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RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495
Posted: 08:44am 12 Dec 2006
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*ding*
I've just realized - you were not proposing a voltage doubler PER POLE - but PER MACHINE.
I take it back, that WOULD be worth trying - but the low voltage and diode drop (and therefore significant loss in efficiency) would still be a concern.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 08:53am 12 Dec 2006
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Alright Ross, no hard feelings. I'm glad your sharing your wisdom with an electrical imbecile like me. Perhaps the 21 may be a little ambitious, how about 14 for now and we'll try 21 later.
How many magnets?
I can do a template in illustrator and space them easy as, then I'll pester you for the wiring.Central Victorian highlands
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 09:05am 12 Dec 2006
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Ross regarding the rectifier wiring, if you look at john's pic, which is only 11 phase the waves are so close so the 21 would be even closer, perhaps to get an even flow in one rectifer have say phase 1 in one ac inlet and the corresponding phase which was at the other side of the wave connected to the other inlet, this would give an even enough flow of pos and minus to be able to connect them in series like a battery.
I don't think I explained this well did you get the drift of what I was saying?Central Victorian highlands
RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495
Posted: 10:06am 12 Dec 2006
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The problem is, if you try to stack diodes on low voltage coils, the forward drop on each is still a problem.
Even if you were to "stack" 10 sets, there will be 20 diode drops across the whole collection.
I'm sorry, but I just can't see any way of reducing the voltage drop losses without (substantially) increasing the number of coils in series - which means of course, fewer poles (for a given machine).
Two F&Ps on one shaft could have coils in series, but it'd be a right sod to wire, and I expect itd be difficult to get the stators and rotors matched perfectly.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 10:49am 12 Dec 2006
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Alright I give in Is 14 going to be feasable or should I just stick to 7?
if 14 how many magnets?Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:34pm 12 Dec 2006
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Regarding stacked outputs, the "voltage doubler" principle has the advantge of halving the number of diodes and the more phases the less demand for capitance.
21 phases has the greatest potential for reducing cogging, well except 42 phases if we could!
With 21 phases adjacent phases are so close they could be treated as one, maybe even three adjacent phases could be treated as one. There will be a drop in output but then I think every attempt to reduce cogging will bring a drop in output.
Highlander, I suggest making a 44 pole rotor, you did say that was easy? This will give an immediate indication of cogging. Then we can try a variety of stator configurations, fortunately you only have to wire a few coils to make useful tests which hopefully we can guide you through.
All hitherto not withstanding, I am not forgetting that it will be a real challenge to get enough volts to be of any use.
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:39pm 12 Dec 2006
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[quote]I take it back, that WOULD be worth trying - but the low voltage and diode drop (and therefore significant loss in efficiency) would still be a concern. [/quote] No problems Ross. Diode drop is a real challenge but we have interacting factors we can juggle somewhat.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 07:17pm 12 Dec 2006
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OK it appears we're getting somewhere. I'll try and do the rotor tonight if not, should be none by Thursday.
Re wiring I'm good at following drawings just do a hand sketch number the coils and scan it, no need for fancy graphics. Central Victorian highlands
Megawatt Man Senior Member Joined: 03/05/2006 Location: AustraliaPosts: 119
Posted: 01:06am 13 Dec 2006
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G'day again Highlander, I now understand where you are coming from with the idea of connecting two rectifiers in series. That would work, but as has been pointed out by others, it would involve the forward voltage drop of two rectifiers and that would be a bad thing. Electrically though, if the rectifiers were perfect and had no voltage drop, your proposal would be the same thing as connecting two coils in series and then passing their combined current through only one rectifier. Bearing in mind the realities of rectifiers, it is better to use only one, so there's only one recifier voltage drop. Am still thinking about magnets, how many, what spacing and 21 phases. Would love to see something from Gizmo on this. Sorry Gizmo!
About rectifier forward voltage drop, there are other postings that imply that this is linearly dependent on current through the rectifier. As you see by my nickname, I am a megawatt man, ie large electrical things like transformers, cables, three phase machinery and electrical safety is my field. But I would have thought that the rectifiers that are in use in windmills are the same as little silicon chip types, in that when they turn on, their forward voltage drop remains constant at about 0.7 volts, regardless of the current through them. Can a milliwatt man clear this point? It is very important for design considerations of course. Megawatt Man
RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495
Posted: 01:34am 13 Dec 2006
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You can "make" some near-perfect rectifiers using power FETs, but it takes 4 FETs and diodes to make a bridge, because each FET gets biased from its partners drive.
A lot of mucking about, esp if you were making 20 or more of them!
Its not linear. But it is real.
