A question for those who use high cell counts as opposed to higher current - RC Groups
Apr 11, 2004, 10:13 AM
Giddy up!

# A question for those who use high cell counts as opposed to higher current

I've pretty much been the type of e flyer that tries to stick with power systems that use the same type of battery pack now that I'm pretty much all lipo. High cell count applications (higher than 3) pose a interesting option, running at lower currents and not beating up my cells. Since watts are what we are ultimately after to fly our planes substituting more cells in series help keep the watts up there. Packs can be added either in series for higher voltage or in parallel for higher current. But the end result is still having a motor that can handle the wattage.

My question for those of you that use the higher cell (series) is heat generated. Do you use throttle management to keep it in line? Is heat a major factor?

Thanks.
 Apr 11, 2004, 10:27 AM Registered User Watts generate heat. You can have lower current and more voltage, or more current and lower voltage, but you'd end up dissipating the same amount of heat.. (variations in motor efficiency at varying loads and voltages excluded) ..a
 Apr 11, 2004, 11:31 AM Giddy up! I guess what I'm getting at here is I'm wondering how many people are using a higher voltage rather than current and how they feel they can deal with the heat generated. The short story is start with a motor that can handle the wattage and mix your cells in series / parallel as you see fit. The power ultimately has to come from the battery so more cells (either series or parallel) will provide the wattage needed.
 Apr 11, 2004, 12:34 PM Motors beat engines! Motor heat does'nt come from total watts, it comes from waste watts, an important distinction. Heres an excerpt from a previous thread, and how to calculate this stuff, but the upshot is amps are far more important than volts when calculating waste watts. The only caveat on this is the extra volts cannot over-rev the motor. Heres the excerpt: "The problem is simply increasing volts also pushes amps right up with it. A 50% increase in volts from 2 cells to 3 also gives you 50% more amps. If you drop gearing 50% though, amps drop back to the original number, but you have 50% more power because you still have 50% more volts. On ips motors, your normally pretty close to the 2.5 amp limit on two cells as it is, so adding a cell without either regearing, reproping or both can push you WAY over your amp limit. Heres the calcs from another thread on ips: Motor eff. calcs. Power dissapated in watts = R( i ^ 2 x r) You need armature resistance and io, plus amps drawn. For 7.2 volt ips motor, io = 0.21 ohms, R = 1.8 ohms. Two parts to formula: Notice volts are a very small part of the heat load. 1) use total amps. With 1.9 amps and 1.8 ohms: 1.9 x 1.9 x 1.8 ohms = 6.5 watts wasted. 2) With 0.21 io: 0.21(Vinput - I x R) =1.59 watts lost. Add 6.5 watts to 1.59 watts = 8.09 watts total loss. With 11 volt input, you have 20.9 watts gross, and 8.09 watts lost = 39% or a 61% motor eff. With 7.4 volt input you have 14.1 watts gross, and 7.34 watts lost = 52% or a 48%motor eff. Notice that in this example, waste watts and therefore motor temp are all but identical. Also, this shows how just a little more amperage really heats up the motor: If you push the amps to 2.5 at 7.4 volts, waste watts jump to 13.05 or 44.5 btu or about 57% more heat from that small amp increase!" Dean in Milwaukee
Apr 11, 2004, 01:20 PM
know it all
I have always preferred high voltage over high current, I start getting unconfortable with anything over 30amps.. I do have some 45amp setups but those are used in gliders with motor runs of 10 secs or so. Low currents also means you dont need heavy guage wire either.

Higher voltage means more rpm, wich means a gearbox to keep the load (current) down.. but you are correct that waste watts is what kills motors.

Here is S. Florida I have killed many escs and motors due to heat.. this first started happening when new nicads batteries with higher than 1200mah capacity came out (this happened a long time ago).. suddenly the motors started to overheat just due to the extended flight times.

The final solution to overheated motors was to go to brushless, the winding in these motors are attached to the outside skin and they dissipate the heat quickly.. and usually (not all the time) they are more efficient thus generating less heat in the long run. Standard brush motors benefit greatly from heatsinks, keeping the magnets cool (important with cheap ferrites) but heasinks do not help the internal windings, and these will overheat too.

There are couple of ways of dealing with overheated escs, get an oversized esc for your application or keep the esc with a constant airflow over it. Having the esc sticking out of the airplane is very effective. The modern escs are very good and will protect themselves from damage.

