Number of magnets Vs number of coils

[daleh007]

I was just reading how the leading edge of one magnet must be nearing a coil as the trailing edge of the proceeding magnet is exiting the same coil in order to achieve max. power output. The dia. I saw shows the center to center distance of both coils and mags. to be the same. I am wondering how this is achieved when some of the designs I have looked at have 10 coils and 12 magnets for instance. I understand that the difference in number helps to allieviate cogging but what does this do to generating efficiency?

Daleh

[DanB]

10 coils and 12 magnets makes for a 5 phase alternator.  It was chosen I believe... because the coil size is about ideal - and there is room there for 10 coils.  So at all times, 2 coils are exactly in phase with one another - and there are 5 phases.

 

[juiced]

Hi Dan.

  I was hoping you could explain: A> Definition of a 'phase', B> purpose of a phase and C> variances between phases...

     

[Flux]

This whole issue is rather confusing, you do get maximum voltage from a coil when its sides are spaced the same distance as the spacing of the magnets,but you can't put all the turns in one place.  Maximum power is another issue and efficiency is also a separate issue.

Having the maximum open circuit voltage will not guarantee you maximum power or efficiency if you optimise things wrongly.

You can not get maximum power or efficiency from a single phase winding.

For AC distribution 3 phase has so many advantages that it is always used.

If you rectify the output you can use any number of phases, 3 is still the common one but 5 has a slight advantage at cut in as the voltages add in the rectifier to give 1.9 x phase volts against 1.7 for 3 phase.

If you simplify life by using a single layer winding with coils touching, how near you get to the maximum volts per coil depends a lot on the spacing and width of magnets. As you depart from a single turn you reach the situation where all of the turns can not be in the position of maximum voltage. As you add more turns your voltage falls below the ideal, but you have more winding to reduce the resistance.

It is better for the turns to fall short of the ideal spacing as the short turns have the lower resistance. You reach a point where the gain in voltage is offset by the extra resistance and this means that you end up with a hole in each coil that is about the width of the magnet or a bit less.

It is all very much a compromise depending on the amount of magnet you want to use for a given power output.

There is probably little to choose between 9 or 10 coils with 12 magnets, perhaps 10 is marginally better and if you use thin wire you can parallel all the coils and have a very simple connection system where all starts are joined and all finishes

go to a rectifier.

Many of us like to keep the rectifier at ground level and in this case 3 phase is better.

Flux

[Ungrounded Lightning Rod]

I was hoping you could explain

A> Definition of a 'phase',

B> purpose of a phase and

C> variances between phases...

As your generator runs, each coil produces a voltage that varies cyclically, repeating the same pattern over and over.  One ideal case would be a coil spinning in a uniform field, in which case the voltage pattern would be a sine wave, and real generators tend to approximate that.  But even if the pattern is some odd shape it still repeats identically.

The "phase" is how far along this cycle (from some arbitrarily-chosen reference point) the waveform is at this moment.

If you have multiple coils in your genny, spaced such that they encounter the magnets at different times, they will produce identical waveforms, but will be at different phases along the waveforms at any given instant.  In that ideal generator case it would be like having several coils rotating, but at different angles to each other (like the arms of an asterisk).

In such a situation you pick one of the coils as a moving reference point and refer to the "phase" of each coil as the fraction of a cycle it is ahead of or behind the reference.  You express this fraction in degrees, as a fraction of a circle.  (Back in that imaginary perfect two-pole generator this would actually be the angle between the coils.)

The number of phases you have is the number of DISTINCT phase angles you get from your various coils (including the zero-degree phase angle of the one you picked for a reference).  Note that in a multi-magnet, multi-coil genny some of your coils may be producing identical waveforms, so the number of distinct phases may be much smaller than the number of coils.

In a polyphase alternator the phases are typically evenly distributed "around the circle".  So a three-phase alternator would have x=zero degrees, y=120 degrees, z=240 degrees.

Any single phase produces a varying output (and a varying drag on the shaft) during its cycle.  But with multiple phases these varying outputs and drags occur at different phases of the cycle, so they add up to something much more smooth, and when one phase is producing little or no power another will be near its peak.  (In fact, two or more phases with a resistive load can be made to add up to a continuous load on the shaft that doesn't vary with angle.)

