How has my brushless motor killed my RX?

[Rob Buddle]

I post this in the hope that the collective can explain this in a way I will understand. Heres the scenario: My 6 year old Kyosho P40 (electric) nosed over on take off at full chat. No more damage done than a snapped prop and injured pride (or so it would appear). The next time I come to fly it I plug in the 5S pack and the 60A ESC starts beeping but it will not bind, and there is no light on the Spektrum RX (I use the built in BEC on the ESC to power the servos). I plug in another models RX, and that too fails to work either in this model or anything else. o I retire to the workshop and replace the RX and the ESC (I assume that the ESC has fried the RXs). Everything works and I fly the model 7 times without incident. On the 8th flight there is a noise like the prop coming loose (screechy whirring) and I loose flight power. I throttle back completely drop the wheels and flaps and call for an emergency landing. It rapidly becomes obvious I'm not going to make it, so I flip the gear up, but nothing happens and the model stops responding to flight inputs and lands itself in the crop, fortunately with no damage. The ESC is beeping (saying it has power but no radio signal) and I have no light on the RX. There is an electrical burning smell coming from the engine and (with the battery disconnected) when I turn the prop by hand it feels like the engine is running in thick treacle. The engine is one of the original Turnigy SK 4250 650kV units and has always been in the model. Can anyone help me identify what has happened? Are the ESCs both actually okay, and is it just the motor and the RXs that are toasted? why did the ESCs allow the (presumably shorted) motor to cook the RX? And if I have a nose over incident in the future, how can I test to see if the motor is okay? Before this "landing" the motor turned over fine and there were no strange noises. I really would like to be sure of what is okay and what isn't before I plug any of the components back together. So far it seems to have killed 1 motor, 2 ESC and 3 RXs. It's almost enough to drive you to glo-fuel

[Denis Watkins]

This needs unravelling bit by bit Rob, as there is a lot going on here.

My contribution is that beeping, usually signifies low voltage or high throttle, and these were present

Non of this equipment tolerates sudden shock well, especially RX.

Motors do not tolerate being prevented from spinning under power at all and bent motor shaft is usual

Screeching is loss of timing, as a result of some sort of ESC reset/ voltage anomally.

More to follow from the guys am sure

[Rob Buddle]

Thanks for the input Denis! The beeping present here is the type you get if you connect the motor battery but don't power on your TX. I should also add that the ESC is a HobbyWing 60A Platinum unit with switching BEC. Not sure there is a bent motor shaft at play as there was no noise or vibration noted in the previous 7 flights. It could be dislodged magnets, but would that allow excess voltage into the RX? And would the ESC also be damaged?

[Martin Harris - Moderator]

Have you measured the BEC voltage? Perhaps an overheated ESC has damaged the BEC allowing over-voltage from the flight battery on to the receiver?

[Rob Buddle]

Sorry Martin, I should have mentioned this in my original post; after killing the first two RXs I did indeed measure the BEC inputvoltage to the RX; it was spot on 5.6v even at the point of motor battery connection (measured with a digital fluke meter). As I believed the esc to be the problem, I was quite surprised at this, so although I replaced the esc for the subsequent flights, I haven't thrown it away yet!

[Chris Walby]

Perhaps two problems

Motor insulation breaking down under high load conditions (could be an inter winding fault) hence burning smell works okay with prop off and low load + explains the "phasing sound", could test tethered for short period at high throttle, but you might kill an ESC in the process

BEC supply killing RX, this is proving a little tenuous, could be when the motor starts to fail, but the motor power electronics should be quite separate from the BEC in normal conditions, but in fault is possible or something else

Could you list the components mfg/model e.g. motor, ESC, RX & servos (retracts)

[Frank Skilbeck]

I had something similar (different radio and motor) caused by loose magnets, which catch against the fixed part of the case, my telemetry warned me of overcurrent and the motor was screeching. In my case the magnets moving caused drag on the motor and it then drew more current to try and overcome this, if I hadn't had telemetry then the ESC would have been overloaded and possibly overheated.

On the ground the motor was very stiff to turn.

[Chris Bott - Moderator]

I would test each component separately from the others.

