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Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 03:36am 10 Dec 2006
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G'day all, I can't find it but somewhere Kiwijohn mentioned 21 phase to reduce cogg.
I can work out the spacing but how many magnets would be req'd for this?
Is it a good idea?Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 08:37am 10 Dec 2006
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Lets see if I can figure this out. We have 42 poles on the stator.
Now just suppose we isolate each of those coils from each other then put a ring of 43 magnets we would have a 42 pole machine! Except that the magnets must be installed in alternating order of polarity so for a continuous series we need an even number of magnets.
44 magnets and our 42 pole stator will give a 21 phase machine, each phase will be two coils and the coils will be directly opposite on the stator. i.e. coil number 1 to be in series with coil number 22 and so on..
I am not sure that 44 magnets would be best, perhaps they would be too far apart. I think that any even number greater than 42 would work as I think the significant criteria is that the number of magnets must be divisable by the number of poles. But not 84 though as that would give tremendous cogging! Also if you have too many magnets the poles would to too close and the magnetic flux would tend to go directly rather than through the air gaps and the stator core, I think.
Now is it a good idea you ask? I dont really know but I do know the voltage would be low though the amps would be correspondingly high. Cogging would be very low I think. A lot of diodes would be needed for the rectifier, 44 I think.
I stand to be corrected on any of this! Edited by KiwiJohn 2006-12-11
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 09:23am 10 Dec 2006
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This is looking good I've got a good feeling about this one.
I just had a look at the 3 and 7 wiring and realised I have no idea how to do 21.
I'm thinking the left from 1 goes to the right of 22,
left from 2 goes to the right of 23 and so on.
The right of 1 through to 21 go to star.
And the left of 23 to 42 go to the phases.
Is that right? Also with the rectifiers, is ther any reason not to put two different phases into one rectifier?
They are basically one way diodes letting +&- out to two points so if the inputs come from two or more phases should that really matter?Edited by Highlander 2006-12-11Central Victorian highlands
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 10:06am 10 Dec 2006
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Hey Kiwijohn, about the voltage being low,just a rough guess, but how low are you talking about?
Perhaps 14 phase may be good, with higher v?Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:10pm 10 Dec 2006
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Highlander, the voltage per coil will be about the same for the same RPM. Highest voltage is standard 3 phase with 14 coils per phase. 14 phases with 7 coils per phase would be about half the voltage but twice the amps. 21 phases would produce voltage about 1/7th of the standard 3 phase with correspondly higher amps.
You can increase voltage by increasing magnetic density, increasing the number of turns on each coil, by increasing RPM.
This is all theory/guess work on my part you understand!
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:13pm 10 Dec 2006
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I think it would be difficult to mix phases in the one rectifier as the phases are not "in phase" so some of the output of one phase would be wasted by back flowing through the other phase. This would be a waste and make the genny harder to turn too.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 08:30pm 10 Dec 2006
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Hi again, the 21 phase looks good but the low voltage may be a problem but is it possible to connect the outputs of the bridge rectifiers in series to increase volts(but lower amps)?
Normally I'de test it and find out myself but everything is in pieces.
Does the wiring sound right?
I'm thinking the left from 1 goes to the right of 22,
left from 2 goes to the right of 23 and so on.
The right of 1 through to 21 go to star.
And the left of 23 to 42 go to the phase output. Central Victorian highlands
RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495
Posted: 05:11am 11 Dec 2006
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There is one more problem with MANY phases and the resulting low voltages.... and that is that the voltage drop across a diode becomes and increasingly high percentage of the generator output, and that this voltage drop will increase with higher current, making the problem even worse.
To put some numbers on it: if you have only (say) 6 volts from a single coil at a given RPM, by the time you put that through a bridge rectifier (2 diodes drop at any given time) at (say) 20 amps, you can expect to see 1.6 to 1.8 volts drop across EACH DIODE. That's 3.2 volts to 3.6 volts, representing MORE THAN HALF YOUR POWER LOST!
Compare that to the same speed, but with (say) 7 coils in series making 42 volts. Current will be more like 3 amps than 20, drop more like 2v, for a waste of about 5%
Of course, there are a bunch of other issues, but this is one I've rarely seen mentioned.
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 05:21am 11 Dec 2006
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Good point Ross.
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 05:45am 11 Dec 2006
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Highlander, this is a way of doubling the voltage (and incidently avoiding one of the diode drops, I think). Through one half cycle one capacitor is charged and the other charges on the other half of the cycle. The output voltage is the total of both capacitors.
