# of Coils vs # of Magnets

[taylorp035]

After a lot of thinking and reading the FAQ section in the wind section, I only have one question about making an alternator.

In theory, you could make an alternator with the same number of magnets and coils.  By doing so, you could make it have only one phase and therefore wiring all the coils in series for the highest voltage output.  But doing this would be a bad move because it would cog very badly.

So, what "reasonable" coil vs mag numbers are out there and how many phases would I need for these?

So as an example, how many magnets could one use for this wiring set up?

[spinningmagnets]

The most popular set-up is 3-phase. You would use 4 magnets for every 3 coils. You have 12 coils so you'd use 16 magnets (if you only use one rotor on one side).

Adding a second rotor with magnets on the opposite side of the stator increases the output, and its only a small percentage increase in the total cost of the PMA, blades, tower, etc.

Each magnet should be smaller than the holes in the center of each coil. The coil sides can touch each other as long as theres adequate insulation to prevent side-by-side coils from shorting out.

Best of luck!

[nunyabeezwax]

I don't think you wired your coils correctly: S F+S F+S F+S F

NunYa

[electrondady1]

 " doing this would be a bad move because it would cog very badly."

cogging is a term used to describe the effects of the attraction of the magnets to an iron core .

there is no cogging in an air core alternator no matter how many phases are created.

in a single phase alternator there is a vibration inherent to the frequency .

[imsmooth]

Flux,

If there is no cogging why use three phase?  Is it that the vibration at high rotational rates will cause noise or destruction of the stator?

[ghurd]

There is a vibration anywhere above cut in.  3-ph vibrates less.  3-ph is a more efficient use of the magnets and copper, because there is no time when above cut in with 0 output.

G-

[TomW]

I think of that growling above cut in as the sound of Raw Power being extracted from the machine. It is there on all the turbines I ever flew. Air core as well as iron core conversions.

Funny, no mater how often cogging is defined people still don't understand what it really is.

Tom

[taylorp035]

Thank you everyone for the input.  Now all I need is some good coil winding techniques.  I will try and research this myself.  Things that I know not to do include 1) not crossing the wire and making a big ball  and 2) to make the coil the appropriate size for the magnet.

[ghurd]

Pay attention to methods of keeping the coils 'held together' when removed from the winding device.

My first 20 tries tended to explode into a rats nest.  

G-

[taylorp035]

Thanks for the heads up.  I just read this part of the web site, which is the only place where I could see exactly how the coils were wrapped (which direction).

http://www.otherpower.com/stator.shtml

My first coil that I made this morning did not work very well.  I think it was due to the wire overlapping each other (I used a dewalt drill) and the resulting coil looked pretty bad.

[Flux]

Don't confuse cogging with torque pulsation.

Cogging can only happen with iron cored machines.

All single phase machines have torque pulsation. With 3 phase machines loaded conventionally with normal loads the instantaneous sum of the 3 phases is zero so at any instant the torque of each phase is balanced and the thing should be perfectly smooth. If waveforms are not perfect then there may be a minor torque pulsation normally at 3 times frequency but it is small. When you use strange loads such as rectifiers then there may bit of extra pulsation but it will be tiny compared with single phase.

Single phase alternators all tend to vibrate but with conventional machines with large rotating mass and high speed small machines at least survive perfectly well.

With wind machines operating at low speed and over a wide frequency range the vibration from a single phase machine can be very destructive and if you hit mechanical resonance it may be bad indeed. You will have noise and vibration problems that you may find hard to live with and a common problem is shaking the tail to bits.

Single phase has no other advantage whatsoever to compensate for these bad characteristics. It works very badly with rectifiers and the efficiency is lower and the alternator needs to be bigger for the same output.

These axial machines are not wound as true 3 phase machines so there may be marginally less gain from 3 phase but typically a single phase machine needs to be 50% bigger for the same power output.

3 phase machines often growl or hum on load but this is caused by the tiny torque pulsation that will be only a few %. Some find this noise a problem under certain circumstances. Try single phase and you will find it hard to live with a real full torque to zero pulsation every cycle.

Flux