afterburner

My Feedback: (18)

Just curious if anyone has tried this. I'm thinking about it. I have a Mega 16/15/3 with a jeti 30-3p SC in my projeti. Looking at the Kokam 3S 2000ma X 2. Should supply 32+ amps. Has anyone put their mega 3 turn on a whattmeter with 9 or 10 cells(or 3s lipo) and the graupner 5.5 X 4.3 cam prop? I assume it pulls more than the 24 amps that a 3s2p kokam 1500 could handle? Anyone with any info if this is a good or bad idea, please chime in.

thanks

marty

Matt Kirsch

My Feedback: (21)

I ran this one through PCalc the other day, using TP2100 cells for another thread regarding this motor. On a 5x5 APC, it was drawing a little over 30 Amps and had a pitch speed of 114 MPH. That's right at the limit of the motor, but PCalc isn't that accurate either...

afterburner

My Feedback: (18)

Thanks Matt. I was figuring that bumping up the voltage would push the setup above the threshold but I have another question that you might be able to answer.

When you plug numbers into these software programs, I assume they are giving you current and power numbers for a fully charged pack at a full throttle setting. The question that I have is, say for an example that the setup that I am contemplating is pulling a few amps over the safety margin for the ESC and the motor. If you put the setup on a whattmeter and see where you're getting close to the "red line", could'nt you just program your throttle setting on your transmitter to not go beyond that setting or even a little lower for a safety margin or even conservative throttle management . I realize this is not a cure all but on a setup that is a little over the max, you would still see benefits in weight savings and duration and still be getting a little closer to getting the max out of the setup. One problem I possibly see is that when the voltage starts to drop on the pack, you might see a dramatic drop in performance because your max throttle level on your xmitter has been lowered. Please let me know if I'm totally off the mark.

thanks

Matt Kirsch

My Feedback: (21)

Electronic speed controllers are just big on/off switches. It's not like a gas carburetor which restricts air, or a diesel injector pump which restricts fuel. They don't restrict voltage or current, so any time the speed controller throws the switch to ON, both it and the motor see full voltage, and full current. When you get a lower current reading on the Whattmeter, it's only because that's the AVERAGE current being drawn from the battery. Electric motors are closer to the old LeRhone rotary engines from WWI in their "throttle" than a modern gasoline engine.

I'm not completely schooled on the way brushless controllers work, but it is my understanding that as you reduce throttle, the current being handled by the controller itself on an instantaneous basis actually goes up. The relationship I heard was 1/2 throttle, double the current. It has to do with the electricity returning from the motor at the same time another pulse is being sent to a different segment of the windings. 40 Amps out, 40 Amps in.

The rating of a brushless ESC denotes the full throttle current, but it is designed to handle partial throttle as well. Exceed full throttle current, and you exceed partial throttle current by twice, three, four, five times as much.

afterburner

My Feedback: (18)

Matt, Thanks for the response. I'm definetely not an expert on brushed or brushless motors or speed controllers but some of the theories don't make sense to me. I do have some schooling in electronics and pulling more current at reduced throttle settings would tell me that I would get more flight time out of a pack flying at full throttle than at half throttle and we know that not to be true. Like you said a brushless controller works like a switch. My understanding was that it switches on and off at different frequencies depending on the throttle setting. I thought that the faster it switches on and off the faster the motor turns, the power being generated goes up and the current increases. Maybe I need some "brushing" up on my electronic theory. Again, thanks for the response.

Greg Covey

One of my pending test flights is the Mega-powered ProJeti on the 3s2p 4200mAh ThunderPower pack from [link=http://www.hobby-lobby.com/thunderpower.htm]Hobby Lobby[/link].

The Kokam 2AH cells were not a good physical fit in the ProJeti. That goes for either the old 8C ones or the new 20C cells.

I'll likely not fly it until the Memorial Day weekend event but i'll report back on it.

Peter Khor

My Feedback: (4)

fwiw I'm quite happy running 16_/15/4 and 5.5x4.3 CAM prop; with either 10 cell KAN pack or 3s LiPo's, I'm running just around 20A which suites the packs that I already have (3s TP2100 and 2 pack of Tanic 3s 1100's in parallel). With the LiPo's you can point the Projeti straight up and it'll just climb. btw, has anyone a real-world comparison btwn 3 and 4 Mega's in a Projeti?

imsofaman

My Feedback: (2)

I have a movie of a guy with a Pro Jetti....it is fast!!!!! Send me a PM with your email address and I will send it to you.


Dave

777

My Feedback: (1)

I'm thinking of a similar relationship with a 16/15/3 and TP2100 3s. I noticed that several people use batteries in parallel too. Is there some advantage to that other than duration? Wouldn't I be satisfied with 2100mah?

