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Author Topic: Cobra C2808-22 runs but there is something weird happening  (Read 1975 times)

Offline Matt Piatkowski

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Cobra C2808-22 runs but there is something weird happening
« on: August 06, 2015, 03:40:03 PM »
Hello,
My setup is as follows:
Motor: Cobra 2808/22 ( Brushless outrunner)
Prop: APC 9 x4.5 (recommended for Cobra 2808/22 and 3S LiPo. Ref: Cobra Motors Propellers Data).
ESC: E-Flite 25 Amps, Brushless
Battery: E-Flite, 1300 and 2200 mAh, 3S, 20C and 30C respectively
Timer: simple push-and-hold type (E-Flite, EFLA172). While holding the button depressed, the motor runs for a short time and stops number of times. When the button is released ( after ,say, five run and stop sequences),  the motor starts, reaches the max RPM and keep running for five minutes.

Something weird: The motor runs at ~9,800 RPM for 3 minutes and then runs at ~5,500 until the battery drains.

There is no excessive heat after run. The motor, ESC and battery reach 130-140 deg. F.
The max load current measured while the motor run at ~9,800 RPM was 24 Amps.

There was no smoke and no smell of anything burning.

I run this setup three times ( 2 x on 1300 mAh, 1 x on 2200 mAh) and the weirdness repeats itself but it is more visible while using the 2200 mAh LiPo.
It is so because on 1300 mAh the entire time is only 3 min.15 seconds.

Any idea what is to check/do?

Thanks,
Matt

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Offline Fred Underwood

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #1 on: August 06, 2015, 04:06:16 PM »
It sounds like "setting timing for flight" here.

http://www.e-fliterc.com/ProdInfo/Files/EFLA172-Instructions.pdf
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Fred
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Offline John Cralley

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #2 on: August 06, 2015, 05:19:22 PM »
I suspect you are hitting the voltage cut off for your ESC. When the voltage cut off is hit many ESCs cut the rpm to allow the plane to be safely landed (RC planes). If you can, reprogram the ESC to have a cut off of 3.2 volts (per cell).

On another note, if you are pulling 24amps through a 25amp ESC, you are pushing the envelope! Also do not drain your battery beyond 70 to 80% of the capacity!  Otherwise you are apt to be buying a replacement battery soon!
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John Cralley
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Offline Matt Piatkowski

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #3 on: August 06, 2015, 08:46:23 PM »
Thank you for your input.

Timing for the motor run is set correctly so we have to exclude this possibility.

24 Amps is the maximum continuous current listed for 2808/22 and I was drawing 24 Amps for about three minutes with measured RPM =~9,800.
The motor itself cannot "sense" the danger so it was ESC that decreased the RPM to ~5,500. I did not measure the current at 5,500 RPM but it was lower, probably around 14-15 Amps. E-Flite website claims that this ESC can withstand the continuous current of 25 Amps and the "short burst" current of 35 Amps.

I agree that I was very close to the Red Line for current and will decrease the max.RPM by turning the knob on the timer C-CW. The amount of this reduction will be set experimentally to get the max current around 22 A.

If after this reduction I will be able to get 6 minutes of the motor run on 2200 mAh battery, the issue will be closed and the verdict will be "ESC protected the motor by limiting the current". Perhaps this ESC has some build-in circuit that automatically reduces the current to a certain level considered safe but why so low?

5,500 RPM is only 56% of max RPM ( 9,800) and the current at 5,500 is roughly 50-60% of the max current.

Question:why did this ESC allow the motor to run at 9,800 for three minutes instead of cutting the current instantly?

If I cannot get 6 minutes with max RPM and the weirdness continues, I will replace the E-Flite 25A ESC.

