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Forum Index : Windmills : Running wires: 3 or 2?
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imsmooth Senior Member  Joined: 07/02/2008 Location: United StatesPosts: 214 |
If I use the same amount of copper wire, is the power loss the same if I run three phases down the tower and then rectify to DC compared to rectifying at the top of the tower and running two wires down the tower? |
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Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
I am surprised at this question as for most builders it is not the the power loss that dictates the 3 phase run to the base of the tower but a need to access the rectifier, either for metering purposes such as RPM, monitoring/maintenance such as heat dissipation & replacement, or switching purposes such as Star~Delta, or switching purposes again such as back EMF braking before rectifiers. But you mention none of these or state the reason so it's hard to give advice. Now to answer your question, the power loss is the same. So if you had 600 x 0.2mm strands made into a 2 x conductor DC wire cable, and the same 600 x 0.2mm strands made into a 3 x conductor AC wire cable, the losses would be the same as the current they carry is the same. Hope that's helped. Gill EDIT: Sorry that's not quite true as the rectifiers get hot so must be consuming some AC power not carried by the DC side. However in the grand scheme of things If I wanted to do one or several of the tasks mentioned above, I would not consider this very small difference a deterring factor in running AC to the tower base. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Gill, are you sure? I am thinking about the question of the 120 degree phase differences. But still thinking though as I dont know the answer either  |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
Harrrr. I'm not often right, but am I wrong this time? Line loss is a product of it's resistance. If the current passes through the same total sized line (be it divided in two for DC or 3 for 3phase AC, as allured to in the initial question), then the resistance is the same and the loss is the same. If the voltage is taken after the rectifier at the base of the tower the reading will be the same regardless of where the rectifier is placed along that line between the generator and the test point. Ohms Law? It is not the same if the same wire is used (as opposed to "the same amount of copper wire"). In this case the 3 phase AC run has less resistance due to 3 wires whereas if done as DC there is a lesser AMOUNT of wire in 2 for DC, hence the resistance is more and line loss is more. If it were to be single phase, it the voltage drop would be the same. There is a subtle difference initiated by the question. Maybe one of our electrical enthusiasts could say "close enough" or "way off" as the case may be?  was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Losses will be identical if using either three wires carrying three phases, or with two wires carrying rectified dc. This is assuming identical wire. Here are some figures. Second diagram on the right hand side for a three phase bridge rectifier. 
If the rectified dc load current is 1.00, the rms current in each phase feeding the rectifier will be 0.82 Losses in the wiring will be I squared R, or directly proportional to I squared, if identical wire is used. Each phase will have an rms current of 0.82 Power loss in the wire will be 0.82 squared or 0.6724 As there will be three phases, and three wires, the total power loss will be 3 x 0.6724 or 2.0172 If the rectifier is placed at the machine, we will have a rectified dc current of 1.00 Power loss in each wire will be 1.00 squared or 1.00 As we now require only two wires, total loss will be 2.00 So wiring losses will be very close to being identical in either case. Cheers, Tony. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Thanks Tony, I knew that "I squared R" would be significant but I was not smart enough to say how! |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi all, I will throw a spanner in the works. I am assuming the intended purpose of this excercise is to charge a battery and that in the DC case, it is not pulsed DC, ie rectifyer and filter caps are on the mill. We will compare based on the same average current to the battery. The current will only flow into the battery when the generated voltage is greater than the battery voltage + rectification voltage losses. The current that flows in the wires in the DC case will be the average current. The current that the wires see in the AC case will be peaky currents not an AC sine wave. The resistive loss with peak currents will always be greater than if only the average current flowed, due to the I^2 x R power relationship. The peak current loss and the average current loss being equal is only valid for 100% duty cycle. This is similar to the problems encountered with using pulsed currents with a mosfet device. cheers, Gordon. become more energy aware |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi Tony, the example from the text you provided was for resistive or inductive loads, as stated at the bottom of the qotes in notes 1., 2., & 3.. A battery presents a much different load. The DC will only exhibit lower transmission losses if the rectifyer and filter caps are at the mill head. The omission of filter caps will leave the losses in the wires equal for DC or AC. The key to being able to reduce the losses with DC is the ability to only transmit the average current. Hi imsmooth, I would err on thicker wire, and as Gill has stated, there are many other reasons you may want AC. cheers, Gordon. become more energy aware |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Sorry but you are quite wrong. The losses are the same. Think of it this way, current can only ever flow down two wires, one source and one return either going into, or coming out of the rectifier bridge. This is very obvious with a dc circuit, steady dc, or highly pulsing with a high form factor makes no difference. With three phases, and three wires, the current still only flows down two of the three wires, but it is "switched" or "modulated" by the rectifier, no matter which end the rectifier is located. The load current transfers from wire to wire as the rectifiers switch on and off, but only two wires are ever carrying current on the ac side of the rectifier. Replace the rectifiers with imaginary relay contacts and you would see that the rectifier just "commutates" the ac side onto the dc side. The CURRENT waveforms are the same, but just chopped up between two wires on the input, and the two wires on the dc output of the rectifier. I am just a poor humble retired electronics engineer with an honors degree in engineering, and forty years of electronic design experience under my belt. I do not claim to know everything, but I am tired of being attacked on this Forum. This is the end, I will never post here again. After the previous attack on the MPPT thread last time I posted here, I thought this thread was particularly interesting, and non controversial, and I may be able to contribute something. I was very wrong to attempt that. Goodbye. Cheers, Tony. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Just before you go, if you please Tony, perhaps you will show where my thinking is wrong? Suppose 1 Amp through 1 Ohm for 1 Hour, is that not 1 Watt/hour? Now if I double the current and half the time is it the same? (2 Amp through 1 Ohm for 30 minutes) I think the answer is 2 Watt hours. This is not my area of expertise. Thanks P.S. You were not wrong to post here as I thought your explanation was spot on. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5116 |
