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Twin brushless config'

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gliggsy28/08/2012 13:14:58
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Hi guys, recently purchased a Twinstar mk 1 for very little money, given it about half a dozen flights on 11.1 lipo and as expected, one of the standard motors packed up on me. Really enjoyed flying it so decided to treat it to twin brushless so got a couple of motors from GS (had a guess at the size and power etc) and these fit nicely with some old 8x4 wooden props, fitted a couple of SS ESC's from HK and on WOT pulling about 150 Watts. So all's well so far, the problem is that one is doing more work than the other by the sound of it, also one tends to start up before the other. Anybody out there configured twin leccys and are there some rules and guidance. I would have thought I need to balance these out a little closer than what they sound but have not maidened yet, any advice on leccy twins would be mucho apprecio.........G

GrahamC28/08/2012 13:42:43
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Yes I've done twin electrics.

You do need to disconect the red wire from the plug on one of the ESC's so that only one is providing power to the receiver.

I was lucky with my setup, and both ESC's and motors seem fairly evenly matched, but I have heard that this is not always the case, and a clubmate had to buy a third ESC in order to get a pair that were sufficiently matched.

I guess you have checked that th programming is the same for both ESC's?

Mark Powell 228/08/2012 14:12:04
430 forum posts

I have a twin electic DH88 Comet Racer. Axi motors, Jeti controllers. These because previous experience has shown those manufacturers to generally produce good quality products, if more expensive than some.. No problem with different performance at all.

As GrahamC says it is important to remove the BEC power connection from one ESC. Also with his comment about programming. On one ESC, not of the above make, I inadvertently introduced a 'power curve'. Done on one and not the other, this could produce the effects you describe, though I suspect it is unlikely in your case.

PS: for a while, Multiplex has a power set for their foam twins with one controller driving two brushless motors. Sounds a little unlikely, but they did. I would think you had better not shut down the motors totally in the air. Whether you risk trying it with yours or not is up to you.smiley

Edited By Mark Powell 2 on 28/08/2012 14:33:10

Chris Bott - Moderator28/08/2012 14:33:03
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Gliggsy if the motors and ESC's are identical then as GrahamC says, have a check that they are both programmed the same. If you haven't touched the programming then they should be.

Other things to check are that the props are indeed identical (make, diameter and pitch) and that you have no poor connections.

Are they both running off one battery?
Are they supplied by a Y lead from the throttle channel?

As for one starting before the other, this might be difficult to solve. It could be because the ESC's can't tell where the motors are until they are spinning so sort of guess what to do to start with. Once they are running though, they should be the same. I'd tale off any brake programming too so they are always spinning in the air, this will help.

John Cole28/08/2012 14:55:48
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You may be able to calibrate the ESCs; depends on the make. You can do this with HobbyWing.

First restrain the model. Before connecting the LiPo, switch on the TX and increase throttle to max. Then connect the LiPo. You should get a "programming" sound, and no running of the motors. Then reduce throttle to minimum and you get the "Arming" sound. Then the two ESCs should be calibrated the same, and when you open the throttle they should produce similar amounts of power.

If your ESCs are not HobbyWIng then make extra sure you keep out of the way of the props as the motors might power up when you connect the LiPo.

If all this works after a fashion but the ESCs are still not balanced then calibrate them separately but when you do the "high power" ESC only open the throltle to e.g. 90% before connecting the LiPo.

GrahamC28/08/2012 19:55:18
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Also it has occured to me....

Have you tried swapping the moters around to make sure it is the ESC's and not the moters themselves.

If you swap them, and the slow motor is on the same ESC as before, then your problem is unmatched ESC's. If the slow motor is on the other side after the swap, your problem is with the moter. Motors can vary. I would imagine manufacturing tolerences on the bearings might not be as fine on cheaper motors, and winding can vary in neatness.