Go do a google on any reasonable bridge (lots of cheapies don't publish specs, so I suspect they're even worse). Most show 0.7V forward across each diode (1.4V for a bridge) at currents of under about 200mA, increasing to 1.4V (2.8V for a bridge) or so towards maximum rated current (at 25 deg C)
Some I've checked specs for over the years have gone to 1.8V per junction.
Bridges are not special - it should be easier to find detailed specs for power diodes.
Megawatt Man Senior Member Joined: 03/05/2006 Location: AustraliaPosts: 119
Posted: 03:38am 13 Dec 2006
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Thanks for that RossWMegawatt Man
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 05:59am 13 Dec 2006
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OK, OK, the aim of this exercise was originally to reduce cogging.
I think everyone agrees that cogging will likely be reduced by reducing the number of poles that are in line at one time, the ultimate being reducing this to just two poles by configuring for 21 phases.
Unfortunately the individual coils on the stators we have produce a fairly low voltage and when a phase has just two coils there is hardly enough volts to be of any use especially when rectification losses are taken into account.
Highlander I am eagerly waiting to hear when you have your 44 magnet rotor just how much cogging remains.
Now just suppose it is all good news I have drawn some more diagrams today:
This is not 21 phase! I have pretty much gone off that idea and been looking at alternatives.
The first diagram is what I think the voltage waveforms would be in the 14 coils of one phase of an unmodified stator with a 44 pole rotor. These voltages are not in phase with each other and the total is zero! As shown by the green line.
The second diagram, however, is the same 14 waveforms but in this case I have reversed the polarity of the second 7 coils. The green trace shows the total of these 14 waveforms (incidently, you only see 7 waveforms because each trace is actually two identical traces).
This looks much better dont you think? However the summed voltage only peaks at 9 volts whereas it would be 14 volts with an unmodified rotor. I guess that is the price of a low cog system.
But Wait! The wiring modifications must be the easiest ever: Connect all the 6 ends of the standard stator together, thats the three phase terminals connected to the star terminal. Now find the point between coils number 21 and 22, there are three wires there and all thats needed to be done is to scrape the insulation off these three and connect the new phase wires to them, do not cut the wires.
You can use these three phases in any of the usual ways, connect in star for about 9 volts** or delta for a bit over 12 volts**
**Note that these voltages are at the RPM that produces 1 volt on each coil and is pretty slow.
I think thats about my dash on this subject.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 07:59am 13 Dec 2006
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Ok template fits, should be a couple of hours.Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 08:38am 13 Dec 2006
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Great, bedtime for me, I will check in the morning for how the cogging feels!
Megawatt Man Senior Member Joined: 03/05/2006 Location: AustraliaPosts: 119
Posted: 08:49am 13 Dec 2006
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Lovely work, KIwiJohn.Megawatt Man
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 10:21am 13 Dec 2006
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F*** I did the template wrong it has 45 spaces so I'm left with a big hole and wrong spacing.
I'll try again tomorrow.
Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 04:34pm 13 Dec 2006
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Thats a booger Highlander, I sure hope my figuring is right after you go to all this trouble.
Thanks Megawatt Man but as you see it is Highlander putting in the hard yakker.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 08:39pm 13 Dec 2006
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Just character building It's not hard just a bit fiddly
RE wiring "Now find the point between coils number 21 and 22,"
I need a reference point to start counting from. If your looking at the stator from the back where the star and phase plugs are, which one are you calling number 1?Central Victorian highlands
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 05:52am 14 Dec 2006
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"Character building"? I must have the best character around!
Find the point between the three star lugs and the 3 phase lugs then count 21 coils from there (either direction) to find the directly opposite, which is the centre of the three phases.
Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266
Posted: 07:47am 14 Dec 2006
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My rotor got distorted when I threw it against a wall So you'll have to wait till the weekend for results.
I'm going to reinforce a new rotor with a metal band 3mm thick around the outside.This should make it stronger for when I use two or three lines of neo's.
I was going to use a locktite plastic metal glue. Eddy current shouldn't be a problem if I'm not linking the metal should it?
"You can use these three phases in any of the usual ways, connect in star for about 9 volts** or delta for a bit over 12 volts** "
Your explanation is for star but how do you wire for delta?Central Victorian highlands
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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The wiring modifications must be the easiest ever: Connect all the 6 ends of the standard stator together, thats the three phase terminals connected to the star terminal. Now find the point between coils number 21 and 22, there are three wires there and all thats needed to be done is to scrape the insulation off these three and connect the new phase wires to them, do not cut the wires.
RE wiring "Now find the point between coils number 21 and 22,"
