I'll give it a try, Woof:

Unlike gravity, which is taught in high schools, electricity and magnetism are usually only subjects of university physics and engineering courses. So very few of us get introduced to this stuff. I count myself lucky to have had the class, and even so it took me a while to get from "theory" to "practice".

First, start with the "FLUX". I've already written about that:

http://fieldlines.com/board/index.php/topic,143565.0.html I hope it's a good start.

The flux is the amount of magnetic field that fits inside a closed loop. You don't need to know a number or measurement of flux directly, but I'll add that it is measured in "Webers" (named after Max Weber who studied it). All we need is the

**electro-motive force** that can be generated. We measure EMF with our multi-meters as a voltage, but ONLY when no current flows.

The formula you need:

EMF = N*F*f*2 Where:

N = Number of turns in the coil

F = the maximum Flux in the coil

f = frequency of change of polarity N-S

In the alternator, the poles pass N to S and N and so on. One cycle N-S-N will flip the field direction, and hence the flux from N to S to N, and the EMF makes a sine wave. Since the flux at the N pole was reduced to zero, then reversed to S, the total flux reversal is twice the amount Flux on one pole. Each magnet is a pole, and most of these alternators have a dozen or so magnet poles. Also, with increasing speed, the flux flips faster, and that makes much more EMF.

The other factor in the equation is N number of turns of wire. Adding turns of wire increases the EMF (and open-circuit voltage).

Try an example:

I have wound a coil of 10 turns of wire and stuck it in the gap of the axial-flux alternator (with the magnets on). Spinning at 60 RPM, I measure an AC voltage on the coil of about 1 volt.

Convert RPM to frequency: (60 rev / minute) * ( 1 minute / 60 seconds) * (12 poles) = 12 Hertz (cycle per second)

Convert measured RMS voltage on the meter to peak voltage (sine wave): 1.0 Volt * 1.41 = 1.41 volt peak

(Note, multi-meters are not usually very accurate at low voltage or at low frequency, so be careful)

EMF = N*F*f*2 > rearrange the equation...

F = EMF / ( N*f*2) = 1.41 Volt / { (10turns)*(12Hz)*2 } = 0.0059 Weber

If I want an open-circuit voltage of 12 Volts to cut-in at 180 RPM, then:

180 RPM => 36 Hz on a 12-pole alternator

N = EMF / (F*f*2) = 12V / {0.0059Weber)*(36Hz)*2} = 28 turns

Let's not forget that if the alternator is going to output 3 phase AC, then there must be 9 coils of wire, 3 in each phase. Each of these coils must work together, and each is collecting the same amount of flux in phase with each other. Divided among these 3 coils in the phase, we will distribute the needed 28 turns:

28 turns / 3 coil = 9.4 turns per coil ... well actually we can either go with 9 or 10 turns.

If we wanted 48V, an also to cut-in at 180 RPM, then we can just scale it:

9.4 turns * (48/12) = 38 turns per coil (if you calculate the whole thing again you get the same result)

If you used the same size of wire for the 48V coils as for the 12V coils, then obviously the coil would be much larger! But actually to get the same amount of V*A power, you can use wire with a much smaller cross-section. With 4x higher voltage, you need 4x less current, and the smaller wire can handle that.

There isn't really a simple (or cheap) way to measure flux directly, so I'd suggest the test-coil method, which is about as direct as you can get, and uses the alternator magnets exactly as you intend them to be used. Also good for diagnosing problems, like one reversed magnet.