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Old 09-02-2007, 10:55   #361
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Quote:
Originally Posted by Kapena
Lower voltage means higher current...power remaining the same. Means bigger wires. Means higher losses. Means lower efficiency. Means more heating.
I am no electrician but I know enough to realize that lower voltage doesn't mean all these things at once.

Don't you mean :

Lower voltage means higher current...power remaining the same. Means bigger wires OR (Higher losses AND lower efficiency AND more heating)

If we were all to take your statement literally, all electrical systems would be better off running at gigavolts and microamps which is clearly absurd.

I'll go for the bigger wires thanks.
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Old 09-02-2007, 16:26   #362
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You don't know enough... I posted an example. Let me clarify if I can.

Power is equal to Voltage X Current P= E*I If you decrease E (volts) then I (amps) goes up to get the same power.
20 KW = 240 volts * 83.33 amps
20 KW = 72 volts * 277.78 amps .....same power. 1/3 the voltage means 3 times the current.

Resistance remains the same. This is a function of wiring, connectors and losses in the controller. I used .1 ohms, a very small amount of loss. 1/10th ohm. Controller losses maybe be a lot higher depending upon configuration. The absolute number DOES NOT MATTER. What ever the loss is---it will ALWAYS result in higher power loss for higher current.

Voltage DROP (or loss) across this resistance E(volts)= I(amps) X R(resistance). This is also a function of the POWER wasted across the resistance. Power (loss) = I^2 (current squared) X R (resistance ohms).
So as current increases the power wasted goes up by the SQUARE of the Current. Double the current and the wasted power goes up 4 times. In the case of 240 volts and 72 volts the wasted power
is 11.1 times higher (3.33^2)

The power loss is HEAT. The energy is converted into heat...exactly how hot water heaters works...resitance element. Same reason your computer, stereo and the light bulb gets hot when operating. Wasted energy is converted to heat. Ever feel how hot jumper cables get when trying to start another car? Use the same jumper cables to run a light bulb and it won't get hot. Less current means less losses.
Efficiency is equal to Power in vs Power out. 20kw input power, 10 kw output power is 50% efficiency. The other 50% is heat. Everybody understands this.

So...Lower voltage results in BIGGER WIRES, higher power loss, lower efficiency and more heating....all at once. All you did was change to a lower voltage. It's EXACTLY how it works. I would tell you to talk to an engineer to help explain this...but you already are.
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Old 09-02-2007, 16:38   #363
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Quote:
Originally Posted by Kapena
You don't know enough... I posted an example. Let me clarify if I can.

Power is equal to Voltage X Current P= E*I If you decrease E (volts) then I (amps) goes up to get the same power.
20 KW = 240 volts * 83.33 amps
20 KW = 72 volts * 277.78 amps .....same power. 1/3 the voltage means 3 times the current.

Resistance remains the same. This is a function of wiring, connectors and losses in the controller. I used .1 ohms, a very small amount of loss. 1/10th ohm. Controller losses maybe be a lot higher depending upon configuration. The absolute number DOES NOT MATTER. What ever the loss is---it will ALWAYS result in higher power loss for higher current.

Voltage DROP (or loss) across this resistance E(volts)= I(amps) X R(resistance). This is also a function of the POWER wasted across the resistance. Power (loss) = I^2 (current squared) X R (resistance ohms).
So as current increases the power wasted goes up by the SQUARE of the Current. Double the current and the wasted power goes up 4 times. In the case of 240 volts and 72 volts the wasted power
is 11.1 times higher (3.33^2)

The power loss is HEAT. The energy is converted into heat...exactly how hot water heaters works...resitance element. Same reason your computer, stereo and the light bulb gets hot when operating. Wasted energy is converted to heat. Ever feel how hot jumper cables get when trying to start another car? Use the same jumper cables to run a light bulb and it won't get hot. Less current means less losses.
Efficiency is equal to Power in vs Power out. 20kw input power, 10 kw output power is 50% efficiency. The other 50% is heat. Everybody understands this.

