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Old Jun 10, 2012, 12:09 PM
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United States, MN, Minneapolis
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High Kv and Low Voltage or Low Kv and High Voltage

Which one do you prefer???
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Old Jun 10, 2012, 12:32 PM
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United States, AK, Fairbanks
Joined Aug 2009
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Extremely vague question...

The Kv essentially equates to the current that'll be drawn in this case. Assuming the goal is the same overall power, are we talking 2V and 50A versus 25V and 4A, or 12V/8A versus 8V/12A?

In theory, high-current setups waste more energy as heat than low-current. This may not always be the case in practice, and there are additional hurdles to get over with high voltage setups.

Are you just asking for academic purposes, or do you have a specific application or setup in mind?
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Old Jun 10, 2012, 09:51 PM
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Australia, WA, Perth
Joined Feb 2003
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I usually pick a kV and prop combo to suit the packs I have to avoid having to buy packs which can only be used in particular aircraft. That way I get more flights with fewer packs. For example I run 6s 3600 in all my bigger planes and 3s 2200 in the smaller ones. I do series or parallel them sometimes.
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Old Jun 11, 2012, 10:39 AM
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Quote:
Originally Posted by C₄H₁₀ View Post
Extremely vague question...

The Kv essentially equates to the current that'll be drawn in this case. Assuming the goal is the same overall power, are we talking 2V and 50A versus 25V and 4A, or 12V/8A versus 8V/12A?

In theory, high-current setups waste more energy as heat than low-current. This may not always be the case in practice, and there are additional hurdles to get over with high voltage setups.

Are you just asking for academic purposes, or do you have a specific application or setup in mind?
I disagree with your second paragraph. If you size a HV and a LV system appropriately (by Kv, #cells per pack), there will be no difference in heating.

I agree with ParkJeff, sometimes there are good reasons to choose HV over LV, and his reason is the one I have used.
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Old Jun 11, 2012, 05:41 PM
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United States, MN, Minneapolis
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Thanks for the replies. I have just been wanting to make a high voltage (5-6S) low amps plane.
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Old Jun 12, 2012, 01:13 AM
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Georgia
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I generally fly models that are between 4.5 and 6 pounds in weight.
The low KV motors often pose a problem, due to the required prop length and ground clearance. There are 3 and 4 blade props out there, but they have limited size and pitch choices, as well as center holes that are usually not of the correct diameter, just complicating matters more.
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Old Jun 12, 2012, 05:00 AM
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Australia, ACT, Kambah
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Originally Posted by PatrickRC View Post
Thanks for the replies. I have just been wanting to make a high voltage (5-6S) low amps plane.
I sometimes fly a Sig LT-25 Kadet on 6s at about 20A peak - a lot less in the cruise. It's running a fair bit below the motor peak efficiency current, but doesn't seem to lose too much in actual efficiency - the efficiency curve is pretty flat a lot of amps either side of peak.

Gives the cheap low C rating batteries an easy time. The only downside IMO of higher voltage lower current setups are the need for an appropriate voltage ESC with switching BEC (much cheaper than used to be, and I use SBECs on all 4 servo models anyway), and the need for a 6S capable charger (also pretty cheap now).

I can't see myself using a low cell count high current setup unless space and weight are big considerations and duration isn't an issue, eg in a hotliner. If I can stay below 10C by using more voltage, that's my preference
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Old Jun 12, 2012, 05:49 AM
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Orleans, MA
Joined Feb 2007
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I prefer higher voltage in the power mix too. I try to set up for no more than about a 10C draw, if possible. This can be done without imposing a significant weight penalty on most models.
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Old Jun 12, 2012, 06:19 AM
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higher voltage is easy to attain using lipo cells in series
amp draw on lipos used to be a problem as getting 10C discharge was either unsustainable or required very expensive cells

thus going up in voltage was the obvious solution - as it results in less stress on the cells

nowadays lipo's are better capable of handling higher discharge rates ... and the technology is advancing all the time

so the trend of the last 5 years for sky-high voltages will probably become relativised ... and people will be able to tailor their voltage more around their ESC or motor choice rather than around battery constraints

that said lipo life will always be extended by sujecting them to lower rather than higher discharge rates
(i still think that 10-15 C discharge is about the practical limit for a reasonable service life - even on cells labelled otherwise)

most modern small esc's are 4s capable and the 60 watt variety are 6s capable so the capacity is there to use

othet constraints may be fuselage dimensions - some gliders won't accomodate a 4s pack
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Old Jun 12, 2012, 02:05 PM
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United States, OH, Cincinnati
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Higher voltage setups are more efficient.

