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Brushless and Li-poly cells

Old 11-12-2003, 10:26 AM
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azfrench
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Default Brushless and Li-poly cells

Im just trying to clarify how the cell count of li-poly's and the Kv of a brushless are related. Will the heat build up of a motor increase or decrease with the cell count of a li-poly? And, Will a lower Kv motor be able to handle more cells? I want to go brushless but I don't want to burn the motor. I want a Himaxx 2025 -4200 and a 2015 - 4100 for a couple of models. How many li-poly cells would work with this? I guess I should add that prop size and gearing is a constant. Possibly using the GWS "C" gear box.
Old 11-12-2003, 04:04 PM
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Greg Covey
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Default RE: Brushless and Li-poly cells

Hi,

To help clarify things, the motor Kv (or gain) is simply the RPMs per volt. Typically, higher Kv motors are geared to convert the RPMs into thrust.

What kills a motors is excessive current draw and heat. Keeping a motor cool and within its rated current and voltage limits is your real goal. In general, brushless motors have a wide dynamic range of operation and like the higher voltage of a 3-cell pack.

The Himaxx 2025-4200 should work well with the GWS EPS-300C "CS" gearbox on a 10x6 or 11x4.7 SF prop using either 8-cells of NiCD or 2-3cells of LiPoly. Basically, you want to keep the current draw limited to 12-15amps.

Use the EPS "D" gearbox for larger props like the 11x6 or 12x6.

Finally, advanced pilots also add good throttle management into the mix for extreme short burst performance. Use a setup that is best suited to your flying skills and habits.

Good luck!
Old 11-12-2003, 08:04 PM
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azfrench
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Default RE: Brushless and Li-poly cells

OK, how do you know that the 2025 himax should only be run to 12-15 amps. I don't see this on the spec page. It does give the Kv and a max of 25 amp for this motor so are you just taking a precentage of the max amps?
Old 11-13-2003, 01:03 PM
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Greg Covey
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Default RE: Brushless and Li-poly cells

In this case, the current range came from a trusted vendor that sells the motors; Troy Goeff at [link=http://www.b-p-p.com/Himaxmotors.htm]Bishop Power Products[/link]

The current range is also in line with other motors of similar size. Basically, it is the gauge of the wire inside the motor that mostly determines the maximum current rating. Other factors are duration of maximum current used and temperature. These can be taken advantage of when using throttle management.
Old 11-14-2003, 10:31 AM
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Default RE: Brushless and Li-poly cells

Greg - the power setup on 2/3 cells is vastly different according to my dyno tests.
Both setups work - obviously - but to keep amp draw reasonable and hold watts in the under 100 catagory - I have to switch from a 9x4.7 to a 11x7 - on a 4100 on a C drive--as an example .
I run each combo -and watch the amps/watts at 12 ozs thrust (I call that "hover thrust) then floor the throttle -so to speak.
the big props will pull over 14 amps on 3 cells - but stay onder 9 on the small props
the total thrust difference is -from around 20 ozs to 24 ozs.
although these are MY figures - they are of course - comparisons .
I have yet to do a O Haus triple beam oz calabration thu the linkage - but if the gram scale is as good as it appears to be --and the Whatt meter is as good as it appears to be - then my results are pretty accurate.
Obviously - I have a preferrence to actual tests-vs- calculated results .
Force of habit
Old 11-14-2003, 04:50 PM
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Greg Covey
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Default RE: Brushless and Li-poly cells

Dick,

I think what you are describing is the thrust diffference between a 2s and a 3s setup. To compensate for the increased voltage, you changed the prop size to maintain a reasonable current level. The net result is that a 3s setup on a smaller prop produces a better result due to the increased power input level.

Add to this some throttle management and you can "reserve" the top 1/4 to 1/3 of your stick travel for when the 3s voltage drops during flight.
Old 11-14-2003, 05:34 PM
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rmh
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Default RE: Brushless and Li-poly cells

Yes -of course - the problem is -- unless you know about the problems of over watting your motor -as you do -of course - it is easy to ruin batts and motors.
I have been thru this on gas engines also - the novice believing a bigger prop equals mor thrust ---
Old 11-14-2003, 11:04 PM
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Default RE: Brushless and Li-poly cells

Higher and Lower voltages:

A wiser motor man than I suggests that running an electric motor at higher than rated voltage will increase RPM, Power and Efficiency. Running at lower voltage will decrease RPM, Power and Efficiency. Motor RATED Current and Motor RATED Torque will be unchanged and or constant.

