Safety plug

[Geoff S]

My DB Cirrus (Gypsy) Moth build is continuing apace (despite no updates for a while on my build thread) and before long I'll be doing the electrical installation. Batteries will be replaced through a hatch under the motor so, unlike all my other conversions the connections won't be easily accessible when the model is on the runway so I think it might be a good idea to fit a disabling plug.

The usual way is to put the plug in the battery circuit so the model is completely inactive (ie neither receiver nor esc powered). I'm not too keen on adding extra battery wire length if I can avoid it but I read somewhere that a disabling plug could be fitted to one of the motor wires.

This would still make the model 'safe' in that the motor couldn't burst into life taking the odd finger with it but would have the advantage of allowing control checks to be made.

Has anyone done this? Is it effective? Are there any disadvantages?

thanks

Geoff

[Colin Carpenter]

Geoff. Have a look at the H King XT60 panel mount kit ! I have used them for ages with no problems . Colin

[Allan Bennett]

I don't think a disabling plug in a motor wire is a good idea; A brushless motor can be ruined if it's powered up in that state.

With my DB Piper Cub my battery is also accessed via a hatch beneath the motor, behind the prop, so I've got an arming plug in the +ve battery wire. It's mounted in the cowl itself, so battery leads are not significantly lengthened.

[Geoff S]

Colin: I already have an XT60 panel mount kit to use. It's just where to put it both electrically and physically.

Allan: What's the failure mode of the motor if one phase of the esc output fails (or, in my case, disabled)? If the throttle is disabled as mine would be (I have both a switch and a verbal warning from the transmitter) then there wouldn't be a current source to damage the winding phases still connected. You may well be right. I just don't know.

I would like to avoid, if possible, an ugly non-scale lump of wire sticking out of the side of the model. I was hoping to put the plug in the front cockpit where it would be better hidden. The esc is going to live in the what would be the fuel tank bay if it was glow powered right next to the front cockpit instrument panel so, if I adopted the more conventional electrical siting I'd be adding about 100mm at most to one battery wire. That's not much inductance, which is the killer rather than resistance.

Thanks for the comments

Geoff

[Andy48]

I have some very long battery leads in my Puppeteer, it hasn't done any harm so far despite all the doom mongers. The safety plug is actually the steering yoke in the cockpit. Not a very good photo I'm afraid, but you can get the idea.

[Geoff S]

That's encouraging, Andy. I guess your power train will be in the same region as mine - about 600 watts on a 4S LiPo. I think I'll just bite the bullet and put the plug in the battery side of the esc and hope for the best. Of course if there's a problem, I'll blame you totally

Geoff

[Denis Watkins]

[Andy48]

Posted by Geoff Sleath on 08/04/2018 21:28:12:

That's encouraging, Andy. I guess your power train will be in the same region as mine - about 600 watts on a 4S LiPo. I think I'll just bite the bullet and put the plug in the battery side of the esc and hope for the best. Of course if there's a problem, I'll blame you totally

Geoff

Er no. Its running a 5S 5000 battery, and a 1200w motor just to get the noseweight right, though due to the prop size you are correct and it is running around 600-700w. It will fly for over 30 minutes on this setup! Almost all my models have lengthened battery leads, in more recent times these have been needed for the in-line current sensor to be fitted. So far I have never had an ESC fail.

I'll stick a claim form in the post.

[Biggles' Elder Brother - Moderator]

I too have been retro fitting arming plugs to some of my larger scale jobs that have been converted from IC and while there is room for the battery access when its on the ground just prior to flying is a bit of a faff. So the arming plug is a good way round this.

Like Andy I place them in the battery to ESC lead - usually on the +ve. I've done this on 4 or 5 models now, the first about 6 months ago. So far, again like Andy, I have experienced no problems whatsoever.

