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Old Sep 21, 2005, 08:42 PM
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high kv low kv high amp low amp

http://www.totalerc.com/rcstuff/highkvvlowkv.htm

im very interested in this so called phenom.

please look at the table at the link.

what i see that is interesting for me at least is that there is a tradeoff for performance v flight time at the expense of weight (more cells). The efficient current range of each respective motor is related to it's winds so for each respective efficiency i cant see the difference.

we all know that transformers step up and down voltage/current to reduce heat losses especially over large distances, but for me i cant see a great benefit in going higher voltage.

am i missing something here? it appears that the bang for buck is in the current carrying capacity of the cell, not the extra voltages..i.e longer flight times.

the heat losses are interesting and i wonder what the inertial losses- if significant are with extra winds, larger gauges etc etc and so on.

seems swings and roundabouts to me.

im sure this will open debate...any gurus care to explain concisely the benefits?

dont get me wrong, not a nay sayer, rather want to quantify the gain precisely.

cheers.
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Old Sep 21, 2005, 09:03 PM
EDF rules... :)
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Watts are watts, no matter how you do the math(low voltage, high amperage or high voltage, low amperage). If you are just taking one size/type fan and juggling motors in it there will be combinations where performance(rpm) is nearly the same, just a personal choice to me. I like lightweight setups and airplanes usualy fly better when lighter. One thing you might look at is the thrust to weight of the fan system, the total weight is more relavent to the aircraft flying performance than just the electrical system and its associated parts. Higher amps systems use larger wire diameters to buss the current without turning to liquid as compared to lower kv so a system where the fan and battery is seperated by some distance becomes pronounced as the wire weight becomes a problem. Just my 2 cents(US).

Eric B.
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Old Sep 21, 2005, 09:08 PM
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Quote:
Originally Posted by AirX
Watts are watts, ......... One thing you might look at is the thrust to weight of the fan system, the total weight is more relavent to the aircraft flying performance than just the electrical system and its associated parts.
Eric B.

Eric, your post has been my thoughts exactly, but i wanted to make sure- and esp the total weight not just the components... (my bad for the charts)

it was more a bee in the bonnet about some i think exaggerated claims that the setups are night and day.

personally, i like the ease and availability of the standard run of the mill 3s pack that i can move from plane to plane/heli to heli.

horses for courses. thanks so much for the reply.

tim.
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Old Sep 21, 2005, 09:09 PM
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Quote:
Originally Posted by AirX
....Just my 2 cents(US).

Eric B.

we call it 2 bob's worth here in Aust (equiv to 20c Au)..which is about the same as 2c US.
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Old Sep 21, 2005, 09:17 PM
EDF Jet Jam 2015 , May 28-31
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This is interesting. For me the advantage usually goes to the low cell count higher current setup. This is due mainly to the cost of high cell count controllers. If you are flying the smaller stuff it isn't as big of a problem. For the larger stuff it adds up quickly, especially on a twin

As far as higher cell count/lower current vs low cell count/higher current goes, it really depends on the motor and batteries selections. The higher current can be hard on batteries and motors if you make the wrong choice. Cooking a motor appeared to be the norm for awhile as one tends to try and fly longer with a high cell count. Everything else being equal.....watts is watts
I'd call it a draw.

What Eric said.....I simply type too slow
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Old Sep 21, 2005, 09:20 PM
EDF rules... :)
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Quote:
Originally Posted by what_the?!
we call it 2 bob's worth here in Aust (equiv to 20c Au)..which is about the same as 2c US.
Yup..

Eric B.
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Old Sep 21, 2005, 09:30 PM
EDF rules... :)
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I am in the process of gathering larger equiptment for a midifan sized jet, the lower kv higher voltage has taken me by surprise as to the level of pricing but overall I think that since I am getting new equiptment I will go that direction as I am not getting new eq to take the place of older eq. I think working on this project when it starts will be fun and enlightening. All my small fans are geared for 3s, seems to be a nice size for me too.

Eric B.
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Old Sep 21, 2005, 09:36 PM
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If you have the $$$ for a high volt set up, then that is the way to go, I'm planing on that for my midi-fan A-10 i can't beat the low weight of a 12s1p set up per side going with the HV as compaired to the high weight 6s3p lower voltage set up. then again it's up to your own preference and wallet size........

Gene
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Old Sep 21, 2005, 09:43 PM
EDF rules... :)
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Gene,

Your right, and that is the direction I am leaning.

Eric B.
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Old Sep 21, 2005, 10:03 PM
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is the high volt setup more expensive?

i think that its the cost of more cells + higher cell count ESC versus the larger , higher amp ESC and the higher capacity Cell pack.

thatd be a nice little equation to see the comparison...maybe when i have time ill do it for fun.

tim.
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Old Sep 21, 2005, 10:31 PM
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HIgh voltage set-ups weren't really practical until Lipos came along. With Nicads, and to a lesser extent, MiMH cells, you gained too much weight trying to build a high voltage system.


Rob
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Old Sep 21, 2005, 10:37 PM
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Quote:
Originally Posted by what_the?!
is the high volt setup more expensive?

tim.
Tim,

With Lipos, not necessarily. You may get away with building a high voltage system with the same total cell count as in a high current system. It all depends on the kv of the selected motor(s). We used to not have much choice in HV speed controls. There was Jeti and Hacker and both of their HV controllers were a bit on the expensive side. Now that Castle has HV versions of their Phoenix controllers, the price of a HV system has certainly gone down.


Rob
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Old Sep 21, 2005, 11:32 PM
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Quote:
Originally Posted by what_the?!
is the high volt setup more expensive?

i think that its the cost of more cells + higher cell count ESC versus the larger , higher amp ESC and the higher capacity Cell pack.

thatd be a nice little equation to see the comparison...maybe when i have time ill do it for fun.

tim.
I'm not sure about your assumption that the hi-volt system has more cells & is heavier. I flew my Midi f-18 with 6s4p (24-cells) before switching to 8s2p (16-cells). Because of the lower amps even at higher power, one ends up with less "P's" & also lower maH for same duration, therefore less weight. Now that CC is into even higher volts & with new 16C to 20C packs, may be time to bump up volts some more. Remember, one still fly at same output watts at cruising speeds, so 9s1p or 10s1p can be viable (if wide format packs becomes available)

Phil
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Old Sep 21, 2005, 11:52 PM
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Redmond, WA, USA
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My take on it is as follows:
* with pre-LiPo technologies, from both a weight and cost perspective, volts were expensive and amps were cheap
* with LiPos, amps are expensive (especially as you cross "Xp" barriers) and volts are cheap.
I fly all my jets on 1p packs. Looking forward to going over the 7p barrier (the practical limit for the non-HV CC ESCs) with one of their 45 HVs.

Steve G.
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Old Sep 22, 2005, 12:04 AM
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Higher Voltage Advantages:
1. Less weight due to thinner power cables and less copper mass in the windings. this is due to lower current needed to maintain the same power level.
2. Less IR (resistance) losses due to lower current needed to maintain same power level. Lower IR losses mean less waste heat, and heat is not good for anything in an rc airplane: ESC, motor, battery, connectors, the plane itself, etc...

These are the main reasons the cordless power tool industry have gone to higher voltages. They save money in copper, and make money by offering a more powerful tool.
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