Just a thought.

gliggsy28/08/2012 22:00:04
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Thanks for the input guys, pretty much as I thought though, already disconnected one of the ESC power feeds to disable the BEC, did do a bit of reading prior to this mod' . Haven't programmed the ESC's, just as they came out of the box, maybe worth doing a flight to see what the effects are before throwing any more dollars at this one, my thoughts are that it probably will fly, and maybe ok and I may swap the motor/ESC around to see the effect or maybe try another ESC, will keep you posted with the results as this may be of interest to other flyers although it could be a couple of weeks before I get home to try it as I have been posted down to Plymouth for a while with work.......g

gliggsy28/08/2012 22:03:50
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Hi Chris, should have added, yes, running of one lipo, 3 cell and two esc's with a 'Y' lead from the throttle channel.......G

Peter Beeney28/08/2012 22:35:14
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May I ask, please, purely out of curiosity, does anyone actually have any knowledge at all as to why two, (or more, perhaps), BEC’s should not be run in parallel? I just can’t help wondering why this idea has always seemed to be so persistent. I certainly don’t have any problems with the fact that someone may wish to disconnect various wires, rather it’s simply that I’ve never seen any specific technical reason to do it, other than a vague reference to the fact that ‘they fight each other’.

I have to say I’ve never been able to find a satisfactory reason why this should be done, I’ve given it some thought in the past and in the very unlikely event of a overheating heating problem maybe, it might possibly even be beneficial; to run them in parallel, that is.

I think my take on this would be, provided, as I said, there are no external factors to change the standard power supply, then it really doesn’t matter if you disconnect one red wire or not. I don’t think that it would ever be possible to tell the difference, the radio will still operate perfectly, on one or two.

Just a thought, and as always, perhaps, we won’t again come to any definite conclusions……

PB

Mark Powell 229/08/2012 06:40:26
430 forum posts

Peter,

not really. I am not an electronics 'expert' but I do fiddle around. Many Hi Fi amplifiers have lots of output devices in parallel, driving the loudspeaker. As do our ESCs. And both the motor power and the receiver power are 'output devices' of the ESC.

Probably it is said because it it 'obvious' rather than correct. But I am not going to blow two up just to check!

Chris Bott - Moderator29/08/2012 07:35:17
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Calibration is a good thought. Although I'd always advise doing it with the props off rather than by restraining the model.

Peter I have two ESC BEC's powering the receiver in my 88" DH88. However I have isolated them from each other using a shottky diode in each red lead. This was done through no particular knowledge but just in case one went faulty and left a short circuit accross the lines.

Even now I can see a minor (very unlikely) fault scenario where one BEC fails leaving just that ESC with no supply, giving me a single motor out. If I had no diodes, one BEC might even supply the other ESC in this event. Who really knows?

Mark Powell 229/08/2012 07:59:38
430 forum posts

Chris has got a bigger one than I have. (My little one flies much better with the new, more washout, wing.)

I would never use a BEC on an 88 inch model. Separate radio battery.

Edited By Mark Powell 2 on 29/08/2012 08:06:03

Chris Bott - Moderator29/08/2012 08:17:13
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Posted by Mark Powell 2 on 29/08/2012 07:59:38:

Chris has got a bigger one than I have. (My little one flies much better with the new, more washout, wing.)

I would never use a BEC on an 88 inch model. Separate radio battery.

Edited By Mark Powell 2 on 29/08/2012 08:06:03

Mine might be bigger Mark but at least you dare fly yours smileyyes

As for using two BEC's on a larger model. I've done extensive tests, and each BEC is more than man enough for the job on it's own, and I have two, so I have a back up. Personally I think that's safer than a single radio battery. But we all make our own choices and none of them are absolutely right.

Must get out with mine again soon, actually I must get out to fly at all crying.

John Cole29/08/2012 09:41:00
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Posted by Chris Bott - Moderator on 29/08/2012 07:35:17:

Calibration is a good thought. Although I'd always advise doing it with the props off rather than by restraining the model.

Peter I have two ESC BEC's powering the receiver in my 88" DH88. However I have isolated them from each other using a shottky diode in each red lead. This was done through no particular knowledge but just in case one went faulty and left a short circuit accross the lines.

Even now I can see a minor (very unlikely) fault scenario where one BEC fails leaving just that ESC with no supply, giving me a single motor out. If I had no diodes, one BEC might even supply the other ESC in this event. Who really knows?

Chris: I'm not sure what you mean by your last point. The BEC does not power the ESC. It powers the Rx.

Chris Bott - Moderator29/08/2012 09:50:54
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John

Yes you're right.

However the ESC electronics, the computer in it that controlls and drives the high power MOSFET's etc, must get it's supply from somewhere and I suspect that it uses around a 5V supply. So it would make perfect sense if that ran off the BEC as well.

