12 magnets/12 coils

[ivandenisovich]

I am getting ready to wind my stator for a 48 volt machine and based on Hugh's design.  I was reading some posts awhile back about 6 phase alternator being a better match for the blades.  So I started fooling around with the math and sure enough, 6 phase does look more effiecent.  If you slightly reduce the number of turns in each coil, then use 12 coils instead of ten, in theory (or at least in math) the blades will be less likely to stall, you will have slightly less resistance per phase, so that is good.  The only draw back I can see it that it also will increase the cut in speed, according to how many turns you reduce the coil.  For my experiment I used 260 turns #20, 12 coils, 12 poles.  My question is this, is there any draw back to using the same number of coils/poles?  Phase cancellation, cogging etc.

[Flux]

Hold on, be careful, with 12 magnets and 12 coils you are going to have single phase.

I think you have missed something somewhere.

Let's consider a simpler case of 8 magnet 6 coil. Normally this would be connected with opposite coils in series so that you have 3 phase groups and these are star connected to make it 3 phase. The electrical displacement between the 3 sets of windings is 120 deg electrical, although to confuse things the mechanical displacement is 60deg but we needn't go into that.

Now we could split this winding into 2 sections each using 3 coils. If we make a star point of the starts of the first 3 coils and do likewise for the second 3 coils we now have two 3 phase windings which in the perfect world we could connect in parallel to get half the voltage of the original winding and 1/4 the resistance.

This is still 3 phase but it is parallel 3 phase.

Now leave the first 3 coils the same but for the second set we make a star point of the finishes. We now have two 3 phase windings, but at the instant when one phase of the first winding is maximum positive, the equivalent in the second is maximum negative because we have reversed the coils.

We can still rectify the two windings separately to dc and the effect is the same as for the earlier case, but we can no longer directly connect the two windings in parallel on the ac side as these windings are reversed in phase relative to one another.

If now in the final case we connect the star points we have 6 phase. The 3 leads of the first winding give us normal 3 phase with an angle of 120 deg. The second 3 likewise but these are 180 deg shifted from the first, resulting in a winding with 6 leads with a separation of 60deg electrical. This is 6 phase and at cut in the rectifier sees 2 coils with a displacement of 180 deg and the cut in is the same as phase volts x 2. In the 3 phase case the voltage is the vector of 2 coils at 120 deg or 1.73 x phase volts.

The 6 phase case therefore gives a lower cut in speed for the same turns. At high loads the rectifier conduction pattern changes and the thing behaves much as the 3 phase case. The overall efficiency is a bit lower as the peak currents per winding are higher but the effect is small. Overall the more sloping speed characteristic of the 6 phase may offset this loss in improved prop matching but the effect is not great. 5 phase falls between the two with a factor of 1.9 x phase volts.

Now with 12 magnets you have a messy situation for 6 phase as you have 9 coils in a single layer winding so each of the 6 phases would be 1.5 coils, messy and more trouble than it is worth.

16 magnet is ok with 2 coils per phase. Overlapped windings with 18 or 36 coils is ok for 12 magnet but a difficult winding with the space available.

I hope you have followed this. With the same number of magnets as coils the coils are all in phase (or 180 out) so it is effectively single phase with all the problems that go with it.

[Smithson]

     I did the same thing.  And I asked the same question.  On a 13 inch mag rotor I moved the mags out 1/4 inch.  The width of one of those spacers [1/8 inch?] that comes between the magnets when you order them.  At that time there was a question of problems with the propeller rotor and the answer was to increase the airgap or use fewer turns.  I only had 16awg wire so I concluded that 70 turns and 12 coils instead of 10 would work as well, and the wire would have less resistance and not overheat as someone said would be the case.

     The answer I got was pretty much the same as what you got.  But I can't see how it is a problem as the magnet sees each coil individually.  How can it be single phase when you are rectifying each coil individually?

     "Also this comment was made about five phase..."

5-phase is really just a gimmick, but it seems to me that it is often simpler to have a lot of coils in parallel in a low voltage stator and I prefer to use separate rectifiers to prevent parasytic currents so they might as well be a lot of different phases as only 3.  You could have 11 phases no problem.  But keep the rectifier near the stator".

Hugh Piggott http://www.scoraigwind.co.uk

     Anyway I never put the stator up but if it would work I might retry it.   Smithson

[ghurd]

Wow!

G-

[Ungrounded Lightning Rod]

How can it be single phase when you are rectifying each coil individually?

