How has my brushless motor killed my RX?

[Chris Walby]

I use a separate BEC other than the ESC, but power it from the balance lead of the same Lipo...simples as you don't have to worry about a RX battery failing.

IMHO its worth having a separate BEC for U/C as likelihood of a stalled servo holding the RX supply down is higher, if its any of the other servos you'll either be able to fly or not! but for the additional weight/one off additional wiring its worth it.

[Martin Harris - Moderator]

Hmm...over the years I've had several balance leads come adrift from the cell terminals so I wouldn't be too comfortable about powering a model from them.

[Rob Buddle]

I think after some time and testing I have a culprit.

After fitting a new more powerful ESC and a new (different make) RX, I connected the wing and a motor battery to test. Smoke immediately began to pour from the (JR) retract servo.

So a stalled servo (caused by a temporary jam in the retract actuation mechanism) seems to have been the cause of this, drawing enough current to make the servos wires very hot to the touch, so presumably cooking the RXs and one ESC.

Testing has shown that both ESC will run an engine, but the BEC circuits on one of them is dead and transmits no power. The other appears fine.

Not sure if there is actually any problem with the engine that replacing the silicone sheathing on the wires wouldn't fix? The bearings and magnets seem fine.

One central retract servo is almost a thing of the past (this model is over 7 years old now) with the advent of readily available servo-less systems. As I can't retro fit, I shall replace the mechanical arrangement with a single MG servo for each side, working directly on the retract unit. I will also investigate the use of a separate power supply for the retracts, a small battery with nowhere near the ampage of a motor pack!

I am not sure I can trust those ESC in a model, one clearly is damaged (the BEC is dead) but seems to work otherwise. Perhaps they could both be used, but only with separate BECs?

Not sure it's worth sending the RXs to Spektrum. I expect they are beyond economic repair, even though replacement with current Spektrum units would be of the order of £50 each. Is it likely to be something they could/would fix if caused my a massive current overload?

Thanks for all your help and input. I hope this helps someone else along the way.

Rob.

[Chris Bott - Moderator]

Rob I'm glad you've found a culprit.

The dead receiver has very probably had a PCB track burned through like a fuse. I'm not sure how easy those are to pop apart? But it might be worth a look. The + & - connections on all the servo sockets will be common right across. It should be a simple repair.
Only you can decide if you'd trust it afterwards though.

[Bob Cotsford]

As you say, fit a small 2S LiFe or NiMh pack to drive the retracts. It's much the safest way with mechanical retracts. Modern electronic units have current limits and auto shut-off but a mechanical unit that jams will just keep loading the servo - as you have found out! It's easy enough to fit a dedicated pack, just use a Y lead between receiver and retract with the battery in the other leg of the Y, then disconnect the red wire from the Y lead plug going to the receiver.

Edited By Bob Cotsford on 17/08/2017 10:45:31

[Engine Doctor]

Why not start again using an Opto ESC and a separate BEC unit or battery for Rx an servos with another battery for retracts. At least that way if the ESC or retracts go us you still have control of the model. Built in BEC are  fine until it goes wrong and takes a model with it.

[Chris Walby]

ED have you not missed the point...or perhaps me!

The failure of the ESC was caused by a jammed/failed servo drawing excessive current and damaging the RX and ESC BEC in the process.

There was nothing wrong with the ESC BEC until it was damaged by the other components.

I agree with all those that recommend a separate BEC for the retracts, but can't see the advantage of adding another battery and BEC, just more components to check and go wrong in my book for a normal installation. As for the "stressed/damaged " ESC Rob has it would be test rig material or bin for me.