BEB

[Geoff S]

I have an in line current sensor, too, but I only use it for the initial flights to give me a feel for the consumption and hence duration. It's currently in my 1/4 scale Mew Gull, which is on 6S and on the short test flight back in late December it was drawing around 20 to 25 amps straight and level. On that model the large battery hatch is easily accessible immediately behind the cowl and that's where I'd put an arming plug. It's just as easy to disconnect a battery lead (I use 4mm bullet connectors on bigger packs).

Anyway, I'll pursue my initial location in the front cockpit of the DH60 with the plug in the battery lead. It'll still be visible on the ground but, as Peter Miller has said in the past, it won't be seen at 40' when it's in the air.

Thanks for all the contributions.

Geoff

[Allan Bennett]

Posted by Geoff Sleath on 08/04/2018 20:25:45:

Colin: I already have an XT60 panel mount kit to use. It's just where to put it both electrically and physically.

Allan: What's the failure mode of the motor if one phase of the esc output fails (or, in my case, disabled)? If the throttle is disabled as mine would be (I have both a switch and a verbal warning from the transmitter) then there wouldn't be a current source to damage the winding phases still connected. You may well be right. I just don't know.

I would like to avoid, if possible, an ugly non-scale lump of wire sticking out of the side of the model. I was hoping to put the plug in the front cockpit where it would be better hidden. The esc is going to live in the what would be the fuel tank bay if it was glow powered right next to the front cockpit instrument panel so, if I adopted the more conventional electrical siting I'd be adding about 100mm at most to one battery wire. That's not much inductance, which is the killer rather than resistance.

Thanks for the comments

Geoff

I'm not an electrical engineer so I don't know the exact failure mode, but I do know (from reading) that if one phase has a poor connection -- not necessarily completely disconnected -- it causes the motor to stutter and the other phases to take too many amps.

My arming plug is a Deans Ultra with the two pins shorted by a bit of brass strip slotted over them and soldered. The protruding soldered pins are then cut off and the whole lot filed down to a smooth dome shape, so no protruding wires. It's still a non-scale lump on the side of the cowl, but it's not really noticeable on the ground even, for it's almost hidden by the protruding dummy engine.

Lengthened battery leads won't immediately have any noticeable effect, but in the long-run they can harm your ESC due to high-voltage spikes that are somehow induced in them. The remedy is to add capacitors across the leads, close to the ESC.

Andy48's solution is a very neat one

[Chris Walby]

I must be missing the point here, but to me its just more things to go wrong?

My electric Wots Wot start up procedure:

Pre flight inspection and turn upside down
Turn TX on and insert battery, connect battery and allow ESC to arm with one hand on the model behind the prop arc (the TX should alarm if switched on with throttle up and ESC should not arm if throttle is not at low position)
Switch throttle hold on TX and check motor will not start (even if it did the prop will strike the bench/ground and not rotate a complete revolution)

Turn plane right ay up and walk to flight line WITH LEFT THUMB OVER THROTTLE STICK

When ready to fly, control checks and throttle hold off, taxi and fly

Perhaps we should walk from the start line to the flight line with the props rotating like the IC guys just to remind ourselves that its a live model?

PS Please see other thread about ESC motor and battery lead lengths or seek professional advice.

[Bruce Collinson]

As a relative newcomer/returnee (2 years) I remain nonplussed by the lack of a simple switch which will handle these currents. You'd have thought by now that the simple demand/supply equation would have prevailed.

My Wot 4 has an ESC with a switch, I recall it's a Jeti, which doesn't alter the fact that the battery is still connected to the motor but does reduce the propensity for accidents involving inadvertent stick movements.

What sort of parameters apply in terms of cable length difference if interrupting the live feed from to ESC?

[Nigel R]

"Pre flight inspection and turn upside down"

Not always feasible.

It's also not always feasible to put the hatch in safe and/or easy reach when the thing is live.

"PS Please see other thread about ESC motor and battery lead lengths or seek professional advice."

Which other thread?

[Biggles' Elder Brother - Moderator]

I'm afraid the currents involved are often far to high for any weight viable, reliable, switch I'm afraid.