The two capacitors could be two batteries of the right voltage.
RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495
Posted: 05:51am 11 Dec 2006
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Voltage doubler, yes... but crap performance at any non-trivial current, and you have to use very large capacitors - and caps of that size generally HATE the high ripple current they are required to pass in this mode.
Sorry KiwiJohn - but not practical in this instance.
Overall, be much better to go high voltage then use a switchmode downconverter. That also significantly reduces your copper losses on the run from the mill to the batteries.
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:11am 11 Dec 2006
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Well I will just go and sit quietely in the corner then.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 07:55am 11 Dec 2006
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Oh crap looks like I opened up another pandora's box. Cheer up KiwiJohn you can come out of the corner Thanks for the info guys but no one answered my questions.
is it possible to connect the outputs of the bridge rectifiers in series to increase volts(but lower amps)?
Does the wiring sound right?
I'm thinking the left from 1 goes to the right of 22,
left from 2 goes to the right of 23 and so on.
The right of 1 through to 21 go to star.
And the left of 23 to 42 go to the phase output.
I was planning on using caps at the star and I'll probably have to add wire.
If it does reduce cogg I could have upto three lines of neo's(I have a heap of them). 1 line on the 7 phase made 20 amps. So maybe 2 lines may be needed for 20 amps on a 21 phase but cogg may still be less.
Megawatt/anyone what do you think about this?Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 05:08pm 11 Dec 2006
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Hi Highlander,
[quote]is it possible to connect the outputs of the bridge rectifiers in series to increase volts(but lower amps)?
[/quote] Yes it is, but what you get out of the rectifiers is not exactly smooth voltage and if you are going to connect two in series for more volts you miss out on a bit if the peaks of once phase do not coincide with the other. In a 3 phase system this variation in phase (i.e. timing) causes the total to be only 1.414 or so volts higher than one phase alone, I think. As the number of phases is increased this becomes less significant but I really could not calculate what it would be for 21 phases. Capacitors smooth out the output from the rectifiers more or less and reduce the effects of phase.
Regarding the wiring, if you really are going to try for a 21 phase machine I think the coils need to be wired reversed to each other. One coil will have a positive magnetic pole passing while the other has a negative pole passing. I suggest making the rotor first then connecting up just one pair (i.e. one phase) and giving it a spin. This might save a lot of wasted effort if the whole idea turns to custard.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 07:45pm 11 Dec 2006
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Hi KiwiJohn, thanks mate.
I just did a template and if you alternate the magnets nsns etc the opposite poles will either have both north magnets or both south. But you could skip one pole and use 1 and 24 to make a n/s combination.
If I had one phase consisting of two poles in series and one had a north and one south wouldn't they cancell each other out?
Or did I misunderstand your instructions?
This is what I had in mind for the rectifiers.
PS I'll set up for 44 magnets unless someone has any bright ideasEdited by Highlander 2006-12-13Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:06am 12 Dec 2006
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Highlander, you are quite right about the polarity of the magnets being the same. So the coils should be in series, that is the wire on the outside of one coil connects to the wire going to the inside of the other.
Regarding rectifiers, if you are using bridges you dont need a bridge for every phase, two bridges will do three phases and every bridge after that adds another two phases.
Connect the '+' on every bridge together, connect '-' on every bridge together. These are your two outputs. Connect one wire of all your phases to one of the '~' terminals (this is the centre of your star) connect each of the phase wires to a '~' terminal each wire having its own '~' terminal. You might have a '~' left over but that is no problem.
Remember, that there is no doubt that you will get a low voltage out of this configuration, how low I dont know so I suggest you be prepared for this to not be a very good idea after all!
Megawatt Man Senior Member Joined: 03/05/2006 Location: AustraliaPosts: 119
Posted: 07:12am 12 Dec 2006
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Hello Gentlemen, I'm back on the air after being away. I think Highlander, that KiwiJohn has answered your question about wiring the coils in series. My initial response was to say connect them in exactly the same way as Gizmo's drawings and photos but hey I'm currently a bit mushed in the head and am not quite with just where the magnetic poles would all be.
About voltage output, for normal rectifier connections, with 3 phases it is, as KiwiJohn says, 1.414 times the voltage of one of the coils. With 7 phases, it's 1.9 times and with 21 phases it would be close enough to 2 times. I don't think that the current output though would be comparable, because the current flow would stop from one pole the moment the voltage on the adjacent pole became higher. This process is called commuting and would result in pulses of relatively high current for 1/21 of the time from each coil. I would have to sit down and calculate for some time.