Thanks,
Brad

afterburner

My Feedback: (18)

Brad, The duration is usually the byproduct of the real need to parallel packs. The cells have a maximum recommended current that can be drawn out of them. You probably have seen a number followed by a letter C. For example: If a 2000ma pack has a rating of 8c, it can safely supply 16000ma or 16 amps. By putting two of those packs in parallel, you can supply 32 amps in addition to now having a 4000ma pack. The mega16/15/3 can handle 30 amps as per the manufacturer and depending on how it's propped will be close to pulling that especially on 3 cells. I'm not sure what the rating is on the TP2100 but I don't think its close to 15c. With Li-po's it's especially important not to push them too far beyond their recommended current rating.

marty

Greg Covey

I have to agree with Marty.

The ProJeti is not a cruising wing like the Zagi, it is an all out speed demon in which you mostly slow down to setup your next extreme fly-by or all out vertical climb!

With that in mind, my 8-cell CP1300 NiCd packs and 8-cell 1950 FAUP NiMH packs would be quite hot after each flight because we are drawing around 30-32amps on full power runs using the Mega 16/15/3 motor with the Graupner 5.5x4.5 CAM Speed prop.

I like to design my Lithium packs so that the continuous current rating equals the full throttle setting. Using a 3-cell Kokam 2AH (20C) pack would work fine because it can deliver 40amps continuous but it just doesn't fit without excesssive fuselage modifications.

The ThunderPower 3s2p pack has a 4200mAh capacity and can deliver about 30amps continuous and 40amps burst. The pack shape due to the 6 cells inside is a rectangular block much like the popular Zagi 400X battery packs.

777

My Feedback: (1)

Thanks for the tips, guys. So if I understand you, Marty, using Greg's TP 3s2p example, his packs would be able to deliver 15a continuously indivdually? So would limiting yourself to 3s be bad? Would you have to go with something in parallel?

Thanks again,
Brad

Matt Kirsch

My Feedback: (21)

Hey afterburner,

Quick clarification on my explanation, the INSTANTANEOUS current goes up. We know through measurement that the average current drawn from the battery does indeed go down at reduced throttle settings.

Brushless motors have three separate circuits in the windings, and work by energizing these three circuits in sequence to push and pull the magnets. When the current flows, it doesn't just flow and disappear into some black hole inside the motor. Current flows from one end of the battery to the other (whether it's plus to minus or minus to plus is another discussion); it goes in one wire on the motor and comes out another.

We're talking in terms of a few ten thousandths of a second here, but the timing at certain throttle levels is such that a pulse is coming back into the ESC as another is being sent out. Hence, the ESC is actually having to handle more than just the full throttle current.

Please note that this is just my best understanding of the subject based on limited knowledge of brushless motor technology.

afterburner

My Feedback: (18)

Brad,

I don't have a full understanding of all the specs that come with different motors but I think I can answer your question(and if anyone can explain it better, please do). Every motor has a specific internal resistance and can draw a certain amount of current when a specific voltage is applied. On the Mega 16/15/3, we know from other users experiences that on an 8 cell pack, we're drawing close to if not over 30 amps. By using a 3 cell li-po, the voltage is even higher than the 8 cell ni-mh/cd and the motor will want to draw even more current. The motor doesn't care that the battery can't supply the current that it wants, it's going to try to suck it out of the battery anyway. The battery will try to deliver that current and since it's only rated for 15 amps but the motor wants 30, the battery will overheat, balloon up and probably be destroyed. In this application, you need to parallel packs until they come out with cells that can deliver 30+ amps. Hope this makes sense.

marty

afterburner

My Feedback: (18)

Greg,
Unfortunately, I misread the measurements on the Kokam 2000 cells and ordered them for the projeti and received them today. It would be easier to strap them on the wings than modify the fuse. Anyway, I saw that you were going to test out your Projeti with the TP2100's around the end of the month. Please post the results as soon as you can. I'm probably going to send back the Kokams and wait to hear your results with the TP's.


P.S. Your construction/review of the Potensky Cobra was very helpful. I bought it last year and upgraded it to a Maxx 2015-4100 with Kokam 3cell 1500. Twenty plus minutes and still have good vertical at the end. Flew it today in 10 to 20 mph wind and it's still a blast.

afterburner

My Feedback: (18)

Matt,
What more can I say except ok. I can understand that explanation.

777

My Feedback: (1)

Thanks, Marty. Great explanation! I didn't understand that deal about amps. I thought all that mattered was volts, & if I understand correctly, that does not change by putting packs in parallel. Please correct me if I'm wrong.

Thanks again,
Brad

Matt Kirsch

My Feedback: (21)

You've got it. In parallel, the capacity of the packs are combined, as are the packs' ability to handle current (Amps). Connect them in series, and voltage is combined.

Oversimplification:
In parallel, two packs will run a motor twice as long.
In series, two packs will run a motor twice as fast.