Motor turning with RX Battery

[Peter Beeney]

With the greatest respect, surely the BEC, of wherever flavour, is rather irrelevant here anyway? The battery is simply taking the place of the BEC and powering up the ESC. Just simple case of Ohm’s Law, maybe? A good schematic diagram of an ESC would help to establish the current path, I’ve never been able to find one but I’ve not looked very hard, either. I’d also say this has to be a pretty low resistance line otherwise I think the motor would never run; so I’m not fully fully convinced yet that all the necessary current flows through the internal resistors in the regulator. When I get five minutes I’ll check out a few regulator facts, such as ‘can a current flow backwards through the device’?

I think I’ve related a least a couple of times in the past in various posts how a colleague proved the hard way how a receiver battery paralleled with a BEC kept the ESC functioning when the ESC/ BEC overheated. When on the one occasion the battery wasn’t switched on the model crashed and was destroyed. This is perhaps not really significant here but it might help to show that the BEC is only operating in parallel with the rx battery.

PB

[Biggles' Elder Brother - Moderator]

Oh I do love being right!

BEB L3....too smug for his own good Dept

Edited By Biggles' Elder Brother - Moderator on 03/07/2012 14:11:18

[PatMc]

Posted by John Cole on 03/07/2012 10:21:32:

Or disconnect the throttle cable from the Rx; that's what's powering up the ESCand (in turn) the motor.

My post was ambiguous. When I said I unplugged the BEC I should have said the ESC. I've never yet used a BEC that's seperate from the ESC.

[Chris Bott - Moderator]

Peter I can see how the Rx battery would keep the signal and microprocessor side of the ESC alive and even keep it going if the BEC overheated and shut down. That's pretty straightforward.

Surely the issue here is that the Rx voltage is also feeding the power input side of the MOSFETs. It must be, as the MOSFET's have to be sequentially switched on and off to make the motor go round. Unless of course its the MOSFET drive signals passing through the MOSFET's to the motor. But I doubt that very much indeed.
I would have thought that normally the flight battery positive feeds only the power input of the MOSFET's and the input of the BEC?
Hence thinking the Rx battery is feeding backwards through the BEC.

[Peter Beeney]

Chris - I would consider it might be exactly the reason the entire ESC is powered up is because the microprocessor and possibly other components are switched on due to the supply from the rx. So there may now well be other paths between the BEC output line and the main battery input line. But I don’t know this for sure, it’s only supposition on my part.

Looking at the schematic drawing of the L78 regulator, it would seem, as PatMc said, there there is a route from the output to the input via R17 and R21, diode D2 and R13. There is also another parallel path via R11 and R12. Resistor R12 I’m pretty convinced is the overload limit resistor, so this will be very low, if the reg was rated at max 2 amp I’d think it would be in the order of 0.3 ohm, but what the values of the other resistors are I’ve no idea. However if the resistance of our motor was say half an ohm, then even if the resistance total added up to only half an ohm there would still only be three volts at the motor; this resistor chain is now in series with the motor. Would the motor start at three volts? I’m still very tempted to think the voltage is fully maintained so the route will be a low resistance, i.e. unlikely through a resistance, perhaps.

Whilst fiddling with regulators in the past I can’t remember ever seeing a voltage on the input when there is one on the output but that is definitely not to say it doesn’t happen. I would certainly have to check this out before I could say one way or the other.

Beyond this I have little idea, but as I said maybe I might be able to have a dabble with a regulator and possibly an ESC to get a little further. Most of what I’ve thought so far has only been guesswork, anything else would just be pure fantasy!

I think to make any further guesstimations as to what is going on I would really need some more information.

PB

[Chris Bott - Moderator]

I think the most important thing is that we know it happens with some speed controllers.

I don't think putting a voltage on the output of an unpowered BEC is a great idea anyway, so I'll continue to remove the speed controllers servo lead from the receiver if I ever use a RX battery.

[Erfolg]

I really do not know what is supposed to be happening.

