alternator

An alternator consists primarily of a rotor, a stator assembly, and a couple of end frames to hold the stator and rotor bearings so everything is properly spaced yet doesn't crash into one another. The end frames are also a handy place to stick a few other necessary parts like brushes and diodes.

The Rotor

In our alternator, we take this coil and core electromagnet and mount it between two iron segments with many interlacing "fingers" which each become "poles". When current is passed thru our conductor, each of the fingers being on opposite sides of the wire, pick up the "Pole-arity" of that pole. Consequently, the fingers are polarized N-S-N-S-N-S etc. When we spin the rotor, the polarity of


the magnetic field passing a given point is alternating between N and S at any given time. This is known as an alternating magnetic field, get it? Add a set of smooth copper slip rings on one side of the core connected to either side of our coiled conductor so we can feed some "field current" into our "field winding", spin the whole shebang, and off we go!

The Stator

The stator is really nothing more than 3 wire conductors spaced evenly around a ring of iron. Which gives us 3 of the coil/core combos with the ring of iron acting as the common core for all the windings. Each of the wires is formed into a number of coils spaced so that a coil of wire made from conductor #1 is followed by a coil from #2, followed by #3, followed by a coil from #1, and so on. This is known as a 120° (apart) three-phase winding. On most automotive alternators, one end of a coil is tied together with an end of each of the other coils of wire and is grounded to the frame. The three remaining ends go to the diodes.

The Diodes

An alternator produces alternating current (ac). To use it to charge our batteries we need to "rectify" it to direct current (DC) The diodes, or rectifiers as they're sometimes called, are a series of electrical one-way valves. They allow current to pass one way and block it from coming back. When installed on a line carrying ac, they pass one half of the ac wave and block the other half, changing the ac to a "pulsating" DC. With the addition of a filtering capacitor to "smooth out" the pulse, we have DC clean enough to charge batteries, play rock 'n roll, or whatever.

The Brushes

The brushes sit on the slip rings of the rotor and maintain electrical contact with the field coil while the rotor is spinning. Wires connected to the brushes and to a battery provide the field current necessary to make the field magnetism of the rotor.

Yer Basic Alternator Bob-O Schultze - KG6MM

Kilowatt for kilowatt, using water to spin a generator or alternator has long been recognized as the most cost-effective way to make electricity. Given that fact, it comes as no surprise that most home power folks who have the potential to generate hydroelectricity do so. By far, the greatest number

of these DC generating hydrosystems use a common automotive-type alternator, just like the one under the hood of your favorite go-mobile. Let's take a look into an alternator and see what makes it work.

©1990 Bob-O Schultze

Home Power #20 • December 1990 / January 1991

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