Hello,
The system containing Cobra 2820/12 with APC 11x5.5 E Multiprop and Cobra 40+ ESC is behaving in a weird way. Specifically, both components are getting warmer and warmer with every flight of my eParrot. Two days ago, I have made five 5 minutes flights and could feel this.
I am using KR governor set for: start delay=31 seconds, RPM=~9,930, flight time=5 minutes and gain=3.

Anticipating the question: the rest period between flights was about 10 minutes and this allowed both components to return to the ambient temperature (21 deg.C).

During the last flight the motor simply stopped mid-air and I barely saved the model with minor damage. I run to the model and put my finger tips on the motor (it is rear mounted and almost completely exposed to the incoming air for better cooling). It was no longer very warm like after fourth landing-it was hot! ESC was maybe not as hot as motor but much warmer than after fourth landing. I have measured the ESC temperature using the probe and it was ~140 deg.F (60 deg. C) on the surface of the ESC aluminum heat sink (I have removed the part of the ESC shrink tubing directly above the heat sink). Heat sink is exposed to the air flow inside the fuselage. The motor temperature readings were inconclusive but between 130 and 150 deg.F (55 to 65 deg.C).

Cobra ESC has the Over Temp Protection: Shuts down the ESC output if temperature exceeds 230F (110C).

???????????

For the last 65 or 70 flights of eParrot, I was using 4S 3000 mAh batteries and the same propeller (11x5.5 APC E Multiprop.), turning at 9,860-9,900 RPM.

Please see the attached. The static current during this bench test was acceptable and the average in-flight current is always smaller ( 3/4 Rule).

I will be bench testing the system today trying to solve this puzzle.

Is this possible that the ESC is slowly dying for...whatever reason and allows for higher current to pass to the motor now than before?

The motor has 40 Amps continuous current limit.

Your suggestions and comments are always appreciated.

Regards,
Matt

Hi Fred,
Hi Motorman,

The run time in the air is 5 minutes.
Charging the 4S 3000 mAh batteries last time (Aug.16) , I "poured in" on average 2330 mAh per battery (the max was 2352, the min was 2309).

5 minutes=0.08333 hour. 2.33/0.0833=~28 Amps average in the air. This number fits my "3/4 rule" quite well.

I have used the Cobra specs. and, like always, made current the most important parameter defining the motor.
Cobra 2820/12 has the maximum static current =40 Amps. and considering that the average current in the air on 11x5.5 E Multicopter prop. is close to 28 Amps., I have considered this motor satisfactory.

I have made 60-70 flights with this motor during the last two and a half month and, until Aug. 09, 2017, there was no problem. After landing, the ESC was warm to the touch (but not hot) and the motor was less than warm.

On Aug.09 the motor stopped in the air in the middle of the vertical eight.
I managed to save the plane and noticed that the motor was very warm.

I have to admit that this fact did not raise the red flag and I went home suspecting my timer (KR governor) and re-programmed it. My reasoning was that perhaps the timer's memory accumulates the minute errors and, by analogy to PC, has to be erased during re-booting. This suspicion remains still only a suspicion - I cannot prove it.  Keith Renecle, that I communicate to very often, was not sure either what was the root cause of the sudden motor stoppage in the air.

Then, I went flying again and had a minor crash, easily repairable, after the motor stopped again in the air when I was upside down.

I run to the model and put three fingertips on the motor (it is exposed and easily accessible), said "Ouch...." and removed by hand quickly. The motor was hot!

Please see the attached.

I have made this static run a moment ago and, in the end, the motor was so hot I could smell something (resin?). The ESC was warm but not very warm.

I know now the motor is dying but I still do not know why.

Patrick MacKenzie made a suggestion that perhaps the magnets are getting weaker, this increases the heat, this makes the magnets weaker and so on.
Keith Renecle said recently the same thing.

Motorman,
I am using 4S. With 16.6 Amps the plane would barely fly level with no wind.
Bearings seem to be ok. as there is the same amount of the side and longitudinal shaft play as two month ago. There is also no extra friction while rotating the shaft by hand.
The prop is APC 11x5.5 E Multicopter. It weights 18 grams (0.64 oz.) and has a smaller hub and a bit wider and thinner blades that "ordinary"
APC 11x5.5 E.
Yes...9,930 RPM is high and apparently too much for Cobra 2820/12 working like this motor was working during last two month.

Last 20 or so flights were done when the ambient temperature was around 70 deg.F. I am flying usually between 6:30 and 9:00AM and this Summer is not very hot in Toronto,Canada.

When the motor started heating up, I extended the breaks between flights to 15 minutes.

It was enough for the motor to cool down to the ambient temperature.

I am considering 2826/12 and the same prop. but the flight time will have to decrease to 4 min.30 seconds to protect the 80% mAh drain rule for the batteries.

Thanks,
M

 

 

Check the motor bearings and see if they are dragging.  A long shot, but worth a look.

Hello again,
I believe this puzzle has been solved.
Please see the attached.

The graphs clearly indicate that the brand new Cobra2820/12 behave properly.
The static run on 4S 3000 mAh was 3 minutes 30 seconds that is 20 seconds longer than the last one with the old Cobra 2820/12.

The temperature of the motor green part immediately after run was 150 deg.F (66 deg.C), the ESC 115 deg.F and the battery surface about 100 deg.F.

3 minutes 30 seconds of static run is a serious abuse of the system and I will not try such a long static run again. It was essential, though, for me to see  full comparison of old and new motor.

Conclusion: the bearing/bearings of my old Cobra2820/12 started failing after about 60-70 flights and this is unacceptable. I am contacting Lucien Miller with serious concerns and will consider the alternative to the cobra equipment.

Thank you,
Matt
 

I believe i read somewhere on here, Paul mentioning that he was going through bearings at an alarming rate and one of the reasons he went to a plettenberg.

Tom