Not being electrically minded, I either supply the system either

a) via a lipo to the ESC.

or

b) via a set of Nicads to the Rx and take disconnect the + feed from the ESC to the Rx.

Not being electrically orientated, I just try and operate everything conventionally.

Am I correct in thinking that what is happening is that the Rx is being powered with Nicad pack, whilst the leads to the motor are still connected?

Why would you do that if I am correct? Or is it something else?

[Biggles' Elder Brother - Moderator]

Yes Erfolg, that is correct. What is happening is an NiMH pack is being connected to the Rx (with the motor also connected to the Rx via the ESC - but no Lipo is in place). Its being done presumably to check the servo function without connecting a flight-power battery. It appears that the ESC can indeed, in some cases at least, effectively function backwards! That is to say the NiMH pack can power the motor via the Rx's power rail and the ESC's throttle connection.

BEB

Edited By Biggles' Elder Brother - Moderator on 04/07/2012 01:24:40

[Erfolg]

I am still at a loss, why use a separate pack if set up for supply via the ESC?

I have read on this forum that it wise to remove the +ve supply to the Rx, if a separate pack direct to the Rx is used.

I think this was suggested as sensible on the basis that two supplies could have unpredictable affects.

I can see this is a little different, yet it appears to fall into the same line of argument.

I am also assuming that no claim that the motor spontaneously sprung into life, irrespective of the Tx throttle position?

[PatMc]

It's sometimes convenient to plug a spare 4.8v battery into an electric powered model in order to set up & check Tx mixes, control throws etc. I do it in preference to connecting the normal drive lipos so as not to have to remove the prop but I always unplug the ESC. I had wondered if leaving it connected would make any difference, now I know that it does.

[Peter Beeney]

I’ve alway thought that it’s very fortunate that the aeromodeller electrical power system in general is very tolerant of a variety of ideas and wide operating techniques. I have my own ideas, which sometimes are slightly different from others.

The salients points as I see them are firstly that this situation as posted is unique, this will only happen when a battery is connected to the receiver to be able to check the servos without connecting the main battery. I suspect this is not really a general practise, so it may not occur that often anyway. So to avoid this battery possibly running the motor and turning the propeller it’s a good idea to either remove the propeller or remove the throttle cable from the receiver. Which is only repeating other people’s sound advice anyway.

If, however, you wish to leave the battery in situ and permanently connected when flying, then that is perfectly acceptable, too. The battery then powers the receiver and servos, and also the 5 volt rail on the ESC, but this is not obvious in any way to the pilot. Or of interest, either. On the other hand, if you think this might not be a good idea and you don’t want to do it you certainly don’t have to, as in the vast majority of cases, I suspect. this is in very much the same way the practise of removing the red wire from one BEC when two ESC’s are coupled to the same battery, for instance. Leaving these connected together is usually frowned upon, too.

With regard to the motor unexpectedly starting, which can be a characteristic of electric motors in models, there is one aspect which I personally think is important but does not generally seem to get much attention. I don’t have very much in the way of electric models at all, but in the past I have persuaded some electrical pilots to gently run their model’s motors, on the ground of course, and then switch the tx off. In every case the motor shuts down, indicating that the ESC will not function without a throttle pulse; and so by definition it will not start the motor, either. This is why I said earlier that I would personally include a switch in the BEC/rx line. If the receiver has no power the motor cannot start. As always, it’s perhaps not 100% secure, but the chance of a fault occurring that will precisely duplicate the throttle pulse is remote enough for me to ignore it. If you wanted to leave the rx live, switching the signal wire instead would do the same trick; by the same token, the ESC would still not be able to receive a throttle pulse from the receiver.

Safety is the Big Issue here, for me, at least. I feel I can never too careful…

PB

[PatMc]

Having a switch on the BEC relies on the user remembering to unplug the battery. If he/she forgets the battery is liable to be run down & if it's a lipo probably permanently damaged.

[Erfolg]

Although it may seem tiresome, does not the use of the Lipo and removing the prop has a degree of certainty and convention that has merit as a preferred method.

