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Old Nov 01, 2002, 07:50 AM
Martin Hunter is offline
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Need help with this speed 300 dd setup...


Hi gang,

I'm running this combo in my zipper:

GWS 370 dd
GWS 4.5x3 prop
8 x 720 NiMH AAA pack
Kontronik Sun1000

I suppose I'm pulling too much current for these poor little 720s, as the LVC kicks in at anything above about 1/2 throttle after about 3 to 5 seconds. What is the best thing to change to keep everything happy? Will reducing the pack by one cell make a big enough difference? Thanks!

Martin
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Old Nov 01, 2002, 08:35 AM
Wright Flyer is offline
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Martin,

I could have got completely the wrong end of the stick on this but I'd have thought that adding a cell rather than removing one would be the solution. Ultimately the motor/prop are consuming P watts and that's made up of V volts times I amps. The reason the LVC cuts in is because the current pulled from the cells causes their voltage to sag - so to deliver the same P for less I you need more V - or have I got this theory completely wrong??

I just put the numbers into P-calc (only motor calculator I have access to from here) and got values of "V to motor" of:

7 cells - V=5.79
8 cells - V=6.25
9 cells - V=6.66

Cliff

PS What's a "Zipper" ?
Old Nov 01, 2002, 09:03 AM
Dave Hederich is offline
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Martin, according to the GWS motor tables, a direct drive 300 with 4x2.5 prop draws 7.8A at 7.2V (6 cells). With a larger prop (in both diameter and pitch) and two additional cells, it is likely that you are trying to draw >10A out of cells that don't like to give out more than 6A. I'd spend some time studying the GWS charts and checking the current capabilities of various cells. The 300 will run at ~10A, all right, but it requires cells that will handle that demand. On the other hand, your AAA NiMH cells will work fine if you prop for ~6A max.

http://www.gws.com.tw/english/produc...tem/edp300.htm
Old Nov 01, 2002, 09:51 AM
mkirsch1 is offline
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Yep, reducing the pack by one cell will make it WORSE...

Wright Flyer's got it "wright." You're drawing too much current (I) from the batteries, and voltage (V) is suffering.

Speed 300s make really crappy DD motors. They're just too high-strung, and need a gearbox to let them really sing.
Old Nov 01, 2002, 10:51 AM
steve lewin is offline
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I think Martin was (correctly IMO) referring to the fact that reducing the number of cells while keeping everything else the same will reduce the current. True it will provide less theoretical maximum power but if it avoids overloading the battery it may provide quite a lot more actual power.

I've run an S300 on 5 cells before now (but not with BEC). OTOH I do agree that in general S300s make lousy DD motors.

Steve
Old Nov 01, 2002, 11:21 AM
dave morris is offline
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I use to fly a speed 300 on 6 cells (500ar) and a 5*2 prop on a DMA flea. Flew well but the motor will not last long. DD is not the way to go with this motor, I would either go BL or a BB280 with 7 or maybe 8 cells (it is a little lower kv). I have heard of a new speed 300 for 7.2 volts, that may work better it you want to use 8 cells. still 720's have hi IR and would be stressing.
Old Nov 01, 2002, 01:09 PM
Martin Hunter is offline
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Quote:
Originally posted by steve lewin
I think Martin was (correctly IMO) referring to the fact that reducing the number of cells while keeping everything else the same will reduce the current. True it will provide less theoretical maximum power but if it avoids overloading the battery it may provide quite a lot more actual power.

I've run an S300 on 5 cells before now (but not with BEC). OTOH I do agree that in general S300s make lousy DD motors.

Steve
Yup, that's what I was getting at - good ol' V=IR.

I have to say that performance-wise, this motor is amazing in dd capacity. I can ROG at about 1/3 to 1/2 throttle from a dirt track. Longevitiy is the question mark though - particularly if I'm drawing more than about 8-9A (which is what I have drawn from the 720s in the switchback in the past, so I know they will unhappily handle it without LVC kicking in). How about my reducing prop size/pitch to 4x2.5 vs 4.5x3 to bring the current down into that range? I've looked for specs on the 4.5x3 prop, but they don't seem to be around anywhere, or at least not with the other GWS prop charts.

Cliff, you can see the specs on the zipper at www.oldschoolrc.com and check out my flight report in this thread.

Martin
Old Nov 02, 2002, 06:07 AM
Bruce Abbott is offline
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Changing to 6 x 600AE's will give more power due to higher voltage and much lower internal resistance. I estimate a full throttle current draw of 11 Amps. The motor is designed to handle 8A (best efficiency is at 5A) so it should be OK if you go easy on the throttle. Dropping the prop down to 4x2.5 will reduce maximum current to about 9A. Pitch speed should be about the same but thrust will be a bit less.
Old Nov 02, 2002, 11:35 PM
Dave Hederich is offline
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>> ..... particularly if I'm drawing more than about 8-9A (which is what I have drawn from the 720s in the switchback in the past, so I know they will unhappily handle it without LVC kicking in).

Do you guys up in Canada use the metric system for measuring current?

Seriously, here's what Dave Thacker at Radical RC has to say about the Sanyo 720mAh AAA NiMH cells: Happy up to 5 amps continuous drain.
Old Nov 03, 2002, 01:36 AM
Martin Hunter is offline
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8-9A is based on measurements done by guys such as Bernard Cawley

Martin
Old Nov 03, 2002, 02:56 PM
AIR MOVER is offline
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i love useing the flexible cox 031.25 on the motor on 8 350n's the motor screams but is happy like its in a gear box so i am seeing over 5min runs on a slick drag free plane doing power dives of over 60mph on hand shaped solid balsa wings.. root is 3/16th cord is 4.5in and 2in at the tips, span 19in. the sound of the prop unloading even faster right after you release .. is as close as i have got to wet power flying...
Old Nov 03, 2002, 07:00 PM
Dave Hederich is offline
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Martin, one interesting things about e-flight is how people manage to come up with such widely varying numbers. Some people say that the 720mAh AAA NiMH cells will not work well at more than 5-6A, while others swear they'll work just fine at 8-9A. That's a big difference. I suppose the definitive answer is out there somewhere. But I have yet to see irrefutable evidence from either side.

Pulling 8-9A out of 720mAh cells represents more than 10C, which is quite a feat with the smaller NiMH cells. If the 1600mAh AA NiMH cells delivered at the same level as the 720s, then they should be good for ~20A. Oh well, I guess it only matters if someone buys these cells expecting them to provide a certain level of current and then discovers that they won't deliver.


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