I understand all that, but why do we call them "alternators", when they
produce direct current?
Jim Ruwaldt
'72 TR6 CC79338U
Bloomington, IN
On Thu, 30 Oct 1997 DANMAS@aol.com wrote:
> WARNING - The contents of this post should be considered adult in nature, and
> may not be suitable for reading by the electrically dis-interested. You may
> wish to delete this message now!
>
> If I may, I would like to offer a bit of clarification on the recent AC vs DC
> discussion as it pertains to ignition coils. An ignition coil is, very
> basically, a simple transformer. A transformer will, believe it or not, work
> on DC just as well as AC, PROVIDED.....
>
> The key to inducing a current into a coil of wire, whether it be the
> secondary of a transformer or the stator windings of an alternator, is the
> relative motion of the coil with respect to a magnetic field. The wire must
> "cut" the magnetic field. In an alternator, this motion is rotary in nature,
> and is produced by the rotation of the engine. In a transformer (coil), the
> relative motion is provided by the build up and decay of the magnetic field
> produced by the current in the secondary. As the magnetic field strengthens,
> the lines of magnetic force are expanding, thus producing relative motion
> with respect to the secondary windings. When the field decays, the lines of
> force contract, again producing relative motion with respect to the
> secondary.
>
> In an AC system, the current flows first in one direction, and then in
> another, producing an expanding and contracting magnetic field in the primary
> winding. If you rectify this AC current, you will produce a DC current, but
> it will not be a steady current, as in a car battery. What you will have is a
> cyclic DC current, going from zero to a maximum value, and back to zero
> again, repeatedly. The only difference between this cyclic DC and AC is that
> the current never changes direction. (Ken Streeter is posting an article on
> the VTR web site, which I wrote, titled "understanding alternators," which
> will have a picture of this.)
>
> To a transformer, though, it looks just like AC, as the magnetic field
> expands and contracts just the same as AC. You only need to go back and look
> at the vacuum tube radios used in automobiles prior to about the 60s for a
> perfect example of this. As the vacuum tubes required about 300 volts DC to
> operate, and the car only had 6 or 12 volts, a way had to be devised to
> produce 300 volts within the radio itself. This was accomplished by the use
> of a "chopper," a electro-mechanical device that turned the 12 volt supply to
> the radio off and on very rapidily, acting as a switch. This produced a
> series of 12 volt pulses, which were then fed to a transformer and stepped up
> to the required 300 volts. This 300 volt output was also pulsed, but a very
> large capacitor was used in a filter circuit to convert to a smooth DC, as we
> usually think of DC.
>
> This is exactly the way a coil works. The 12 volts DC is switched on and off
> by the points, producing the required expansion and contraction of the
> magnetic field around the primary, which is coupled to the secondary to
> produce the high voltage output.
>
> As you can see, it is not the reversal of current flow that is the factor
> with AC transformers, but it is the build up and decay of the magnetic filed
> that is important. If you can create this same cyclic action with DC, it will
> work just as well.
>
> I hope this has been of interest to some, and has not been an irritant to
> any!
>
> Dan Masters,
> Alcoa, TN
>
> '71 TR6---------3000mile/year driver, fully restored
> '71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
> http://www.sky.net/~boballen/mg/Masters/
> '74 MGBGT---3000mile/year driver, original condition
> '68 MGBGT---organ donor for the '74
>
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