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What kind of havoc can a bad servo cause?

#1
I'm trying to diagnose a problem I came across the other night while assembling my new Clouds Fly. (Only my second plane, so I have limited knowledge) I was just about complete and then it seemed like the receiver and/or the ESC shutdown then restarted. It seemed weird but I ignored it and proceeded to adjust the elevator. A little while later I smelled something burning and the rudder servo (and maybe all the servos) started freaking out, moving back and forth to extreme limits. I pulled the battery to kill the power, and the ESC was super hot. The battery was cool and so was the motor. The throttle was off and no inputs were being made on the TX.

My initial thought was that the ESC was faulty, but then I got thinking about the servo, specifically the rudder servo. From the very beginning, it has always been "jittery", especially when just turned on, it just kind of vibrates. I'm wondering if maybe this servo is faulty and is pulling way too much juice and over taxing the BEC on the ESC??

Any ideas?

Thanks!
 

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#2
Servos are cheap (even if they're a pain to swap out) so I live by the moto, if in doubt, swap it out.

A bad servo will kill a plane before you even have time to figure out you don't have control anymore.

I'd certainly do a sniff test, to find what was overheating. Replace that as well.
 
#3
Agreed, the servo will be replaced. What I'm wondering is... can a servo do this? Can it be faulty in a way that would cause it to pull much more electricity than it is intended to? Can a bad servo cause the ESC to overheat?
 
Last edited:

colorex

Rotor Riot!
Mentor
#4
Agreed, the servo will be replaced. What I'm wondering is... can a servo do this? Can it be faulty in a way that would cause it to pull much more electricity than it is intended to? Can a bad servo cause the ESC to overheat?
Shorting out the BEC? Why not?
 

colorex

Rotor Riot!
Mentor
#6
Agreed, the servo will be replaced. What I'm wondering is... can a servo do this? Can it be faulty in a way that would cause it to pull much more electricity than it is intended to? Can a bad servo cause the ESC to overheat?
If the power cables are shorted inside the servo, the BEC will also be in that short circuit, causing overheating and damage... I wouldn't trust that ESC if it got too hot.

Are you answering a question with a question? Surely there is a rule against that somewhere?? :)
(Is it better now? LOL)
 

Emparri

Junior Member
#7
I read that BEC ratings as stamped on the component can be misleading. For the AXN clouds fly, my guess is that if it is stock ESC, the BEC rating is 2A MAX. MAX is the key here since as I understand it, it refers to a 2S setup. If you plug in a 3S, the extra battery voltage (11.1V vs 7.4V) cause the BEC to generate more heat when it steps down the voltage to 4.8V (for servos). In essence, the BEC Amp rating is reduced.

A simple mathematical GUESSTIMATE;

At 2S and 2A current
2000 x (7.4 - 4.8) = 5200 mWatts
At 3S and equivalent power (heat generation or dissipation) would mean
5200 / (11.1 - 4.8) = 825mA => 0.825Amps !!
Not exact science but you get the gist of the current ratings drop.


9g servos typically operate at around 200mA. But in stalled conditions, and depending on brands, they can go up to as high as 750mA (From http://homepages.paradise.net.nz/bhabbott/Servo.html)

So yeah, faulty servo/s causing your BEC to overheat is a huge possibilty.

All the guys I know who fly the Clouds Fly, use 3S with their stock setup and haven't had problems yet; so safe to say that in normal operating conditions, it seems to work just fine. My 2cents.
 
#8
clouds fly servo

I'm trying to diagnose a problem I came across the other night while assembling my new Clouds Fly. (Only my second plane, so I have limited knowledge) I was just about complete and then it seemed like the receiver and/or the ESC shutdown then restarted. It seemed weird but I ignored it and proceeded to adjust the elevator. A little while later I smelled something burning and the rudder servo (and maybe all the servos) started freaking out, moving back and forth to extreme limits. I pulled the battery to kill the power, and the ESC was super hot. The battery was cool and so was the motor. The throttle was off and no inputs were being made on the TX.

My initial thought was that the ESC was faulty, but then I got thinking about the servo, specifically the rudder servo. From the very beginning, it has always been "jittery", especially when just turned on, it just kind of vibrates. I'm wondering if maybe this servo is faulty and is pulling way too much juice and over taxing the BEC on the ESC??

Any ideas?

Thanks!
I have two of these, the first had two faulty servos and the second a faulty motor. Also two ESC's burnt out resulting in the loss of plane!
 
#9
Good to know, thanks everybody for the replies. I did pull out the servo in question and, sure enough, it had burned a hole in the bottom of the casing. I've gone ahead and ordered some new servos and a ESC while waiting to hear back from HobbyKing's warranty department.

I know things, in general, are supposed to run cool. But, say you're in your workshop and just testing the servos... should the ESC get warm to touch with just the servos going back in forth. I'm trying to figure out what "normal" is.

Thanks!
 
#10
AXN Cloudsfly Floater Jet Stock Servo

On two separate occasions I have had a stock servo on the AXN short circuit causing loss of control due to the shorted power supply. These servos are not good!
 

Non Action Man

Nose Landing Specialist
#11
I know things, in general, are supposed to run cool. But, say you're in your workshop and just testing the servos... should the ESC get warm to touch with just the servos going back in forth. I'm trying to figure out what "normal" is.
really depends on your perception of warm, but I would say no it shouldn't