I have an "8S plane" that I've been running on 10S. Most guys use 5000mah 8s packs. I use 4000mah 10s packs. The 10s pack is actually lighter than the 8s pack. I pull 2300-3000mah per flight and everything stays cool. I ran the 8S setup and was pulling 75+ amps and draining over 4000mah of my packs.
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Old Jun 13, 2012, 11:10 AM
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efficiency is relative to the motor Kv / prop choice / flying style

a motor with 1000Kv on 3s (say 10 volts for arguments sake... 3.3 volts per cell) will be just about identical in terms of efficiency and power as the same motor wound for a 740Kv on 4s (say 13.5 volts in this example) both will attempt to spin at 10,000 RPM so with the same prop (load) they will be the same

change any single parameter in all of this and you may get a more favourable result in one direction or the other

going from say 3s to 4s on the same motor will of course deliver a wildly different result, most likely requiring to prop down a size, but effectively you now have a completely different drive train
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Old Jun 13, 2012, 11:41 AM
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High voltage, low current. That is my pref.
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Old Jun 14, 2012, 05:05 PM
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Quote:
Originally Posted by big bird View Post
efficiency is relative to the motor Kv / prop choice / flying style

a motor with 1000Kv on 3s (say 10 volts for arguments sake... 3.3 volts per cell) will be just about identical in terms of efficiency and power as the same motor wound for a 740Kv on 4s (say 13.5 volts in this example) both will attempt to spin at 10,000 RPM so with the same prop (load) they will be the same
Incorrect.

For the two systems to put the same power to the same prop, the lower voltage system will have more resistance in the circuit, which is why things get hot when you run high voltage setups. Lower voltage is less efficient because more energy gets wasted to heat.

It also the same inside the battery - It is a lot easier to get 2000W out of a 40V (50A) battery than it is from a 20V battery (100A). Working a lower voltage battery at higher current puts a larger drain on the battery (you will see more voltage sag) and reduces it's useful life.
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Old Jun 14, 2012, 06:05 PM
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Moab, Utah, USA
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Originally Posted by gaRCfield View Post
Incorrect.

For the two systems to put the same power to the same prop, the lower voltage system will have more resistance in the circuit, which is why things get hot when you run high voltage setups. Lower voltage is less efficient because more energy gets wasted to heat.
Wrong. The lower voltage, i.e. the higher Kv, motor will have fewer turns with larger diameter wire. That will give it less resistance. To put the same power to the same prop, the high Kv, low voltage motor will pull more current than the low Kv, high voltage motor. This higher current draw will offset the low kv, high voltage motor's higher resistance so the overall efficiency will be about the same with both motors.

Larry
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Old Jun 15, 2012, 07:35 AM
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I'm not an e-engineer, maybe I got part of that wrong.

What I do know is that I have 2 setups for my contest plane, both spin about 7300rpm, the same 18x12 APCe prop. The 8S 5000mah pulls ~75A, the 10S 4000mah pulls ~50A. The 10S runs cooler, has more consistent power, less voltage drain, less heat. The 10S setup is lighter (10S 4000 packs are lighter than 8S 5000packs), gives longer flight times, and is clearly less draining on the system.

I figure it's probably somehow related to power transmission - the reason they pump 110v to our houses (or even 220 in Europe) - because there is less loss moving power at high voltage.

Compare the wires in your household (typically 14ga) that run 110v. No problem running several large appliances or a few thousand watts. Then think about a few thousand watt amplifier in a car, with ~2-4ga wire. The lower voltage, high current amplifier setup can create enough waste heat to burn right through the 14ga wire.

Heat is wasted energy, and adds thermal stress to components; running your plane to the point where the motor is warm, the esc is warm, and the batteries are warm is wasting energy all over the place. If you can get the same power from a higher voltage setup, you will use less energy and everything will last longer.
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