So...Lower voltage results in BIGGER WIRES, higher power loss, lower efficiency and more heating....all at once. All you did was change to a lower voltage. It's EXACTLY how it works. I would tell you to talk to an engineer to help explain this...but you already are.
You're still confusing poor old me. In your example, you keep the resistance constant and the math works out. No different from your first example. By keeping the resistance constant, are you not implying the wires are NOT bigger. If I do make my wires bigger, the resistance drops and your examples have to be worked out with the NEW resistance. Hence my point - you end up with bigger wires OR power loss, lower efficiency .....

Where am I going wrong?
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Old 09-02-2007, 18:46   #364
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So thinking about this a little further (takes me a bit of time - sorry). In fact - let's turn the thinking around and compare the two configurations spacially.

On the Lagoon, the connections between the batteries and the motors is very short. Easy enough to run nice very low resistant wires (big ones). The run to the generator is not so bad as this is at a higher voltage.

Now compare this to the Leopard where you now have to run the cables from the motors all the way back to the genset. I'm gonna guess this about 10 times the distance - could be more. I can see why this can't be done at lower voltages - so you have to step the voltage up to avoid the losses that would come about from long cable runs.

So your efficiency is going to be far closer than your original math would suggest - even with it's assumptions of constant resistance and contradictory conclusions of bigger wires (heat expansion perhaps).

Now let's talk about something that really has fixed resistance - my body. The only two electrical equations I remember are V=IR and W=IV. Combining these I get W=V*V/R. If R is constant then going from 72V to 240V (a voltage ratio of 3.33333) gives me a power ratio (through my body) of 11.1111x.

So if you look at both boats, you'll probably see very short, thick cables on the Lagoon compared to slightly thinner and much longer cables on the Leopord with a 11x power difference should I accidentally become part of the circuit.

Thanks for considering me an engineer but that is a little generous.
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Old 09-02-2007, 22:10   #365
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ess,

I agree with you. If the cable size is constant, the lower voltage system will waste more energy as heat. All lagoon needs to do is have large gauge running from the batteries to the motors. Problem solved.

Now, there is another factor to consider in the Lagoon design. As you said, part of the system is AC. The Lagoon system is similar to the power distribution to our homes. There is a high voltage generating source in AC that can transmit power over distances efficiently. AC has the advantage of being easy and efficient to convert to a lower voltage. I’m too tired to look it up right now but I believe the controller is a direct AC to 72 volt DC system. This controller/charger AC/DC system looks like an excellent solution.

Initially I could not understand why Lagoon went with such complex system, but now it looks quite eloquent though I’m not an electrical engineer.

Any real electrical engineers care to comment on Lagoon’s design?
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Old 10-02-2007, 00:38   #366
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If you look at my post you will see "This is a function of wiring, connectors and losses in the controller." Also "Controller losses maybe be a lot higher depending upon configuration."

I just give the information. I am not discussing the Lagoon or Leopard implementation. How you misinterpret is up to you. Facts remain...240 volts will be more efficient, much lower weight and cost less...for the same implementation in 72 volts. Why do you suppose no one is trying to do this at 12 volts?
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Old 10-02-2007, 03:47   #367
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Quote:
Originally Posted by Kapena
Why do you suppose no one is trying to do this at 12 volts?
If higher voltages are universally better, why don't you ask yourself why aren't people building systems at 480V, 1,000V, 6,000,000V?

I'm sure you think the above question has the same obvious answer as the question you asked. There are upsides and downside at each level. Lagoon can run at a much safer 72V because the runs are shorter. The Leopard has much longer runs and has to go upwards to remain efficient.

Another question for you. If your purpose was not to draw a comparison between the Leopard and the Lagoon, why did you drop an example in the middle of this discussion quoting exactly the same voltages as these two boats?

