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Beneifits/detriments of additional winds on AXI

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Beneifits/detriments of additional winds on AXI

Old 02-26-2004, 12:44 PM
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P-51B
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Default Beneifits/detriments of additional winds on AXI

Being new to this electric area, I have a question about the outrunner AXI motors. The last number of a motor in a given AXI series seems to indicate the number of turns in the motor. My question is what benefit/loss does the extra turns give? Is it strictly the ability to turn larger props at a sacrifice of runtime. I am curious from the "watts to the prop" standpoint.
Old 02-26-2004, 02:13 PM
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Greg Covey
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Default RE: Beneifits/detriments of additional winds on AXI

This is a very good question and one that I have been thinking about for months now.

For non-outrunners like the high Kv geared brushless motors (Aveox, Kontronic, MaxCim, Hacker) the effect of turns over Kv is more obvious. Additionally, you have another variable to play with in the gearbox reduction ratio. Adding more cells adds more power quickly.

On the lower Kv outrunners like AXI motors, I have not seen a benefit to using the higher turn versions as the increase in cell count provides little power increase when you factor in the extra weight of the cells.

Looking at the specs for both the [link=http://www.hobby-lobby.com/brushless_axi.htm]4120 and 4130[/link] motors, the power output to the prop is very similar, only the weight of the motor, cell count, and the RPMs are really different.

Basically, the larger, heavier motor allows you swing a bigger prop at the cost of even further efficiency reduction which is compensated for by adding cells (and therefore weight). The real measure of how well your plane will be powered is by counting the watts to the prop which is more commonly seen as watts/lb.

The 4130 has slightly more power than the 4120 and also allows you to swing a larger prop. Ideally, it would shine more if you could use Lithium cells. Lithium cells provide less weight and higher voltage for a greater power density.

When using NiCd or NiMH cells, the additional cell count from the 4130 may negate the power benefit. For example, 16-cells on the 4120 produces 900 watts and 30-cells on the 3oz heavier 4130 produces 980 watts. The weight of 14 extra sub-C cells plus the 3oz from the motor is about 1 pound! The net result is 80w extra for an additional pound...virtually nothing.

My current feeling is that the best benefit comes from using the lower wind version of each size AXI motor. The motor design itself brings additional benefits of simplicity (no gearbox) and maintenance-free operation (no screws to loosen or gears to oil).
Old 02-26-2004, 04:12 PM
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Matt Kirsch
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Default RE: Beneifits/detriments of additional winds on AXI

To get 900 Watts out of a 4120/14 on 16 cells, you're pushing over 50 Amps.
To get 980 Watts from a 4130/20 on 30 cells, you're only pushing about 30 Amps.

Prop the 4130 up to draw 50 Amps, and you're talking about 1500+ Watts. You're dragging along an extra two pounds in battery weight (Sub C cells weigh ~2oz each), but you're also getting 600 Watts with that two pounds. 300 Watts per pound is a pretty good power loading

What I would really like to see is an even larger AXi capable of about 2000 Watts, myself. Something that would turn a 20-22" prop direct drive, but doesn't cost a fortune. I've looked at the big MaxCims, but they're waaaaay out of my league.
Old 02-26-2004, 05:12 PM
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Default RE: Beneifits/detriments of additional winds on AXI

Thanks for the input guys. This electric power area certainly is "interesting".

I am comparing a couple of the AXI motors on the Hobby Lobby site and in looking at the current draw vs watts to prop for the various props, the benefit/detrimant effect is not always obvious (for a beginner in electrics anyway).
Old 02-26-2004, 07:16 PM
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Default RE: Beneifits/detriments of additional winds on AXI

Matt you are thinking like me on the AXI 4130/20 with 50 amps but it is only rated up to 40 amps...
Do you know someone that has pushed it that far?
The $150 versus the $409 make it look attractive if it will do the 1500 watts and not be killed in 1 flight..
Old 02-27-2004, 08:37 AM
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Default RE: Beneifits/detriments of additional winds on AXI

P-51B,

Don't use the Hobby Lobby numbers. They're wrong. I've emailed them repeatedly on the issue, and they keep telling me that they're going to fix it, but it never happens.