If the motor spins freely once disconnected from the ESC, the stiffness when connected would indicate a short in the ESC. If it is still stiff then you know there's a motor problem.

I'd check the voltage of each ESCs UBEC without it connected to a Rx and I'd re-test each Rx using a known good Rx battery or other good supply.

One other thought, it is possible for a BEC to shut down if it gets too hot. This would remove the supply from a Rx. It might seem ok again later when things have cooled.

[Denis Watkins]

It is unlikely Rob, that excess voltage passed to the RX

There would have to be catastrophic breakage of components and insulation damage which is very unlikely

A possibility, though am clutching at straws is

As the current collapsed abruptly in the motor windings and surrounding cables, that

this induced a damaging magnetic field

But this is pure conjecture

Edited By Denis Watkins on 14/07/2017 09:36:10

[Nigel R]

Things you do know, at a high level:

1 motor definitely dead

2 possibly cooked ESCs

3 possibly cooked RXs

There is an awful lot of stuff inside an ESC and an RX. I'm tempted to suggest, don't throw good money after bad - bin the lot and replace.

As an aside:

"I did indeed measure the BEC inputvoltage to the RX; it was spot on 5.6v even at the point of motor battery connection (measured with a digital fluke meter). "

I'd want to scope the BEC output at varying loads, after this collection of failures. You may find high (5S level) voltage spikes, or that the supply is now very noisy, causing RX issues - it is a more complex switching BEC not a simple linear job in the Hobbywing 60A I believe.

[Chris Walby]

Chris B off topic, but IMHO

"If the motor spins freely once disconnected from the ESC, the stiffness when connected would indicate a short in the ESC. If it is still stiff then you know there's a motor problem."

May not be totally correct, I have a few perfectly good ESC's that apply a slight drag/brake on the motor when powered up, so not clear evidence that the ESC is knackered.

Although the rest of your post is spot on with possible shutdown of the ESC/BEC under excessive load/fault conditions

Cheers

[extra slim]

Had back EMF blow the throttle channel out on a RX once, rest of RX was ok, but throttle channel goosed.

[Peter Christy]

A little bit of lateral thinking here, and its difficult to diagnose at a distance, but....... I note that the BEC is a "switching" type. These reduce the voltage by switching the output on and off rapidly, and varying the mark/space ratio to provide the correct *average* voltage. A capacitor is usually used to smooth the output voltage.

Without this capacitor, the average voltage (as measured by an analogue meter) will still be 5V, but the peak voltage will be much higher - typically the input voltage of the flight pack.

If the smoothing capacitor has failed, then the receiver would be subjected to the full flight pack voltage many times a second, but because it is being switched on and off rapidly, the *average* voltage will still be within limits. Its the peak voltage that would cause a problem.

Did you use an old-fashioned multimeter or a digital one to measure the voltage? A digital one would probably give erratic readings if the switching output was un-smoothed, whereas an old fashioned multimeter would read the average.

You would really need to check with an oscilloscope to see clearly what is going on, but the above scenario might explain your problem.

--

Pete

[Rob Buddle]

Thank you also much for your input, all of it worth considering.

Here are the components involved:

Motor - Turnigy SK4250 650kV

ESC - Hobbywing Platinum 60A with switching BEC

RX - Spektrum AR7000 (all 3 of them were!)

Servos - 4xInoLabs mini digitals, 1xJR retract servo (sorry, don’t have all the details)

Meter used to check the voltage from the ESC is a digital Fluke meter. No fluctuations were noted.

[Rob Buddle]

Binning the lot is tempting! However I will test (again) all the RXs to triple check they are all unresponsive, and then I shall send them off to Spektrum in Germany (as I have nothing to loose).

Testing the ESCs will be more difficult and risk killing another RX. I am still concerned that some apparent failure on the motor has been able to get through to the RX. Aside from brown outs or more catastrophic short outs of the ESC, I din’t think that was supposed to happen. So the temptation to put them both in the bin is quite strong, but it’s a shame as I am waste averse and they were good controllers.

