RC India

Folks,

I have a scratch built pusher trainer with which I am learning to fly (http://www.rcindia.org/beginners-zone/i-finally-got-to-build-and-fly-a-pusher-trainer/msg42641/#msg42641)

A couple of times I have noticed that suddenly the plane start to loose power. Initial reaction was that it is hitting low voltage cutoff condition ( i have soft cutoff programmed). Yesterday the model was flying at about 45-50% throttle and about 4 mins later the nose started to dip. Divay, my instructor had problems in landing it. On landing, i checked with the hobbyking watt meter. The LiPo checker function showed the battery at 65%.

Later in the day I conducted a static test. With the timer on I ran the motor at about 50% throttle. At first it was consuming 7-8amps and at about 80watts, slowly the wattage dipped to about 67 watts while still consuming 6 - 7 amps. Then around the 5 min mark, the current consumption reduced abruptly - down to about 3 amps. and the wattage at around 34. I disconnected the battery and noticed that both the esc and the motor were pretty hot to touch. I let it cool for about 10 mins and then tried again with the same battery. Again the motor started consuming 80watts at about 55-60% of throttle.

Now,this means that cooling is solving the problem, so reverse logic would be that the heat is causing it. Now the question how to determine this for sure and then which component needs to be fixed - motor, esc, battery, connecting wires etc. How to isolate the problem?

pankaj

If you are using the 9x4.7 prop, try an 8x4.7 if you have one.  A higher C battery is also worth trying.

Anwar,
Normally, the batteries are marked as I am using 3S1P 1000mAH with 20-30C discharge and as per the existing lipo marking, I can safely draw 20 amps on a continuous basis. Recall I said the amp drawn was just about 8 amps initially. So I am not able to understand why the battery needs to be checked ( not that I cannot), but needed to understand all the same

regards
Pankaj

Since the problem happens after the battery is discharged to around 60%, I was wondering it was just unable to deliver the amps required.  If you have a bigger capacity battery lying around, you can try another test to see if it lasts longer before exhibiting the same behavior.

I do have a 1600mAH nano tech 25-55C discharge, will try it out and post the result. However how do you explain the fact that the SAME battery is able to delivery the amps required after the setup is allowed to cool for about 10 mins. Also the only things heating up are the motor and the esc, the battery stays cool.

Try a smaller prop, and see if it lasts longer.

Also, when you say 65% discharged, how are you measuring that ?  You are looking at individual cell voltages ?  Or are you looking at how much mAH are you putting back when you recharge it ?

Most importantly, when you run it after cooling it, how LONG does it run fine again using the same battery ?

I dont have an explanation, but i have a battery that behaves like that. I strongly believe that you have a faulty battery.

Well I had tried this on 3 different cells (2 were 3S1P 1000mAH 25C and 1 was 3S1P 1000mAH 20C) and all were almost the same.
Well actually, the 65% remaining is what the watt meter told me, so I guess it is based on the voltage levels. Let me do the following
1) recharge the cells
2) time the problem properly
3) notice what the watt meter displays and then compare the same with the charge needed to recharge fully.

Am also going to put a larger heat sink on the ESC

Looks more like a motor/ESC problem. More likely the former.

You are now just two steps away from isolating the problem, check with a different motor and then check with a different esc.

Ok, the following observed.

The static test with 3S1P, 20C lipo. Nothing changed - same battery, same motor, except the ESC taken out in the open (earlier it was inside the fuse).
ran on 50% throttle,
the setup ran continuously for about 9:45mins without a major dip in the output, watt meter recorded a consumption of 900mili amps.
does this indicate that the esc being inside was not getting adequate cooling and was heating up and hence causing the dip in the power output?

Most ESC's have inbuilt protection to reduce power when temp rises. This is to protect the FETs in the ESC.
Probably, since your ESC was getting hot due to lack of ventilation, the power got reduced as a failsafe measure.

Also do try what Anwar has suggested. It could be that the motor is getting overloaded with a bigger prop.

What is the rating of the ESC and the motor ?

i had the same problem with 2s1p 1000mah battery of 25c,this is due to bigger prop(over load).........and when its completely charged make sure that all the 3 cells(for 3s) have the same voltage....when i charged my 2s battery fully(i charged till 8v,but the given rating is 7.4v),it runs fine

Noticed another problem.

I am using Turnigy Bell Motor 2408-21T motor with 18amp ESC. Has been running fine. The motor has been running fine but just today I noticed 2 things
1) At very low throttle, the motor starts spinning then slowing down much like a sine wave -if you know what I mean
2) At times there is a scratching sound like something is stuck.

How do I isolate the problem

either you have something stuck in the bell or you have a dry solder somewhere. most probably a dry solder.