If your intention was just to say that identical electrical configurations (identical means same guage, length of wires) at different voltages have different efficiencies then thanks for the lesson. Because it is just possible I am not the only one who maybe thinking you are trying to contribute to the discussion between the two boats, your conclusions and argument should be open to challenge - sorry if you find this difficult.
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Old 11-02-2007, 01:30   #368
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Simple...it's the limits of electronic devices. All have a finite breakdown voltage, especially semiconductors and capacitors. Then the insulation of components also becomes critical. Toyota uses 500 volts for it's hybrid, Ford 330volts.

The reason for my post was in reply to the remark that 72 volts was better then 240 volts. This is absolutely not true. If you read my first reply it says I don't understand any advantage of the Leopard. Has nothing to do with the CONFIGURATION....just the system advantage for higher voltage.
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Old 11-02-2007, 05:03   #369
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The problem is is that your original statement was not put into context and was erroneous on at least 2 points.

First of all, context makes all the difference. On a car, I'm not going to be messing with the electrics myself, on a boat, it is likely I'll have to. My regular A/C power I can turn off before I touch it, banks of batteries in series present a different problem. Managable I'm sure but the risks are different. When making statements like which is better, context MUST be considered. To simple choose one aspect (ie efficiency) and make an argument based on just that doesn't cut it.

Secondly, your two errors.

1) Lower voltage results in larger wires OR lower efficiency. If I halve my voltage and double the width of my wires I get no loss in efficiency (I've been reading)

2) In your second example you stated the absolute number (when refering to resistance) does not matter. Well, in the interest in putting this statement into context, I did some reading and came up with this. If your statement is true then putting different numbers into the equation shouldn't make a qualatative difference to the result.

Googling I found this site : American Wire Gauge table and AWG Electrical Current Load Limits

Based on my recollection, the wires I saw on the Lagoon were somewhere in the region of 6mm, maybe fatter And I'm going to guess they were no more than a meter long.

Using this table, I get a resistance of approx .5 milli ohm. No idea what the controller resistance is but let's double this to make this 1 milli ohm.

Putting this into your original equations (you may need to correct me as this is the first time I've done this), I get an efficiency of 99.97% for 240V and 99.77% for 72V. Now, if I use 10kW instead of your 20kW example (which is more realistic for the Lagoon), I now get an efficiency of 99.98% for 240V and 99.81% for 72V.

So let's at least get this straight. The resistance does not matter if all you want to do is compare efficiency only for two identical systems. 240V will always be better than 72V and 1000V will always be better than 240V.

But in context, using real numbers (subject to my doubling assumtion), the actual efficiency trade of is 99.98% -> 99.81% (not 96.53% -> 61.42% as your example impied) for a system where I'm less likely to harm myself during maintenance. There are no doubt other factors but when comparing systems, I prefer to deal with all the aspects (not just efficiency) and to use numbers which are closer to reality.

Seriously, I appreciate this discussion as it's making me research and learn things that I am benefitting from.

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Old 11-02-2007, 07:41   #370
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You are right ESS. With wire size a constant and the length of the run constant, higher voltage is better as resistance will decrease with an increase of voltage.


As voltage is decreased the gauge must be increased or the run shortened to prevent losses.

As long as you continue to increase the gauge of the wire with the lower voltage you aren't giving up a thing (except weight of the wire) to the higher voltage system.
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Old 11-02-2007, 09:46   #371
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Doesn't resistance stay constant until the wires start heating in which case the resistance increases.
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Old 11-02-2007, 10:22   #372
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From my research I come up with the following resistances/foot for AWG 0 guage wire. Copper= .000100 ohms/ft. and Alum.= .000164 ohms/ft.
Does anybody know if Lagoon used copper or alum cable?

Also. SailVI767, stated the the old 410 and the new 420 have similar sail plans, even though the 420 weighs more. Not true, the standard sail plan on the 420 has more than 100 sq. ft. more sail area, and its mast is 4 1/2' taller.