Chris - the issue is two fold:

1. Many clubs have a rule to the effect that electric models must not be armed except in the pits and held by a physical retraint - exactly the same as IC. A "hand" will not do! Some clubs even go as far as to state that the model can only be armed "on the flight line" - though that is unusual.

2. If you have a model designed with electric power in mind this might not be so much of a problem. But I have a number of scale models - converted from pure IC. Their designer never gave electric power a thought, why should he the designs are quite old now. OK I can fit a battery in - usually where the fuel tank was supposed to go. But fuel tanks were never intended to be easily removable on a routine basis! So I make a hatch or opening, but often access will be very confined because of design constraints. Yes I can get a battey in and connect it - but I need to work through a narrow opening often, by necessity, in an awkward place. OK, I can lie down on the grass in the pits alongside my model and do the psuedo-gynecological proceedure which is connecting its battery - but it's awkward and not a lot iof fun when the ground is wet. Or,...I can incorporate an arming plug, which allows me to do te battery connection in comfort with the model on a cradle on a table and simply retain the arming plug to be inserted on the flight line. Everyone is happy! Yes I still use a programmed throttle cut as well - belt and braces - but I couldn't under my club's rules depend entirely on that, nor would I be completely happy to do so.

BEB

[Simon Chaddock]

Posted by Geoff Sleath on 08/04/2018 20:25:45:

If the throttle is disabled as mine would be (I have both a switch and a verbal warning from the transmitter) then there wouldn't be a current source to damage the winding phases still connected.

In this situation is not a plug on one motor wire rather redundant? What you are actually protecting against is the failure of your existing safety systems.

Should they fail to throttle 'on' (not even full) then your plug would indeed provide protection from the prop (it would not rotate) but not from burning out the motor and/or the ESC.

An ESC starts a motor by delivering a short full voltage pulse to a phase at random and 'reads' the induced voltage from the other phase to ascertain in which direction the motor is rotating and then alters the phase sequence accordingly. If the third phase is not connected the ESC simply repeats the pulse several times a second expecting a reading from the other phase.

A full voltage pulse into a stationary phase draws a high current so continuously repeating it soon exceeds either the capabilities of the motor or the ESC.

Of course a "twin" plug cutting two motor wires would completely isolate the motor as no circuit would exist

[Andy48]

Posted by Allan Bennett on 09/04/2018 12:21:00:

Lengthened battery leads won't immediately have any noticeable effect, but in the long-run they can harm your ESC due to high-voltage spikes that are somehow induced in them. The remedy is to add capacitors across the leads, close to the ESC.

All I can say is it must be a very, very long run, because I've never had a problem with ESCs failing and some have been used regularly for the last 10 years with extended battery leads!

[Allan Bennett]

Andy48, I can only go by what I've read in forums, in particular a very long and detailed thread in RCGroups. The key to protection from the induced voltage spikes seems to be the quality and quantity of capacitors installed at the ESC's battery leads, and I'm sure some ESCs are much better specified in this respect than others and are thus able to withstand more 'abuse'.

The thread in question seems to suggest that 8" is about the safe limit, but I've certainly gone significantly over that when the length of the battery leads themselves, plus a series or parallel harness, and a bit of slack to make connection easier, is taken into account. But I do put extra capacitors on for good measure if I'm going over about 18". Normally I put the ESC close to the battery, and extend the motor leads.

[Geoff S]

From a look at the capacitors fitted at the battery inputs of the only esc I have readily to hand (60 amp Black Mantis) they are aluminium electrolytics (2x 220mfd 35v in parallel). I'm pretty sure they're of the wound type but without pulling them to pieces I can't be sure. If they are what I think they are, they're not very suitable as decoupling for high frequency spikes because they are notorious for being quite inductive because of their construction. The only electrolytics I would have used when I was gainfully employed to decouple high frequencies would be solid tantalum but they're quite expensive - or were when I used to buy them (don't ask me the price. It wasn't my money )

I know it seems odd for a capacitor to be inductive but nothing's perfect and I do wonder how effective they are.