Thanks,
Matt





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Offline Keith Renecle

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #4 on: August 07, 2015, 01:22:03 AM »
Hi Matt,

It sounds like the other guys are correct and the esc is shutting the rpm down.......for you to land in one piece!  #^ Changing the esc for a more powerful one is not really the answer in this case, because the motor will never hold that full rpm anyway without a governor system of some kind. It will just run slower as the battery runs down. The other simple timers  like the old JMP or Will Hubin's timers that add rpm with time, do to some extent help a bit, but as you climb and dive the rpm will fluctuate. You and I have e-mailed this back and forth a few times already and you have the data from me on a good .15 size setup that works very well. It is also posted on this forum as well.

The 4.5" pitch is not enough for 3 cells to fly a small C/L model nicely, and that's why I went to 8x6 or 9x6 APC E props. The 3 cell 1300 mAh will then fly a good 3.5 to 4.0 minutes which is fine for novice stunt type of pattern, and if you need to fly longer then a 1500 mAh to 1800 mAh Lipo will do the job. My best motor for this size has been the Black Tiger BT 3530C with a 1400 Kv from RSM. Your Cobra has 1330 Kv so it is a bit lower, but could still work with some more pitch on the prop. Enjoy!

Keith R
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Keith R

Offline Matt Piatkowski

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #5 on: August 07, 2015, 03:26:13 PM »
I have solved the problem with the help of my friends ( Beatles, 1967, Yellow Submarine) from my local hobby store.
It had nothing to do with using the governor or E-Flite timer.

It was simply balancing the brand new battery.

One cell of the brand new 2200 mAh, 3S battery was lower on voltage than the other two when I run the first test that delivered the "weirdness". Consequently, the ESC protected this cell of the battery when the voltage of this cell went below the 3.3 V level.

In effect, the motor run on two cells after 3 minutes.

I have run the test again a moment ago with freshly balanced 2200 mAh, 3S LiPo having the initial fully charged voltage of 12.6 V and the results are excellent:

RPM on 9 x 4.5 APC prop  = 10,350
Current @ these RPM = 23.1 A
Power: ~  256 W= 23.1 x 11.1 ( I am taking the nominal battery voltage; it had 12.5 V when the test started.
The RTF model sits right now at 680 g = 24 oz. ( 190 grams is 2200 3S E-Flite 30C LiPo).

I am assuming 10W per ounce...it will fly like hell!
 
Time of stable motor run = 5 min 45 sec. and then gradual shut-down .

I was monitoring the voltage of all cells during this test and they all ended up at 3.3 V.

2808/22 temperature after the prop stopped was 140 deg.F. Battery ~ 130 deg.F, ESC ~140 deg.F.
The battery is being charged now.

Conclusion: each brand new LiPo must be cycled a few times to balance the cells.

Regards,
M
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Offline John Cralley

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #6 on: August 07, 2015, 06:11:25 PM »
Well that's strange! Didn't you do the initial charge from storage to fully charged AND balanced?  n1
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John Cralley
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Offline Matt Piatkowski

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #7 on: August 08, 2015, 03:50:14 AM »
Yes, I did but, at that time, I did not check the individual cells voltages.
Perhaps the cells voltages differed to much and the charger could not FULLY balance the cells but only completed charging.

Program "LiPo Balance" used by this charger may have its limitations that I am not aware of yet.

Some people claim that the brand new LiPo must be charged and discharged several times on the bench to achieve balanced cells and I will do it with all my batteries.





 
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Offline Target

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Re: Cobra C2808-22 runs but there is something weird happening
« Reply #8 on: August 08, 2015, 09:10:10 PM »
An easy equation would tell me that at 24 amps draw, 6 minutes (1/10 hour) would use 2.4a, (2400ma).
So 1200ma would be used in 3 minutes, and that's all of the 1300 battery!
4 minutes to 4 minutes and 20 seconds should be safe on the 2200. The 1300 should be timed out at 2 minutes 30 seconds.
That should use 1000ma if my math is correct.
If it will pull the plane, an 8" prop might save you some battery, lowering the amp draw considerably.
Best of luck Matt.
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Regards,
Chris
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