Power input variables or changing the voltage, necessitates choosing a different propeller.

Voltage increases typically require the use of a smaller prop. Static thrust may increase providing the prop turns at a higher RPM.

Decreasing the voltage typically requires using a larger prop at a lower RPM...

Uh, Dick if the engine/motor turning the larger prop disc is spinning it at the same RPM as the previous prop example thrust increases are typical, RIGHT? Of course this not an atypical scenario...
Old 11-14-2003, 11:16 PM
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Default RE: Brushless and Li-poly cells

Michael, your punctuation made me re read a little bit ( who am I to say anything about punctuation!) but yeh --IF you mean that the larger disc represents a bigger load.
One crazy thing I have learned about props, is that what you see is NOT what you get, in some cases.
My 29 lb Extra flies best on a smaller prop- with more demonstrated "in flight" thrust.
Props can be very decieving.
Old 11-15-2003, 07:43 AM
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Greg Covey
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Default RE: Brushless and Li-poly cells

Well, I think that by now we have (unintentionally) confused many readers.

My assumption is that in Dick's last example, his 29lb Extra uses a glow engine (not an electric motor) and the reasons for using a smaller prop deal with loading.

Without seeing Dick's data in a table format, it is easy to misinterpret what he wants us to see.

Michael's statements look true with the exception of motor efficiency. It only changes significantly with current draw.

Ideally, the e-power setup allows you to change the gear ratio as well. In higher end systems like the MaxCim brushless motors, the MEC gearbox needs only a simple pinion gear change to compensate for increased cell count or a prop change. In lower end systems like the GWS IPS and EPS series, you can either buy a different gearbox ratio to suit your needs or settle on the improvement that is gained from a cell count or prop change.

Dick is right about beginners needing a power system design that will not be damaged by use of full throttle. This is why I stated to "Use a setup that is best suited to your flying skills and habits".

Overall, the electric power systems have many ways to compensate for power and performance changes. Some if us don't consider it much fun unless we are pushing what we have to its limits. The subject has enough interest that I have a new column in Quiet Flyer based on this quest called "Hop Ups". My column premiered in the December issue and has already flooded me with e-mails. Everyone loves to hop things up!
Old 11-15-2003, 10:14 AM
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rmh
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Default RE: Brushless and Li-poly cells

Oh oh I really did switch "horses"
the big Extra - had a 100 cc gas engine -
I could lay out the info I get from my "thrust stand " (How clumsy of me to call it a dyno )
It would read like the GWS charts-which I really like .
What I was trying to show was how easy it is to check out the loads on batteries /motors and controllers - with a simple setup involving a good combination test meter (The Astro Whatt meter) and a stand that shows the thrust produced
Although static thrust is NOT a gauge of prop performance at different flying speeds --it is a perfect instrument for doing comparative data.
In this case - the motor under full throttle load shows amps/watts and effective voltage under a known load. (not some theoretical load.)
For the "hover crazed " guys, good hover control needs to have thrust that is half again, to twice the full weight of their model.
The excessive thrust really simplifies things. The "dyno" does tell you that.
Once you find the safe combo which gives you a 2-1 thrust -static--you are set to go
Then -- try increasing pitch and reducing dia , still holding about the same watt load. Static thrust will drop some.
You will find that in actual flying - a higher pitch can give better overall performance.
Just watch that you don't exceed component limits by very much.
If you are after a flight performance which does not include a lot of start stop stuff- you can use MORE prop pitch than we suggested-simply because the prop unloads in the air and flight can be maintained on a much smaller amount of power.
The BUT here, is you can't use this setup a lot for hover as the load gos up and your batt/motor and controller may go with it.
Old 11-15-2003, 05:49 PM
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Default RE: Brushless and Li-poly cells

ORIGINAL: Greg Covey

Michael's statements look true with the exception of motor efficiency. It only changes significantly with current draw.
Greg

Take a look at my comments again, Give some additional thought to what I noted... Consider that an electric motor will only pull rated current, this is a given. Efficiency is enhanced by the application of higher voltage... Higher current loads will decrease efficiency... More power can be realized with higher loads but efficiency will suffer.