Herm

Quote:
 Originally Posted by Sal C I guess what I'm getting at here is I'm wondering how many people are using a higher voltage rather than current and how they feel they can deal with the heat generated. .
 Apr 11, 2004, 01:21 PM Giddy up! Very good point Dean. The reason driving this discussion for me is trying to get maximum watts out of the new series Mega 16/25/x motors. Although they are excellent at spinning large props direct I'm contemplating gearing one & using more series cells (6 or maybe even 9). By gearing it I plan to keep the amps down yet wondering what kind of watts I can squeeze out of it. We were getting almost 300 watts out of the 16/15/x motors when we geared them so I'm hoping to get maybe 400-450 out of the longer 480 type...
 Apr 11, 2004, 03:13 PM Lifetime Beginner Sal, I'll say this even though I think you know it already. Take the Axi 4130/16 I just got. It will turn a 14X8 on 24 volts (anyway you want to get 'em) at just over 40 amps for about 900 watts. The 4130/20 on the other hand will turn the same 14X8 prop on 30 volts at just over 30 amps for 900 watts. Both motors are at about 8500 RPM. This gives the option of more smaller batteries for the 4130/20 or fewer larger for the 4130/16. Now you pays your money ... that being said higher voltage and lower current is always more efficient. The above examples (previous posts, not mine) given were assuming the same motor at different voltage, if that is what you have fine, but if you plan you can play with the numbers. But you know that. This is one of the reasons that electric drives hobby shops and suppliers nuts. You can make so many combinations of motors, gearbox ratios and props and all will work. I purchased the 4130/16 and not the 20. My rational was that I was already using 3s packs in my hotliner and Magic, the 4130/16 will run nicely on a 6s pack (two 3s inseries) so I can get the most use out of those wonderful but expensive big Lipolys. As far as gearing the Mega, go for it, the only down side is you will have and educated guess and may have to play with the gearing and prop sizes. (love when you dynamic types do the research for me, by the way did you fix the tail of the Blade yet?) Jordan
 Apr 11, 2004, 04:59 PM Renaud Ecalle Wannabe I must be missing something because at Mega's website it states that the mega 16/15/3, which I'm using for an example, is rated up to 16 cells with a max amperage of 30. With the proper gearing doesn't that equal out to over 500 watts? I've read on a number of occasions people rating these motors at only 180 watts, but when I do the numbers I just don't see it. It seems to me that most people are running the mega 16/15/xx motors at only 50% their potential with a 3s configuration unless of course you're running dd. If I'm wrong and often is the case, please give me some insight on the numbers given above.
 Apr 11, 2004, 05:52 PM Lifetime Beginner spit, You are in the thick of the idea of this thread. Yes, you could put 16 cells on some of the 16/15/x motors and yes you can pull 30 amps on a 16/15/x motor but you can't do both at the same time! I don't know what the maximum watts that that Mega can absorb but I would guess 200 or so is pushing the motor. So 30 amps at 7-8 cells or 15 amps on 16 cells. If you could get both together you would be putting out the power of my Hacker B40 8s on a little 2.5 ounce motor. Wish it were true. Jordan
 Apr 11, 2004, 06:51 PM Motors beat engines! "The reason driving this discussion for me is trying to get maximum watts out of the new series Mega 16/25/x motors. " The limitation on this concept is likely going to be max allowable rpm, ( lower kv motors will let you go higher on volts, = more watts), both for the motor and what the esc can handle. If you play with the formula above, you'll find it only takes a very modest amp reduction to make up for a major voltage infusion while keeping the motor temps the same or even reducing them. You are quite correct that this is the way to squeeze the most and the most efficient power from most any motor. If its the mega series motors you want to use, I would try motocalcing the lowest kv version to see how high you can go on volts without exceeding allowable rpms loaded to 75% of unloaded rpm, ie, if running a 1000 rpm volt motor on 10 volts for nominal 10000 rpm, you'de likely be loading it to about 7500 rpm. The loaded rpm is what you need to plan around, unloaded is irrelevant. Then its just a matter of jacking up voltage till loaded rpm either comes close to the max the motor can handle, or hits the limit of what the esc can support. Dean in Milwaukee
Apr 11, 2004, 07:34 PM
homo ludens modellisticus
Quote:
 Originally Posted by Andy W Watts generate heat ...
Current generates heat in the coils due to the resistance of the wire.

Groeten Ron
Apr 11, 2004, 11:41 PM
Permanently Banned
Quote:
 Originally Posted by Sal C I guess what I'm getting at here is I'm wondering how many people are using a higher voltage rather than current and how they feel they can deal with the heat generated. The short story is start with a motor that can handle the wattage and mix your cells in series / parallel as you see fit. The power ultimately has to come from the battery so more cells (either series or parallel) will provide the wattage needed.
Here is a thread discussing HV vrs HA setups. Lots of formulas and references.

https://www.rcgroups.com/forums/show...5&page=1&pp=15

In my experience, higher voltage is far better than hi amps. However, you do need to select a motor that can handle the rpms and you need the ability to select the correct gear ratio.

I have run a Mega 22/45/3 at voltages from 14 to 32 and amp ranges from 22 to 45. The hi voltage steups run far cooler. The esc and batteries also run cooler. I also have a Hacker B2036S that is much happier at 4S and 5S voltage levels and lower amps.

Good luck!

Larry
 Apr 12, 2004, 01:40 AM Does anyone hear a cat? Sal, you are onto a great idea with the 16/25 series of motors at high voltage. I am about a week away from buying one that i plan to test some excessive high voltage setups on. While the 16/25/3 for example can spin a large prop direct drive for excellent results, the numbers get excessively staggering when you gear the motor and really push some voltage to it. I called mega motors USA on this and asked about the voltage limitations and was told the limits are ~55,000RPMs. The 16/25/3 is only 1700kV so in theory we should be able to shove 9s lipo's at it and it should handle it. My first test setup is going to be pretty conservative (hehe), running 2 tanic 2200 packs in series at around 20amps but 20 volts. Playing with motocalc, it looks like geared 3.33:1 (promaxx planetary) it should be possible to spin a 10x7 at ~10,000RPMs for ~55oz of thrust and 70mph pitch speed, keeping current under 20amps. I am going to try a 10x5.5 which motocalc predicts 16amps for ~50oz thrust and 55mph. 320watts input, 260 output is predicted. Things get excessive when you look at a deep gearing spinning a large prop. 8s but geared 5.2:1 spinning a 12x6 prop shows over 70oz of thrust at 50mph for the same 16 amps...