I do not know what others think, with respect to folding propellers, I normally restrain the prop by use of an elastic band. This stops the motor timing being achieved, if the throttle is accidently knocked open. If this occurs I quickly move the stick to minimum.

[PatMc]

Posted by Erfolg on 04/07/2012 16:25:50:

Although it may seem tiresome, does not the use of the Lipo and removing the prop has a degree of certainty and convention that has merit as a preferred method.

I assume your refering to the situaition of the OP.

In which case not connecting the drive battery whilst unplugging the ESC is more convenient & gives absolute certainty.

[John Laverick]

I apperciate that unplugging the esc lead or removing the prop whilst messing with servo's is the safe way of doing things, however as can be seen by the differing opinions, I actually thought using a seperate battery on to the RX whether the ESC lead was connected or not, was safe? As Steve says, It didn't spin very fast but I wouldn't want to get my fingers/nose /face in there.

Guess the outcome is Take the prop off no matter what.

thanks guys

John

[Peter Beeney]

With respect but I’m rather afraid I’m not very convinced that switching the BEC is going to make any difference to anything as regards the batteries going flat. I’d tend to think that if any batteries are left connected to any electronic components they will eventually be totally discharged, to avoid this the battery has to be disconnected anyway.

Assuming a lipo is connected as normal, a switch in the positive BEC to receiver wire simply disconnects the BEC supply from the receiver, thus making the receiver inoperative, it doesn’t disconnect the input supply to the BEC. This only prevents any throttle signal starting the motor. But if the lipo is left connected it will still flatten the battery. A friend did this, and then went to Australia for eight weeks. He said the battery recovered up to voltage in the first instance, but I don’t know if it continued to be ok. A switch in the signal wire here will allow the receiver and servos to function as normal, but the motor cannot run in this case, either. The advantage of this would be is that you can still check the servos for correct movement etc. but not accidentally open the throttle until you are ready to take off.
If the situation is as the OP describes, using a temporary rx pack, then a switch in the positive will only prevent the current from the temporary rx battery getting back onto the ESC. If the switch is in the signal wire from the BEC to the the rx then the temporary battery will still power up the ESC but the motor will be unable to start because the throttle signal will not reach the ESC. I’ve personally always found switches to be extremely reliable, so I’d have no worries using a standard switch.

With regard to leaving the lipo's connected and switched on, when I first tried a 2S as a receiver pack for an i/c model I connected in a flashing led to indicate it was switched on. But by the same token for an electric model it could be read as an indicator that the model is fully armed and ready to fly!

Surely the system is flexible enough to be be able to accommodate more or less whatever idea you think is the best for you? I don’t know, but I suspect by far and away the biggest cause of motors starting unexpectedly is the throttle stick being inadvertently operated. Do faults occur that start and run the motor without warning? I never heard of it happening, but I’m sure it’s possible.

But I have to say, I would most certainly leave the prop off until all is proven to be working ok, as well!

PB

[Steve Hargreaves - Moderator]

Posted by John Laverick on 05/07/2012 07:55:15:

I actually thought using a seperate battery on to the RX whether the ESC lead was connected or not, was safe? As Steve says, It didn't spin very fast but I wouldn't want to get my fingers/nose /face in there.

Guess the outcome is Take the prop off no matter what.

I thought it would be too John but there you go.....just shows you can't be too careful.

Removing the prop is the ONLY way to ensure you are safe.....

[Erfolg]

I do agree that I have not been aware of any motor suddenly bursting into life spontaneously, other than the throttle nudge described. I have read of anti electric people suggesting that all sorts of unexplained issues with motors starting, to-date I have dismissed these as pure hearsay, due to lack of evidence.

I do agree that operate the ESC conventionally, via lipo, or separate power source with disconnected +ve to Rx, and remove the propeller when working on the model. This is a safe practise, minimises risk.

[Biggles' Elder Brother - Moderator]

Oh, chaps I'm afraid motors can "burst into life" without touching the throttle stick - as me how I know!