The number of phases has nothing to do with whether you rectify the coils individually (except that if they're out-of-phase with respect to each other you'll typically do better to rectify the phases separately).

The number of phases has to do with the number of distinct phase angles you have between a coil and the magnets.  (And that's counting angles between 180 and 360 degrees as if they were between zero and 180.)

If you have the same number of coils as you have magnets, both equally spaced, the magnets all come across the centers of the coils simultaneously.  They're all in phase, and you have a single-phase machine.

[Flux]

Smithson

Let's try to look at it another way. If you have one coil per magnet the voltages are all in phase, they all rise to the peak at the same time. The things can be connected in parallel or series that doesn't matter. You will only get current into your battery when the peak coil voltage is higher than the battery voltage. The charging current will be a series of pulsating lumps.

The whole concept of looking at coils is a strange one, from a casual glance of what people do you may form the opinion that any combination of magnets and coils will work well. Most of the windings here are strange by commercial standards and you are unlikely to see such things in normal motors and generators.

To overcome the pulsating current of single phase you go to polyphase machines.

These are effectively more than one alternator sharing a common magnet system but displaced with respect to each other so that when one single phase winding is not producing then another phase is. The normal is 3 phase where you have 3 identical windings spaced equally such that when one set of coils is directly over the magnets one of the others is approaching the magnets and the other is leaving them. The 3 phase bridge rectifier connects the highest positive voltage to battery pos and the lowest negative to battery neg. Every 60 electrical degrees a diode swaps over to another which has now reached a higher voltage so that current flows continuously with the pair of leads with the highest voltage being connected via the rectifier.

This is normally done in a star connection of the windings but the star is not connected to the rectifier.

For ease of manufacture with the axial designs we use it is common to leave parts of each of the single phase windings out to make room for the other phase. This results in many less coils than the conventional windings but at any instant you still have coils spaced so that one is under the magnet and one is approaching and one leaving.

Extending this to many phases is a bit confusing but essentially you have more single phase windings spaced to peak at smaller angles.If you have an odd number of coils there can never be the case where all coils are under a magnet at the same time and if you star connect and rectify you will end up with some sort of working polyphase winding. If you choose an even number of coils wisely it can still be polyphase but you have to watch it.

Rectifying coils individually with a bridge for each coil is beyond the level I want to go into here, in the 3 phase case it comes out the same as a delta connected winding and a 3 phase bridge. You remove circulating currents up to cut in but if you measure efficiency on load it is lower than the star case and is the same as delta.

It is a pity you didn't try that stator, the surest way to learn when in doubt is to try it. Otherwise why try your own ideas when there are tried and tested designs that have been shown to work. I assume people try the strange things they ask about to learn. If you just want to make a working machine it is safer to stick to a proven design, many of the things posted here even if they work at all will never be very good but the usual comment is that it is fun to learn.

I think we have flogged this to death and I hoped it has cleared the air a bit but don't be too worried if you haven't got to grips with it. I was in the electrical industry for many years and anything other than single or 3 phase was never used and never a 3 phase winding with half the coils missing.

Flux

[ivandenisovich]

Thanks Flux,  I knew there was something wrong, which is why I asked the question. I think I was looking at the 12 magnets as one giant magnet (in other words, I was adding up the surface areas of the magnets and calculating the flux at .6 tesla) and did not account for the changing polarity of the magnetic fields.

I understand why you do not want single phase generators.  It makes sense that you would want to "spread the blade power out" smoothly to reduce vibrations under high load conditions.  Thats why I was looking at 6 phases.

When I was working this out I noticed a few interesting things.  The 5 phase design by Hugh works better at matching the "Blade power" to the alternator than a 3 phase (assumes wye or star connection) design. That is, you can wind a stator that has a nice low cut in speed, but it seems less likely that the alternator will stall the blades in the 10-15 MPH range.  If you do the same calculations for a 3 phase, there is a tendancy to create an alternator that will overpower the blades in that velocity.

It seems the wise thing to do is stick with Hugh's plan except I am going use a larger blade.  I believe if I wind my coils wedge shaped, I should be able to use #20 instead of #21 wire, and get a few more turns in to accomidate the slower turning larger blade.

Thanks again for your input Flux.

[Smithson]

    Flux / Ungrounded Lightning Rod.   Thanks for that information.  I never looked at it that way before.  I'll have to print this and sort it all out.  It makes sense.  I guess you are never to old to learn.    Smithson

[kitno455]

flux to the rescue!

allan