Motor turning with RX Battery

Hi Guys, don't know if I'm losing it but I've just been testing the throws on one of my planes, switched my TX on connected a 6v rx battery to a spare channel and the motor started singing?? no battery to the motor, I thought "that's odd" moved the throttle and the motor sprang into life, admittedly it wasn't turning fast enough for flight but it was still turning at quite a speed. I've never seen that before! Normally the motor just bleeps to say it's picked up the signal for the RX.

Is this normal? After all these years have I been checking servos with my face near the prop and the motor can spring into life with a RX battery?

John

Hi John, does this set up normally use separate Rx and flight-power batterries? If so then have you cut back the red lead of the ESC?

If the model uses a single combined battery for servos and Rx, and you are just using a NiMH pack to servo test, then yes of course provided the package voltage is high enough it will trigger the ESC and hence the motor - why shouldn't it?

BEB

I've seen it once before, and that is why I take the prop off or disconnect the motor even for servo setting.

Olly

hello john - would have thought that if everything is connected up the way it normally is then you connect a batt up via the RX..... then as far as the motor is concerned.....it is armed and ready... if you really want to connect with out worrying about the motor....you should diconnect it.....dont want to spoil your good looks etc...

ken anderson ne..1 .... good looks dept.

3 great minds have posted together.......who is correct ?

Edited By ken anderson. on 02/07/2012 15:02:58

Most odd indeed......I would have thought there would be some electrical isolation between the BEC circuitry & the power circuitry with the BEC drawing power from the main ESC battery input before regulating it to 5v (or so) so I can't see how this would happen.....What sort of ESC is it? I can possibly think how it might work with a linear BEC but not a switched mode BEC...

I've never actually done this though....I might go & try it in a bit...

I don't know about a motor turning with a receiver battery, but I got a painful whack in the midriff from the little prop of my ST Models ASW28 last Thursday, with the battery totally disconnected!

I'd just landed it and was carrying it back to the pits - the wind had spun the prop up silently to considerable rpm until it got close enough to me to give me a nip! Luckily I had a fairly heavy sweatshirt on and there was no damage but unprotected skin might have suffered a bit more badly...

What seems strange to me is that all the esc's I have will not power the motor, until armed with the throttle closed. If the throttle is open, even a jot, they complain lodly, until the throttle is closed.

Not one will start with part throttle or full, without that arming process. I have a range of about 7 badges at least (if not manufacturers).

John this is very odd behaviour indeed. I would expect that current could only pass one way through the BEC circuit, and that would be from the flight battery through to the receiver. (having been dropped in voltage)

In this case it seems to be flowing backwards, from the receiver connection to the power side.
It must be an unexpected "feature" of the BEC design.

Just goes to show how careful we need to be.

Mmm. Interesting. I would not have assumed that it wouldn't work the other way - why not? Hence why I was not so surprised. I'm not saying all ESC's could be reversed, but I see no fundemental reason why it should never happen? So I'd start from the assumption that it will! (Murphy's first law - if something can go wrong, or work in an unexpected way, it will!)

BEB

Edited By Biggles' Elder Brother - Moderator on 02/07/2012 15:32:15

I would suggest to either disconnect the bec (throttle connector) and use a RX battery now or disconnect 2 motor leads and use a flight battery. Both would fulfil H&S requirements for testing your servos and I leave the discussion of reverse BEC to the more clever guys...

VA

Edited By Vecchio Austriaco on 02/07/2012 15:56:42

Well I've just tried it on one of my ESCs...I connected a 4 cell NiMH battery to the Rx & the radio powered up fine....servos all working as expected....but the ESC started beeping too & although the motor didn't turn there was definitely SOMETHING going on because the motor "brake" seemed to be enabled.

What I can't see is how power from the Rx battery can get back into the "Motor Power" side of the ESC.....the BEC will draw power in this way but I would have expected a semi conductor or two to stop it going back the other way..........as Chris says it just shows how careful we need to be....

If it's a simple linear BEC such as an LM7805 there's only the "series path element " between input & output of the positive rail. & of course ground is common.

I don't think the problem would arise with a switched BEC.

78xx schematic.......

I can't see an obvious path.......

(Picture courtesy of Chris Bott by the way....)

Additional to the above - a 4 cell nimh would always give a voltage below the ESC's LVC threshold (except for single cell indoor ESCs) so should never be able power the motor. OTOH a 5 cell nimh could present a voltage above the LVC, depending on the ESC settings etc.

Output - R17 - R21 - D2 - R13 - input.

'Twas a very simple ESC Pat with 2 setting...."ON" & "OFF"......

I tried it with a 2 cell LiFE battery into the Rx too because I realised that the 5V or so from the 4 cell pack might be too low....same result....

Steve, it doesn't really matter how simple the ESC is there's always some LVC setting even if it's automatic. If the voltage drop due to the resistance of the backwards connected BEC in parallel with the motor windings drops the voltage seen by the LVC the case you described of a beeping ESC with the brake active would be the result I'd expect & probably more likely in most cases than the OP's experience.

Yeah I know Pat.......tongue was thrust firmly into cheek whilst typing......

I have to say I am surprised by this though as I didn't see how it could happen. I must try it with a switched mode BEC equipped ESC......I would be really surprised if that did the same......

Thanks for all the reply Gents, Looking at your replies it seems it shouldn't happen so just to make sure I've not imagined it, I've just done it again with the same result.

the ESC is a Black Mantis 50 amp SBEC programmable, there is world of info about programming it on the Giant Shark site. perhaps some of you more technically minded might be able to glean something from that.

Biggles I don't use a seperate battery only for testing the servo's, I "thought" it was safer than using a Lipo attached to the motor

Thanks

John

I'm surprised that this could actually cause a motor to turn, I suppose it's most likely with a small, low KV with high res windings motor or maybe a small indoor brushed type.

Something similar - occasionaly I've charged a model's battery in the car with the charger plugged into the lighter socket & had the radio playing. With my car it's necessary to have the key in the ignition & turned to the first position for the socket to be live, ditto for the radio. If I've switched off the ignition before removing the charger & battery the radio stays on. This means that the battery that been charged is powering the radio & any other ancillaries else that might be connected to the socket.