Does that help?

[Flux]

As this is related to my post below I will have a go at it.

One phase is effectively one alternator winding. If you put a coil under each magnet you have one phase or single phase as it is normally called.

You can not put all the windings in the right place to make effective use of all the space and as you try to fill the centre of the coil the voltage gets so far out of the ideal position that it comes under the next magnet and you get cancellation.

With single phase you can only usefully have coil sides that are as wide as the gaps between the magnets.

You can wind another separate winding in the gaps of the first winding and increase the output. This would be a 2 phase alternator.

If you make the gaps half the magnet width, the ideal single phase winding will take up one third of the space so you have room to put 2 more completely separate windings in and this becomes three phase.

For standard AC distribution the 3 phase winding has a real advantage over all others in that it can be distributed with only 3 leads. This depends on the fact that the 3 windings are reaching their maximum voltage in sequence 120 degrees after each other and the vector sum of the voltages at any instant is zero so that three of the 6 leads can be joined to form a common neutral wire. With a balanced load this neutral can be left out as no current flows in it.

If you rectify the output to dc you can use any number of phases, but the different numbers of phases affect the rectifier in different ways.

Hope this gives you some idea of why we use different numbers of phases.

Flux

[juiced]

This thread is -GOLD-.

   Thanks alot for the technical definition, and then the practical def. I think the thread answers all questions a newbie could have, regarding phases and coils.

  I will litterally reread this thread, in all probability some 100x.

   (as we say on some other boards)

  K++

[Harry Luubovv]

I am trying to make the explanations here as plain and as simple as possible, and from a different angle.

"Phase" pertains to "A group of coils working in harmony". Different groups of coils can work at at different "Timing" to each other, in relation to the rotations of the rotor. But a phase itself, all the coils in it must work in harmonous timing or else the output will be reduced, or worst, the coils overeworks and be heat destroyed.

Effectively, a "Single phase" generator has no "Phase" to talk about as such, we worry about phases only when there are more groups [Of coils] forming together to work as one power producing union. The reason why we want more unions [Phases] to work is that, we can arrange the coils to effect a somewhat higher output as compared to single phase, if properly designed, given the same internal available space of the alternator. And then again, there will be less vibtrations as the rotor turns. Think of it this way, a one cylinder engine gives lots of vibrations. But as we increase the amount of cylinders, the more we increase, the smoother the engine will be running. This is so because all the power strokes are arranged at different timing from different pistons [Phase, or union] in relation to the rotation of the crankshaft which, in our case, the equivalent of the rotor.

Thanks for reading.

Harry.

[devoncloud]

Back to the original question at hand, I think that for the best efficiency AND least amount of cogging is a single rotor unit and 36 coils to twelve magnets, like Windsutuff now's design.  See his posts for the best descriptions on how to make your stator, but having three legs under each magnet at one time really increases the output of your machine, and each magnet is always under a load so cogging is not a problem.  I think this is a great way to do it.  You hook it up as three single -phase stators.  In other words, you are rectifying three sets of twelve coils.

Devon

[troy]

Yup,

Everything is a compromise.  Single phase is much easier to understand, 12 magnets, 12 coils.  All magnets pass all coils at the same time.  But it's somewhat less efficient.  At the other end of the spectrum are complex polyphase machines.  But if these aren't designed exactly right, you can get voltage cancelation between phases with terrible output and a big waste of time and effort.

If you use diodes (rectifiers) on each seperate phase, that prevents voltage cancelation between phases, but every rectifier costs you approximately 0.6 volts.  The worst case scenario is a polyphase machine where every coil needs a rectifier.  If each coil is making 2.0 volts and you lose 0.6 volts changing from AC to DC then you lose at least 0.6/2V = 30% of your output to losses in the silicone.  So there is no magic answer that's automatically the best.

Good luck and have fun!!

troy

[Cure]

Devon,

I'm interested in reading more about the 36 coil x 12 magnet design.  I've searched through the the board with no luck, can you provide me with the specific link to related discussion(s)?

Much appreciated,

Rob