ORIGINAL: Michael Glavin

running an electric motor at higher than rated voltage will increase RPM, Power and Efficiency. Running at lower voltage will decrease RPM, Power and Efficiency. Motor RATED Current and Motor RATED Torque will be unchanged and or constant.
Old 11-15-2003, 09:56 PM
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Default RE: Brushless and Li-poly cells

This is kinda related--
way back before time began - I worked with large industrial equipment - 220 V 3 ph stuff
Some complete factories wer setup with all motors at 440 v- to reduce size /cost of conductors thu the plant -a sizeable saving as I recall.
Now if understand any of this -- can't we use smaller conductors with higher voltage? assuming we are after equal watts?
Said another way - if we setup for 11.1 -instead of 7.4--don't w reduce any possible heat losses where wires are on the small side?
further - My first cars were 6 v battery machines- equipped wit hfairly large wires toall instruments and accessories ( cigar lighter etc..)
My current pieces of crap cars, have very small conductors - well to be fair ,so do my good cars -- all 12 v machines.
So -are their LESS heat losses in these little motors when using higher voltage (again assuming same total watts ?)
Old 11-15-2003, 10:59 PM
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azfrench
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Default RE: Brushless and Li-poly cells

Wow, I did not mean to start an all out technical war here gentlemen. I thought it was an easy question with a simple answer. But, still I find myself a little confused. I do understand what you all are saying but, how do you know the rated amps of a particular motor based on the numbers given by the manufactuer? For example: the max amps of the motor. How should a newbee like me know how far to push it or not push it? It seems I may have to start studying electrical theory to get a clue. As a new flyer of electric, I just don't want to burn my equipment before its time.
Old 11-16-2003, 06:45 AM
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Default RE: Brushless and Li-poly cells

azfrench,
This is not gonna sound good, but I've been studying this stuff for three weeks now almost every waking minute of free time I have, and the more I study it, the more I realize how much I have to learn. I'm in the same shoes you are right now. The best way to go about what you are trying to do is find a setup that somebody else has tried on your application and go with that. I am studying alot of this technical stuff, so that later, I can do some modifying to yelid some serious performance. I'm getting into the little 3d Foamies and am probably gonna go with a Himax 2015-4100 (maybe a -5600) on 3s1p Li-Po's and GWS 'D' gearing on a 12x6 prop. That is kinda riding the thin line on amps, but max power would only be used for short bursts, and I am used to using the throttle alot because I fly IMAC birds. Right now, I'm just making sure that is what I want, and reading up so that I make an informed buy. Another thing to note is the batteries, some will handle current draw better than others, and some have a max C rate that is very high and can actually handle 20 amps in very short bursts. I'm also waiting for some new batteries to come out to see what they will be able to deliver. Based on what I already know they should be pretty much the cutting edge...we'll just have to see.
Old 11-16-2003, 09:19 AM
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Greg Covey
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Default RE: Brushless and Li-poly cells

az,

Don't worry, it's not a "war" but merely a discussion. Differences of opinion are usually the result of interpretation issues. E-flight discussions are fun stuff!

Michael,

I think where we differ (if we even do differ) in opinion is in the motor rated current spec. Surely you can pull more current through the motor than what the spec claims. This is merely a reasonable rating to be used when calculating efficiency. Typically, a motors highest efficiency is at a lower point than its maximum current rating. Look into how Plettenberg rates their motors. It is a system that everyone should follow but they purposly don't so that it makes comparisons to others difficult.

The tradeoff here is in longevity. A typical canned Speed 400 motor may last for 75-100 flight when used within 8-10amp current limits. You can push this motor to 12amps for better performance at the cost of longevity. In this case, the cost is low at about $8.

Jason Shulman beat the crap out of his Hacker brushless motor at the World Championships in an all out effort to sacrifice longevity for performance. After the event, the motor was trashed but he, and his sponsors, had proved their point...electrics are here to stay!

Aero330LX,

If you have already learned that much in only three weeks, it is an impressive feat! Good luck on your project!
Old 11-16-2003, 11:06 AM
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Default RE: Brushless and Li-poly cells

The little Whattmeter sure helps -as long as you remember to record info
My little "dyno plus the Whattmeter really speeds up finding out what combos do what to whom and for how long.
(sounds like the end of an old Limerick)

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