Now nobody has responded either way to a statement I made some time ago on a couple of other threads and I'll now make it again. Come on, agree or disagree fellers, if you know differently please tell me so I can make better contributions.
Cogging doesn't have anything to do with the coils or their connections. It has to do with the number of magnets that coincide with a stator pole at any instant. The proof of this is that the mills won't start in low winds because of cogging - at that time there's no current in the coils and there's no little elf in the machine telling the coils how they are connected. There may as well be no wire on the coils.
The 56 magnet/42 coil arrangement gives 14 poles coinciding at any one time. The 48 magnet/42 coils arrangement gives you only 8 coincident poles at any one time so has less cogging. That's why Gizmo had success with his 7 phase arrangement, because it used the latter combination. In F&P's later machines they have 48 magnets and 36 poles and this gives 12 coincident poles, so cogging should be intermediate to the above two cases. But Mr F&P skewed his magnets and reduced cogging by that stategy. More than likely though, he did it to reduce noise in his washing machines. More later.Megawatt Man
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 07:50am 12 Dec 2006
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Hey Megawatt, from my lttle understanding your right about the wiring being trivial. It's all about the magnet numbers and spacing, but as a by-product of that you need to wire the stator differently to suit the number of magnets. The 7 phase rotor which lines up less magnets gives no output on a 3 phase stator.
It appears the more phases the less number of magnets are lined up that's why I thought the 21 phase was a good idea.
RE wiring the rectifiers I initially thought of doing it as per the 7 phase(just continue on till I reached 21) but I came up with the other idea to increase volts, but it appears that's not a good idea. That's how you wire up batteries to get higher volts so I figured the same for this, is my understanding wrong?
Mega how many magnets would you suggest?
I was going to do 44
What's your gut feeling on this? Perhaps 14 phase would be a better option.
making the rotor and spacing the magnets is a piece of cake, what confuses me is the wiring. Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 08:37am 12 Dec 2006
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Megawatt Man, Highlander I even sat down today to try and draw the combined waveform of a many poled machine and needless to say it is really quite smooth. Not only are the peaks close together but the peaks are actually quite flat. 21 poles was too hard to plot but here is 11.
Considering how flat this output looks I think it would be interesting to try variations of the voltage doubler circuit I drew earlier.
MM, regarding current flow, I think there would be a fairly smooth transition between ceasation of current in one phase and commencement of flow in the next. Current flow has a voltage drop associated with it and I think once current was being taken from the machine the waveforms I have drawn would flatten out and there would be more overlap.
Yes, I agree with you both, it is the number of aligned poles that determines cogging and thats why I thought 21 poles would be the ultimate for reduction of cogging although I do see serious considerations of low voltage that might make it useless.
About puting outputs in series to increase volts. Although it is best to add in-phase for maximum voltage I would not totally discard the notion of adding slightly out of phase and see what happens, we could even get a fair idea by calculation, but it is bed time here!
RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495
Posted: 08:40am 12 Dec 2006
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I didn't mean to knock the idea guys, really.... but I'm an old, grumpy bugger - and I hate to see others waste time on something that's already been done. There's a whole NEW bunch of stuff out there to waste time on :)
Yes, it works on batteries because they are a continuous source of supply
If you look at the circuit from an AC perspective, on one half of the cycle you get current flowing through one diode (and nothing through the other). Then, the ac cycle goes the other way, and the negative half cycle goes through the other diode, and nothing through the first diode. So without the capacitors, you would measure *NOTHING* from cathode of one diode to anode of the other because the + and - are never there at the same time.
This is where the trouble comes in - in order to "carry over" each half cycle, you need a capacitor to the mid point. But that capacitor then has to deliver the power while the opposite half cycle is charging its cap and delivering power. The lower the frequency, the longer these caps have to hold up for.
And as we all know, the energy in caps is limited - the more current you pull from them, the quicker they discharge. So at "reasonable" currents, these have to be VERY big caps.
But that also means there are lots of amps in the discharge half cycle, and lots of amps when they recharge again, so the "ripple current" is huge - which large caps generally don't like.
So please don't take my comments to heart, you - or kiwijohn - I wasn't trying to teach anyone to suck eggs, just pointing out that what "looks like it should work" in theory more often than not doesn't translate.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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is it possible to connect the outputs of the bridge rectifiers in series to increase volts(but lower amps)?
