E&M Physics Question

[jparmer]

Hi,

I am trying to verifiy that the turbine in Piggot's Axial Flux Wind Turbine Plans will really produce in the ballpark of 100 W. I don't doubt that it will, but I enjoy physics and would like to work it out anyway. Here is my approach:

(1) The magnetic flux density of the magnets is known to be 12,000 gauss. With the magnetic flux density (B), I calculate the magnetic flux (P) by P = B*Area. I use the area of one coil.

(2) Knowing P, I can now calculate the flux linkages throughout the coil by multiplying P by the number of turns.

(3) The emf is equal to d( flux linkages )/dt.  I assume that the magnet passes the coil in ~1/10 of a second, so I set 1/10 to dt. I set d( flux linkages ) equal to the value calculated in (2). So now, we have the voltage induced in one coil.

(4) Using the resistance/foot of 18 gauge wire, I calulate the equivilent resistance in the coil, and then with Ohm's law, the current induced.

From here on, I'm much less confident.

(5) I calculate the rms V and I values and find the rms power. I use rms values because the voltage and current are rectified. Since on the opposite side of the stator, the other coil constituting the phase is being simultaneously induced, I figured I should multiply the calculated rms power by two. (Maybe this makes no sense).

(6)  Since Piggot's machine is "5 phase," this is the rms power of just one of the phases. So, to get the total rms output power at this rpm, I multiply by 5 (maybe this also makes no sense).

In summary, we went from the rms power in one coil, to the rms power of one phase (multiply by 2), to the total power output of the turbine. (multiply by 5)

If someone could affirm, set me straight, or at least point me to a helpful reference, I'd be greatly appreciative.

Sincerely,

Jack

An Aspiring Physics Student

[stephent]

Number 6. the 5 phase power "multiplier" is the square root of the number of phases.

i.e. 3 phase multiplier is sq root of 3 or close to 1.73.

sq root of 2 phase is 1.4 more/less.

sq root of 5 phases is...?

The phases are not all in equal phase "power level" at the same time nor is the power continously produced by any of the phases.

[tecker]

One of the finest things in building one of the air core machines with these monster magnets is the power transfer of the flux that you can feel and that could make a mess of a finger go to work on the coils you just wound Each one output is different minor as it is . What I'm saying is the forces and vectors  make for a rich mix you have to see to appreciate .

[Flux]

I am leaving for Christmas in a few minutes, but you have a lot of errors and wrong ideas in what you are doing.

If you look through past postings, there is a lot of information on how to predict the output of these alternators.

Your flux density figures are not correct for the magnet layout and you seem to take no account of the effect of a battery load. Also batteries work on average current not rms.

You are right in that it is parallel 5 phase and the opposite coils add current and double the output. You don't multiply by 5 for the 5 phase.

The little 3 phase alternator in the plans can do well over 100W. The 5 phase version can peak at about 1kW so you should get your 100W.

Flux

[tecker]

To continue a bit .A lot of the power factors are determined in design and building process . If Coil winding and placement , rotor build and air gap give are all done with basic criteria you get  an average for your rms .If you copy the Piggot say 2005 edition to the letter you'll get the averages you seek to make your calculations . It's all built to an average at it's best and if you miss the alignment you get no differentials to calculate .  

[jparmer]

Hi all,

thanks for your feedback. So far, it seems that I need to multiply by 5^1/2 (not 5) and I need to somehow account for the battery load. Flux, if you could elaborate on the latter I'd greatly appreciate it - I thought the induced I and V was independent of the load. Thanks again you all.