Take for example the 4120 numbers: They claim 600 "Watts to prop" on 12 cells at 40 Amps. On a good day, the motor is 85% efficient. That means to get 600 Watts out, you need to put 705 Watts in. On 12 cells at 40 Amps, the cells need to hold a voltage of 1.47 Volts each! Since even the hardiest NiCds peak at 1.4 Volts per cell, and drop to 1.1 Volts per cell under a 40 Amp load, the figures are completely wrong!

Instead, use the figures at http://www.modelmotors.cz. These are real-world test numbers using middle-of-the-road NiCds (RC1700s in most cases, not the most robust, but not the least either). My real-world testing matches theirs closely.

toysejr,

Doesn't the 4130/20 have a "30-second" rating of 50 or 60 Amps?
Old 02-27-2004, 10:49 AM
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Default RE: Beneifits/detriments of additional winds on AXI

In Steve Neu's review of the AXI 4130 motor in "Quiet Flyer", he pointed out that while the motor case was cool, the windings were much hotter. ( I believe that he said they were pushing 180 F) He was suggesting a max current of 40 amps.

John
Old 02-27-2004, 01:36 PM
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Default RE: Beneifits/detriments of additional winds on AXI

Ahhh, this explains my dilemma above. The numbers posted on the Hobby Lobby site for the 4120 have errors. I'll get this fixed immediately.

As for an AXI motor larger than the 4130, it is already being tested in Nashville.
Old 02-27-2004, 02:04 PM
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Default RE: Beneifits/detriments of additional winds on AXI

Yeah, I've been looking at the numbers from the model motors site. Right now I am looking at the smaller motors in the 22 series.

I am very interested in the potential larger motor "being tested in Nashville" you mention Greg, once I learn a bit more on the smaller systems.
Old 02-27-2004, 03:21 PM
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Default RE: Beneifits/detriments of additional winds on AXI

Matt I e-mailed Model motors and this was the reply:

This motor can handle more than 40A. Only condition is to let it
cooling.
If your instalation let motor to be cooled it can draw safe 50A.

Best regards,

Ales Pelikan


Greg I would too be interested in the:

"As for an AXI motor larger than the 4310, it is already being tested in Nashville"
Old 02-27-2004, 03:33 PM
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Default RE: Beneifits/detriments of additional winds on AXI

For most, if not all, of these AXI motors, I would initially favor the lower wind versions.

The assumption here is that you will only use full throttle for short burst periods. This allows you to momentarilly have a tremendous burst of power when needed for take-offs or vertical runs. This is in sync with Ales Pelikan's reply. The main issue here is heat dissapation as mentioned by Steve Neu's concern.

On my [link=http://www.gregcovey.com/wipa_firecat.htm]Wipa Firecat[/link], I initially used the 2820/10 motor and had more than enough power. As we started to abuse it longer and longer with 3D moves and longer vertical runs, the motor eventually broke from melted internal windings. My fix was to replace it with the lower Kv version, 2820/12 motor. A form-fit replacement without changing prop size or battery cell count. The result gave me a little less power but I still had more than enough for long vertical runs. The lower current draw kept the motor cooler and resulted in longer runtimes.

My point here is that you should design it a little hot initially and use lower cell counts with outrunner motors. Often, a 3-cell Lithium pack works best on a 8-10 cell NiCd setup.

Use higher cell counts for high Kv geared motors on larger e-conversions to increase the power while keeping efficiency high. Again Lithium works best here as the voltage of 9 LiPo cells equals 30 cells of NiCd. Power levels over 1000 watts are easily obtained.
Old 02-27-2004, 08:53 PM
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Default RE: Beneifits/detriments of additional winds on AXI

Hmmm...something else to think about. I would definately be the type to "abuse" the motor by pushing at as far as I could. Maybe I should think about the middle of a given series as a place to start to avoid one of those meltdown situations.
Old 03-02-2004, 09:50 AM
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Default RE: Beneifits/detriments of additional winds on AXI

Hobby Lobby has fixed their Web site numbers for [link=http://www.hobby-lobby.com/brushless_axi.htm]AXI 4120 motors[/link].

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