I've discovered two ways to do this is as follows:

1. I'm a Futaba user, now my electric experience is not extensive but its growing nicely. One thing I have noticed - I've yet to find an ESC where the Futaba's throttle direction doesn't have to be reversed. So, you connect everything up and you get the "programming bleeps". If you now go into the menu and reverse your throttle setting - instant excitement results as your model decides to try to take to air inside the shed!

2. You connect up your model, you're a lot smarter now and reverse the Futaba throttle in advance! You then suddenly realise you've made a mistake - you intended to set the model up as a two servo wing - ie with the ailerons on separate channels. You go into model setup switch the model to separate ailerons - and bingo instant excitement again!

Now I fully accept what you are thinking - what a whally, reprogramming his Tx with the model connected up and live. But if you come from an long established IC background you've been doing that for years and its never been a problem! So initially it comes as a bit of a shock I can tell you. Those of you who have "doing electric" for years wont fall for this but us "late developers" do!

So its not quite a case of "motors never start unexpectedly" - and I suspect a lot of the ic guys claims in this direction may well relate to experiences similar to the ones I have had. I now never work on a "live" model unless I have to and then I always remove the prop just in case I revert to my old ways!

BEB

[Steve Hargreaves - Moderator]

BEB you are not alone......I too have done exactly this......

I had a "live" electric model sat on the bench & reversed the throttle channel by accident (I was aiming for the rudder channel). The model took off like a scalded cat & smashed into the workshop wall before thrashing itself into a frenzy on the floor.

I stood there like a nit not believing what I was seeing & then when I realised what I'd done not believing I'd actually done it.

Luckily it was only a small model with 120watts or so......I shudder to think what might have happened with a larger model.

Now I know there are few people as daft as me & that you would never do anything like that but please learn from my experience & take the prop off......to paraphase.....a spinning shaft gathers no fingers (or arms, cheeks, sleeves, noses etc).....

Edited By Steve Hargreaves - Moderator on 05/07/2012 11:07:34

[Peter Beeney]

It’s not really the bench testing that I’d be too concerned about, Steve, in the general scheme of things I’d have thought the prop would not be on the motor until the last moment anyway. It’s the unrestrained model all on it’s own in the pits that suddenly bursts into life that I’d be taking a closer look at. I know this can happen, I’ve actually seen it on three or four occasions, fortunately with no harm done, although in one particularly bizarre case, a pilot was struck on the back of his hand by the prop on his Easy Pigeon as he assembled it, he took himself off to A&E but they just stuck a plaster on it and he was soon back flying.

All of these proved to be operating errors, though, nothing that was caused by faulty equipment. At this stage, with the model ready to fly it’s quite unlikely that the prop would be removed so I’d treat as potentially “live” all the time.

So I think you’d agree that in all these cases, and also the ones quoted in the posts above, an inline switch would have prevented the motor starting. I will agree that the battery should have been disconnected, but that does not always happen; and in a case such as the reversed rudder/throttle incident the battery would have be connected anyway. A switch would normally be off, unless you specifically wanted it on, as in the case of a standard i/c model. So in your case when you reversed the throttle the motor would not have started but you would have realised something was wrong because the rudder would have still been reversed; also whatever other actions you may have taken the motor would not have started. Please don’t even consider that I’m being critical of your actions in any way, though, it can certainly happen to anyone, and there but for the Grace of God…. But it is a good example of the advantages of an added Safety Addition!

As I said, I’m not much into electric now, but at one time I was involved with hotliners and I also owned and flew one myself for some time. These were not BEC powered, of course, always a separate rx pack, so unexpected starting was never a problem. But they still closely hold your attention apiece, when you are holding the model a geared Aveox swinging a 16 by13 folding prop at around 10 thou is intimidation with a capitol I! One mistake could be one too many! As I’ve said before, here the benefits of a good programmable transmitter are manifold, on the MPX it is a simple matter to assign a switch to the throttle control so that the throttle stick will not operate until it’s switched on. I always called this the ‘Throttle Lock’ and all my electric models were so provided for.

