Intermittent Motor Cutoff

[senaaa]

Hi everyone,

I’m seeing a weird issue with the SunnySky X3520 (520KV) motor.
Same ESC (Flycolor Flydragon Lite 120A), same 6S 5000mAh LiPo battery — but sometimes the motor cuts off during the first throttle-up, especially drawing around 45–60A.
What’s strange is:

I tried two different 6S LiPos — one Profuse 5000mAh 60C, the other Forceup 5200mAh 25C — both show the same behavior.

But with a 6S 12000mAh Turnigy Graphene, the problem never happens.

Also tested with an X4120 (550KV) + 15×8 prop, same ESC, same 5000mAh battery → draws 80A with no cutoff.

The X3520 uses a 14×8 Eolo prop, so it’s not even under heavier load.

Most of the time, the cutoff happens on the first run. After that, the motor usually works fine — even with the same battery.
Seems like a current spike or voltage sag issue, but why would X4120 handle it fine while X3520 fails, even under lower load?

Any thoughts on what could be causing this?

Thanks

 

[WickedFog]

If I am reading this correctly, it sounds like you have eliminated everything but the motor. Except for the Turnigy battery not showing the same behavior. That is the only thing throwing a curve ball here. Maybe there is a bad contact somewhere, which would be a bottleneck in the amperage, and the higher mAh Turnigy pack is able to push through that? I would replace the motor and see if that solves the problem.

Is this on a drone, or airplane?

Oh, and hop over to the Introduce Yourself forum and say hi. Pics of your RC's are encouraged

 

[senaaa]

If I am reading this correctly, it sounds like you have eliminated everything but the motor. Except for the Turnigy battery not showing the same behavior. That is the only thing throwing a curve ball here. Maybe there is a bad contact somewhere, which would be a bottleneck in the amperage, and the higher mAh Turnigy pack is able to push through that? I would replace the motor and see if that solves the problem.

Is this on a drone, or airplane?

If I am reading this correctly, it sounds like you have eliminated everything but the motor. Except for the Turnigy battery not showing the same behavior. That is the only thing throwing a curve ball here. Maybe there is a bad contact somewhere, which would be a bottleneck in the amperage, and the higher mAh Turnigy pack is able to push through that? I would replace the motor and see if that solves the problem.

Is this on a drone, or airplane?

Oh, and hop over to the Introduce Yourself forum and say hi. Pics of your RC's are encouraged

Thanks for the reply.


Yes, this is on an airplane setup.


I actually already tried swapping to a different X3520 motor and a separate power line, and the issue still happens — but only with the 5000mAh packs. The same Forceup and Profuse batteries consistently cut off during the first throttle-up, even with the second X3520. After the first run, it usually works fine.


So the motor doesn’t seem to be the problem — it's more like the initial current demand causes a voltage sag that triggers a cutoff, but only with these smaller packs. Still strange that the X4120 pulls even more current on the same 5000mAh battery without shutting off.


I’m wondering if the X3520 is more sensitive to initial startup surge or maybe has higher inductance/resistance?


Appreciate any further thoughts.

 

[Jerold]

It's entirely possible the batteries you have aren't up to it. Not all batteries are equal and the C ratings are completely bogus. The other thing is the Turnigy is a 2x the capacity, which should provide more current.

Electrical and mechanical properties change with temperature. Something could be changing after it gets a few turns and warms up.I would run it with the Turnigy for a bit and see if the problem gets better or worse. The worst case is you would replace electronics, which would probably happen anyway.

 

[CertifiedMike]

Do you have a servo binding during take off?
End point setting not set correctly allowing the servo to brown out the system?
Servo starting to die?

Just a few thoughts.