Wow, just read this tread for the first time, and I think I will need to read it a few more times before I get an understanding of the thinking going on here. I dont think anyone is intensionally attacking anyone. What appears to be a simple question by imsmooth is just proving to be more difficult to answer than first expected, and its easy for the thinking to go in different directions. Its one of those questions that is easy for some, but others need to work it through and "discuss" their thinking. Me, about now I would grab some wire, a F&P, rectifier and battery and actually test it. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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Trev Guru Joined: 15/07/2006 Location: AustraliaPosts: 676 |
Thanks everyone for your input, very interesting reading. It really is great to have so many very knowlegable members. Tony, take it as though it is only one grain on the sea shore. There are millions more watching and reading....silently. We appreciate your input, as much as we do everyone. I just wish I had more time to read. My ability is more practical than electrical. I know the question say's "using the same amount of copper", but I am wondering if the question should have read "using the same amount of copper in each wire". If this is the case then 3 wires are better than 2 wires. The resistance becomes less by the additional 3rd wire. Trev @ drivebynature.com |
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| brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548 |
Tony,, don't dispair, some of our discussions run parallel to others ,,some disagree.. I've read your posts and all the others with interest and feel enlightened with the responses. I feel there is probably not much difference in either side of the discussion ,when you take into cosideration our meager requirements of our small home made wind generators.--just run your cables as large as you can scrounge or afford in each case. Thats what I've done. So,,to all of you that have posted your thoughts,thank you The old crusty retired 63 year old telecoms engineer that still forgets to take his tablets .... Bruce Bushboy |
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| imsmooth Senior Member Joined: 07/02/2008 Location: United StatesPosts: 214 |
Wow. I thought I was only going to see one or two comments. My question was based on the wires having the same *total* amount of copper so the total resistance of the three wires equals the resistance of the two wires. THis way the question only focuses on the affect of phases on the current carrying capacity and not the intrinsic resistance of the wires for a fixed load. |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
To send power from say Brisbane to Cairns over the state power grid, it would be done at very high voltage. The higher the voltage the less the line loss over a given distance. After rectification the voltage is lower, so in THEORY for the long run down the tower the higher AC voltage would have less line loss. In reality, would I ever own a meter sensitive enough to to meter this effect over such a small distance? I don't think so. What of inductive coupling between the wires? I have seen this discussed on a forum somewhere before, but I tend to feel the theory has little if any practical consequence to our applications. I'll leave the finer theory debate to the academics. For me I see no discernible difference for my type of windmill uses. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| petanque don Senior Member Joined: 02/08/2006 Location: AustraliaPosts: 212 |
My thoughts are: Every thing eventually needs maintenance. Simple stuff generally needs less maintenance than complicated stuff. It is a lot easier to work on stuff that is on the ground rather than up a tower. So the less stuff that is up on a tower the better. Therefore the more stuff that is on the ground the better (assuming there aren’t thieves about). Also electronics seem to do a lot better when they are well protected from the weather how well you can do this up a tower would depend on many things. I will also suggest that resistance is a simple calculation however inductance and reactance are more complicated. |
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MyCattMaxx Newbie  Joined: 24/02/2008 Location: United StatesPosts: 8 |
There is a difference in the amount of power a wire can handle between ac and dc. The higher the frequency skin effect comes into play. But at what we do skin effect is not an issue. But multi conductors will handle peaks better than a solid. Using the same amount of copper in a 3 wire means the gauge is smaller, hence more line loss. |
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Mertz Newbie Joined: 03/04/2008 Location: United StatesPosts: 15 |
There is a lot of good electrical engineering here but isn't it a fact that we aren't using DC, which was Edison's proposal, and are using AC, Tesla I believe, because it will transmit over longer distances with smaller looses. BTW I had the same question. I used (2) 3 wire 12ga extension cords for my 2 wire DC generators to try to equal a number 6 to 8 wire. Mertz |
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| BjBlaster Regular Member  Joined: 04/04/2008 Location: AustraliaPosts: 55 |
If you put a transformer at the mill, and then step it up to 200 or 300V then run it over the smaller cable all the way to the battery end then step it down again to lower volts and then rectify it that should surely be more efficient than a DC run from the mill to the batt bank assuming the same size cable. It really depends on the size of the cable and the distance form the mill to the load/batts. Can we assume some distances and sizes of cable here? I still personally like the idea of AC as the "bearer" because I don't need any electronics at the mill. Cheers Bj Check out my projects here in: Bj's Shed |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Bj, yes I believe a transformer has the potential to gain efficiency but in practice it may not be easy to do. For the three phases either 3 phase transformer or multiple single phase transformers would be required. I am assuming that the F&P would be left in it's native form which provides a few hundred volts and transformers would be required at the low voltage end only. Transformers have a frequency band over which they are most efficient and I doubt common mains transformers that we might be able to acquire would be efficient at the frequency range encountered by our typical mill, maybe someone else can comment on this aspect? Mention of transformers on this site in the past have shown a strong culture against them which may be justified but I just dont really know. There is the safety issue too with a high voltage link and the mill to ground connection (slip rings or whatever) must be able to handle the volts safely. However, I have 'played around' a bit with transformers on an F&P and whatever the efficiency losses are they were not apparent on my simple test environment. Nonetheless, for an extreme situation, say you have a hydro or wind mill half a mile or so from the house a few cheap transformers might give you a system that actually did something useful whereas I expect a low voltage DC link not using hideously expensive cables would provide just a little less than bugger all. |
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