Geoff

[Dwain Dibley.]

Hi Geoff, Here is my arming plug on my Moth to save you trawling thru the thread, altho you will have to scroll down the page a ways.

Easy to make as per the diagram Dennis posted, as you know. Makes life easy and safe in the pits.

You know it makes sense Matey.

D.D.

Edited By Dwain Dibley. on 09/04/2018 23:05:40

[Geoff S]

Posted by Bruce Collinson on 09/04/2018 13:29:53:

As a relative newcomer/returnee (2 years) I remain nonplussed by the lack of a simple switch which will handle these currents. You'd have thought by now that the simple demand/supply equation would have prevailed.

My Wot 4 has an ESC with a switch, I recall it's a Jeti, which doesn't alter the fact that the battery is still connected to the motor but does reduce the propensity for accidents involving inadvertent stick movements.

What sort of parameters apply in terms of cable length difference if interrupting the live feed from to ESC?

Is that a brushed Jeti esc? If so, I had a few of those and just removed the switch and joined the wires together or simply left the switch on. I would never trust a disabling of the circuit that drives the MOSFETs that supply the motor and simply assumed that once the battery was connected the motor could spin at any time. I also always had a throttle disable switch and on my Taranis and Horus transmitters a verbal warning when the throttle switch is operated.

A 'simple' switch capable of carrying the currents our motors draw would need to be very substantial. Even a lightweight 3S foamie like (say) a Wot4E draws around 30 amps maximum - that's nearly 3 times the current draw of your 3kw electric kettle at home. My bigger models draw over 60 amps.

Geoff

[Trevor]

I think that a fairly modest switch (e.g. from the car accessory shop) can actually carry quite a hefty current (e.g. 20-30a). However switching such a current is quite a different matter. On the face of it, you might think that would be okay, since you would normally arm the model with the throttle closed. However, ESCs have a very large capacitor across the input side so, when you throw your arming switch, this capacitor gets charged up - very fast. In fact the current involved can be many times your full throttle current draw. I believe that it is this switch-on surge current (and the arcing that goes with it) which is the problem in finding a suitable switch for our use.

An arming plug (preferably one designed to cope with arcing without damage to the mating surfaces) is therefore a good solution. That said, I have to admit that I don't use them. . . !

Trevor

[Biggles' Elder Brother - Moderator]

20 to 30A is quite a modest size electirc model these days when 60-70A is becoming more common.And I certianly don't think that is really viable with a switch!

BEB

[Geoff S]

My brain must going soft. Earlier I suggested sold Tantalum electrolytic capacitors for decoupling the battery inputs to an esc but I forgot that they aren't available in both high capacitance and with the voltage working we need - my excuse is that when I was working I was more interested in milliamps than amps Tantalums are very low inductance however.

Anyway, last night I mulled it over and found this interesting article which covers the issue quite well. As a result I ordered some 220 mfd 35v low ESR (effective series resistance) capacitors from RS. I was amazed at the price - £1.43 for 5 post free! I've been dealing with RS since they were called RadioSpares and their catalogue was a just a few sheets of paper. The rep used to call at the shop regularly when I was a kid. I haven't bought much from them personally since I retired but I think that's good value.

I intend to add at least one more to the esc I'm using for my DH60 Moth and put the arming plug in the battery lead. The plug will be fitted in the cockpit.

Geoff

[Andy48]

Hmmm!

The article is now 10 years old, and probably based on earlier information and ESCs almost certainly will have changed since then. Also 10 years ago battery technology was mostly NiCad.

If voltage spikes higher than the battery voltage can kill the ESC over time, is this the actual battery voltage, or as I am inclined to think, the maximum battery voltage of the ESC? Surely then these voltage spikes will also destroy any extra capacitors you put in circuit, which being electrolytic will have a low voltage.

Finally, the lithium battery itself acts as a capacitor surely?

Is this another one of those issues which through time has become an old wives tale?

 

 

 

 

Edited By Andy48 on 10/04/2018 14:07:50