Of course, it will be considered that someone will always forget the switch, and that is undoubtably true, but modellers generally are well conditioned to switching their models off when it’s landed, so I see no reason why this could not be true of electric models also.

Hope this is at least of some interest…

PB 


[Erfolg]

Since restarting modelling some 5-6 years back now, I have only flown electric. The club I am a member is electric/glider only.

In that time I have never seen anything approaching a spontaneous motor start.

I also fly Futaba, I am aware that the throttle max is reveresed with Futaba , relative to ESC's. Non of the ESc's I use and most are budget, will start, unless the the expected minium setting is experienced. When first setting up, if the ESC function is not reveresed, all my ESC's sit there complaining, giving the I am receiving a signal level I do not expect. I then disconet the Lipo, reset the Tx and repeat. Thereafter when powering up, unless the minimum value is detected they complain, until the setting is at the minimum level expected.

My main concern when arming the set up, is that I keep behind the propeller with my hands and face. This is not always easy, due to the lipo and plug invariably being near the motor.

Have I ever done anything stupid, erm, well, yes. According to folklore, the world should have imploded at least twice. I once managed to connect two batteries together, I instantly knew what I had done, and disconnected them, one battery had become warm, thankfully it did not explode. The other stupid thing I have done is advance the throttle to full, whilst setting up on the bed, I have been assured that doing anything like this would shred everything. No it does not, the prop hits the duvet, stalls, you think what the hell, and follow up by closing the throttle, and think, why did I do that? Damage done nothing, thank be to God, as it could have been life threatening if my wife knew.

I think there are dangers, yet not half as serious as some would propose. That is why I am advocating sticking to standard, simple methods of operation, which should minimise accidents. I guess the other aspect is I am deliberately staying at less than 400w, 250 being about normal

I did think a few weeks back, do parents let their kids play with IC motors today. What would Rospa make of any parent who did?

[Biggles' Elder Brother - Moderator]

Posted by Erfolg on 05/07/2012 17:33:42:

In that time I have never seen anything approaching a spontaneous motor start.

I also fly Futaba, I am aware that the throttle max is reveresed with Futaba , relative to ESC's. Non of the ESc's I use and most are budget, will start, unless the the expected minium setting is experienced. When first setting up, if the ESC function is not reveresed, all my ESC's sit there complaining, giving the I am receiving a signal level I do not expect. I then disconet the Lipo, reset the Tx and repeat.

Ah, but there is the key element Erfolg - you disconnect the Lipo - how very sensible! The problem I'm talking about is that long accustomed ICers like me don't necessarily think to disconnect the lipo before we start reprogramming the Tx - and that's what causes unpredictable behaviour and , I'm suggesting,leads to reports of "magic start-ups"!

A second point, you talk of no damage - very good. But how much punch was in this power train? Yes a small motor wih a 1300 3s behind iy might just stall against the duvet cover. A 60 equivalent powered by a 6s and throwing out over a kilowatt wont! It will rip anything in its path to bits! Including parts of you! I don't think electric powered models should be afforded any less caution and respect than their IC cousins.

BEB

[Erfolg]

Not even I am stupid enough to operate an electric motor over 300w in the bedroom.

For my part, the more powerful the set up, the more cautious I become.

I do have reservations about the set up with some models. I have just purchased a Parkzone Albatross. It is very tempting, to connect the lipo while holding the model upside down. Unfortunately, you end up with prop, potentially inline with your wrist. Although reluctant, I place it on the ground and approach from the back. Not as convenient as many models, although the battery hatch is far less intrusive.

There is another issue, if the motor were to try and start, it can, as where normally my propellers are in contact with ground, restricting rotation, thereby preventing establishing timing. In this configuration, it would be of, until it struck the ground, breaking the propeller and then potentially wrenching the motor out of the model, due to out of balance forces. Having models shed a blade in flight, I have seen the damage that happens.

These things have not happened to me, but I do worry, what might happen.