


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. 




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 



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.



Milwaukee Wisconsin, United States
Joined Feb 2001
4,564 Posts

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 overrev 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 


Miami, Fl
Joined Nov 2003
2,949 Posts

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:





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 400450 out of the longer 480 type... 



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 



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.




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 78 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 


Milwaukee Wisconsin, United States
Joined Feb 2001
4,564 Posts

"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 



Quote:
Groeten Ron 




Quote:
http://www.rcgroups.com/forums/showt...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 




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... 



Sal: As you probably have noted I've been trying the same tricks with can motors. Sadly I don't have mega 16/25/3 unless you care to donate one
However in the case of a brushless, it seems that iron losses are proportionately far higher than can motors...and in fact to e.g. operate this motor at 40K RPM means its likely to be unsafely hot on no load whatsoever... Now I can't measure all this stuff, but it is consistent with some of the results I have had on my MPjet brushless  the thing gets pretty hot on 3sLIPO almost irrespective of what load I put through it. So be careful. I reckon a motor this size can safely ditch about 45W only without getting too hot...so... It would seem the motor is probably capable of 200W input with 20v and 10A being a target figure, 40K shaft RPM. (DEEP gearing required) BUT its actually happier at 10v and nearer 40A  its actually cooler! If the idle current and resistance figures I have are correct (which is not neccessarily the case). In fact best power out for 45W loss occurs somewhere around 12v as far as I can tell  3s or 4s LIPO operation  and somewhere in the region of 35A. Motocalc predicts an efficiency of nearly 90% at 12.7v and 35A (12 cell operation) with 440W input and a shaft RPM of almost exactly 20,000. That's about as good as it gets for this motor  more volts and you are throwing away power in the idle current, more current and the copper loss wastes you. Output power is a shade under 400W. So a tad over half a BHP or something like a sport 2530 2 stroke. I stress these are predictions from the figures I have in Motocalc, which may not be accurate. and a guess at the allowable dissipation, at 45W. You will note that with a 1.9A idle, and 12v, the iron losses at 22.8W, and the copper losses at 35A and 17 milliohoms resistance, are similar. I.e. at that voltage, its not only operating at near maxiumum power as far as heat dissipation goes, but at maximum efficiency (for that voltage) as well. This may in fact take us to a simple rule of thumb for extracting the absolute maximum out of a motor. (i) determine its maximum heat dissipation  in this case we assume 45W. (ii) take half that  22.5w, and divide by idle current to get operating voltage  in this case about 12v. (iii) take the other half, and apply to the copper resistance, to get maximum current. In this case about 36A. If the indicated RPM is safe for the motor, that is the the BEST efficiency for the 45W heat loss you will ever do. Period. The only way forward is to get rid of the heat better by e.g. heatsinking, or running in short bursts. If for example, you can get rid of 100W of heat, then reapplying the formulae, 50W of iron loss gets us at 1.9A idle current 26volts input. and 50W of copper loss takes us to 54A operatng current. 1.4Kw input and 100W loss allegedly, and shaft RPM of 42,000 Assuming the resistance doesn't go up and so on when the motor gets hot. Frankly I don't believe it Conclusions? (i) with a high iron loss to copper loss motor, more volts WILL make it MUCH hotter  maybe unsafely hotter  even under no load. (ii) for a given heating effect, maximum efficiecny at that power loss  arrived at by tweaking the voltage and current to get max efficiency  will always get you best output power. (iii) Ability to dump the waste heat makes HUGE differences to the safe current and voltage of the motor. By increasing allowable dissipation from 45W to 100W, output power jumps from 400W to over 1.1KW. In fact we can maintain the efficiency at similar levels by adjusting current and voltage so what ultimately limits us is either RPM limits or the dissipation limit  we never should or need to operate the motor outside its highest efficiency point at all. Final point, this does not take controller efficeincy etc, or heating effects on the internal resistance into account. So in the end you have to use a thermometer and a whattmeter to get the best out, but it does serve to indicate roughly where to start. PS. I thought it interesting to apply this to a can 400 motor as a theoretical exercise, but, at sensible power levels, I needed to operate the motor at nearly 60K RPM to get to the best efficiency best power figure. I think that therefore, with cans, we run into RPM limitations before we can get to the absolute electrical best power levels. C'est la vie. 


Canada's East Coast, "An Ocean Playground"
Joined Apr 2002
4,239 Posts

wow, that was a tough "post" to follow .....
so no one did .. Bill 
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