The motor I am pretty sure is toast. I will pull it apart as soon as I can get it out of the plane and report back what I find. I guess magnets could have come loose on the nose over but not caused a problem for the ensuing flights somehow, until they jammed the rotating casing causing the second short out?

Any other ideas/suggestions are of course welcome.

Thanks again!

Rob.

[Chris Walby]

Hi,

This is what I would do!

Motor - Strip and visually inspect, testing the windings will be tricky as the resistance will be very low, but any difference is an indication that there is a winding fault, especially if there is a reading between any winding and the metal casing

ESC - Hobbywing Platinum 60A with switching BEC, I have a test rig that uses a servo tester to drive the ESC and motor (I can measure static thrust), I will test the ESC without killing an RX

RX - Spektrum AR7000 (all 3 of them were!), good idea and nothing to loose

Servos - 4xInoLabs mini digitals, 1xJR retract servo just can't see these producing any of the described symptoms, test in aircraft for any unusual performance

[Peter Christy]

It certainly sounds as if the motor has triggered the failures you have suffered. However, you are quire right that this shouldn't work its way through to the receiver! But just because it shouldn't happen doesn't mean it won't!

Does anyone in your club have a 'scope? That's what you really need to check the BECs. No need to plug a receiver in, just connect a motor and flight pack and see what's coming out of the BEC.

Of course it may be load or heat triggered as well, but just putting a 'scope across the BEC output would be a good place to start.

P.S. I'd be interested to know how you get on with the AR7000s. When I sent one back to Horizon UK a year back, it was returned un-repaired with a curt note saying it was obsolete and they couldn't help!!!

--

Pete

 

Edited By Peter Christy on 14/07/2017 22:28:19

[Frank Skilbeck]

Posted by Rob Buddle on 14/07/2017 21:15:45:

The motor I am pretty sure is toast. I will pull it apart as soon as I can get it out of the plane and report back what I find. I guess magnets could have come loose on the nose over but not caused a problem for the ensuing flights somehow, until they jammed the rotating casing causing the second short out?

Any other ideas/suggestions are of course welcome.

Thanks again!

Rob.

Rob, when the motor was stalled it would have seen a surge in current and got hotter, the glue that keeps the magnets in place may have been weakened and over time the magnets move back, they still stick to the case by magnetic force, but slowly slide out of position until they rub on the fixed part of the motor case.

[Nigel R]

Don't risk an rx when testing the esc. You can get servo testers from eBay for a couple of pounds.

[Rob Buddle]

so finally managed to get some time to strip out the motor and disassemble it. The motor wire insulation inside the case is falling apart (see pictures), so possibly the source of the burning smell and the short? No sign of any magnets being loose though.

[Nigel R]

That must have got quite hot!

I imagine there could be quite a bit of arcing going on with all that exposed copper.

[Frank Skilbeck]

it's like the stator rotated against the back plate and pulled the wires through the back plate severing the insulation.

[Peter Christy]

Looking at the damage in the motor, and knowing how much current they draw, I can understand how that may have fried the electronics in the speed controller.

Now that *shouldn't* get through the BEC, but bearing in mind that these things are built down to a price, not up to a specification, I can understand that it might!

Personally, on anything more than a small model, I prefer a separate battery pack for the Rx, rather than rely on a BEC. At the heli world champs a few years back, the reigning world champ had a solder joint on one of his LiPo connectors melt part way through his flight, shutting the motor down immediately. Luckily, he was running separate power for the radio, and was able to make a safe emergency landing, thus preserving a very expensive model! (He went on to retain his title!)

I learned a lesson from that, as did many others who were present!

--

Pete

[Martin Harris - Moderator]

I experienced a similar event flying a friend's Extreme Flight Extra - a sudden dead stick (yes, it is electric) after which I landed as you'd expect and immediately on touchdown lost all connection to the model. On retrieval, we found that the more than adequately rated ESC had failed (motor was fine) and must have basically shorted the input as both solder connections to the board had melted.

Had the model ended up as a suitable filling for a bin bag, who knows what the diagnostic inquest would have concluded...

[Chris Bott - Moderator]

I have seen electric models go in BECAUSE they have used a separate battery for the Rx. So you pays your money and makes your choice..