Fair Sailing _(\_
Quiet Riot 420
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Old 11-02-2007, 10:24   #373
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From my pics I would say that Lagoons wiring is a least AWG 0 guage
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Old 12-02-2007, 10:14   #374
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Very good ESS, except for YOUR errors...you only chose a single component of the system. A short run of wire. To get the same efficiency you did not double the wire...it's 10 times the size (back to the resitance thing again). Wire gauge for 83 amps is #7. Wire gauge for 277 amps is #0000. No switch, no connectors, no loss of the controller (the major loss of the system), battery terminal connectors. Kind of like making an analysis of the total current consumption of your boat by looking at a single light bulb. And you did not keep the resistance of this wire the same. You reduced it 10 times.



A good test is to measure the voltage directly on your battery terminals on your standard boat today. If you have a refrigerator or other high current item then measure the voltage directly at this device. Compare the two. The resistance will be (V1-V2)/I "I" being the current in amps of the unit. This will be the wiring and connection loss.
For the controller we have to look at designs from the same manufactor, possibly same model but lower or higher voltage and current rating... This would be a good comparison alone if data is available. Especially cost might hold some interest in this regard.

Mistake...72 volts is somehow "safer" then 240 volts. Both will kill you easily. So will your 12 volt battery if the conditions are right.
I will be gone 2 weeks...if you wish to continue this I will reply at that time.
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Old 14-02-2007, 08:20   #375
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[quote=Quiet Riot 420]From my research I come up with the following resistances/foot for AWG 0 guage wire. Copper= .000100 ohms/ft. and Alum.= .000164 ohms/ft.
Does anybody know if Lagoon used copper or alum cable?

Also. SailVI767, stated the the old 410 and the new 420 have similar sail plans, even though the 420 weighs more. Not true, the standard sail plan on the 420 has more than 100 sq. ft. more sail area, and its mast is 4 1/2' taller.

Fair Sailing _(\_
Quiet Riot 420

I took the sail area numbers from Lagoon. The 410s2 was listed at 1011 with the 420 listed at 1054. Light displacement on the 410 is almost 8000 lbs lighter then the 420. Sailing ability was important to me in this boat choice. I read all the reviews I could on the 420 and the few sailing reviews mentioned light air performance as an issue. Magazines rarely have negative comments on boat test since the don't want to upset the manufacturers. You have to read between the lines to try and get useful information. The combination of a heavy weight and small sailplan relative to that weight concerned me for what I was looking for. The real point however is that everyone has different things that want and seek in a boat. With the price many paid for the early 420 delivery positions they are getting an amazing bargin. Had I been looking then I might have made a different choice. The small sailplan can be offset with genniker/spinniker to some extent. The mast height mentioned was also an issue for me. I wanted a boat that would be intercoastal capable. That will not be an issue for most buyers. I am also big into fishing and diving. Both require good water access off the back of the boat. The 420 does not offer that. Again not an issue for many people but an issue for me. Cats that offer great water access often have low bridgedeck clearance. Someone else may feel bridgedeck clearance is far more important. Every boat has good and bad points. The boat It looks like I will purchase does not offer near the interior comfort of the 420. In fact I don't think there is another 42 foot cat built that can even compare with the 420 in that department. Again however for me there were a lot of things that drove my purchase choice. Every buyer will have different issues. I will watch the Lagoon 420 closely. It is the future. I want the boat to succeed and hope that it serves as the basis for my next boat in 5 years. It was just not the boat for me at this time. Any negative comments on the boat seem to be taken unusually hard for some reason. They should be taken for what they are, comments from one person's perspective. When I read about a boat I want to hear the good, the bad and the ugly. Once the 420's start hitting the oceans I will be following how they work out very closely. I am convinced it is the future and I want the 420 to work out great.
George
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