If this is the case, and if one of the BEC's in one of my ESC's say - overheated and shut down. Then not only would that supply disappear from the receiver, but the computer in that ESC would shut down and that motor would stop. (Which spells disaster with this model).

If I connect the two red leads directly together instead of using diodes, there's a good chance that the ESC with the dead BEC would continue to work as an ESC.

So, I wanted a backed up BEC supply, so use them both for the receiver. I protected against the scenario of one going short circuit causing the other to fail by adding the diodes. But I can still think of a fault scenario where a different arrangement would be better. It's all a compromise.

I quite like Peter's thinking written elsewhere of using a small Receiver battery and a BEC together. I will do my own tests on that sooner or later.

Peter Beeney29/08/2012 10:11:16
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John, - With the greatest respect, but I think you will find that the primary task of the regulator, the BEC, is to supply a regulated 5 volts to some components on the ESC. Powering the radio is just an afterthought; and it does have it’s difficulties. In other words, the radio can function without it’s BEC supply, but the ESC will cease to operate if the 5 volt supply disappears. Here, I think, the ESC 5 volt supply definitely came before the radio supply.

I’m now just off flying for the day, but I’ll be back later to enlarge on this if necessary. I have been over it before quite compressively, in at least one other thread.

PB

Mark Powell 229/08/2012 10:55:47
430 forum posts

'Receiver battery and BEC together'

The Kontronik 'Jazz' ESCs, (if BEC equipped, which some of mine are), recommend precisely that in their instructions, saying that the BEC should be considered only as a backup for the battery. So presumably they would not mind 'seeing' the 5V from another ESC either. Which is where we came in, or part of it.

I avoid BECs in any other than my small planes. Ever since the ESC melted in my Flair Atilla. It is so stable that I did not even notice at first. No harm done, but I had a long walk.

I am sure Peter is correct. The logic in an ESC will require a steady 5V. So why not supply it to the outside world too? Of course, the opto coupled ones don't - it can be asking a lot to supply an unknown current demand at 5V from a possibly 50 volt battery (damn these EDFs) and if you forget that you can use the advantages of opto coupling.

But we still don't have any firm answers to the OP's problem.

Simon Chaddock29/08/2012 11:22:32
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Peter

To answer you question

I ran 4 very cheap small motors and ESCs (all off the same LiPo).

First as recommended with only one red wire connected. Not very successful at all. Very rarely did all 4 motors even start. The motors that did not start just squealed.

Reconnected all the red wires and it was much better. The motors would start together most of the time but had very different slow speed characteristics. One in particular would continue to run at a minimum throttle setting where the other 3 had already stopped.

Full power balance was reasonable and any difference in thrust was not a major issue.

In summary

There is likely to be a considerable difference in the characteristics of cheap motors and ESCs.

And disconnecting the red wire in a multi ESC set up is not necessarily essential.wink 2

ps

The two best motors ended up in a 'tandem' arrangement so a failure to start in the air would not be not so serious (it only happens at minimum air speed otherwise the props rotate anyway). It has happened once but fortunately it can manage quite happily on one.

C Norton29/08/2012 11:28:32
170 forum posts

I've never had any problems with paralleling linear BEC's. I have a tricopter where all 3 are connected to the controller board and it works fine, as well as my Twinstar. I'm sure the semiconductor junctions in their output stages are meaty enough to resist a little back EMF? I'd be nervous about paralleling switch mode ones though, they baffle me!

With regard to the original problem, there are tales out there of identically labelled motors being anything but identical, though that would be difficult to test without test equipment or substituting a third "identical" motor. Even a reputable retailer is only as good as his supplier.

Chris Bott - Moderator29/08/2012 11:46:07
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Posted by Mark Powell 2 on 29/08/2012 10:55:47:

'Receiver battery and BEC together'

it can be asking a lot to supply an unknown current demand at 5V from a possibly 50 volt battery

Yes is can be asking a lot.
But if can also be perfectly safe.

In my example the BEC is designed to do just that, is rated properly, mine are rated at 5A each. They have been tested thoroughly without showing any signs of warming. I also have telemetry that records lowest Rx volts and highest ESC temperature during flights, which I always have a look at after landng.

To follow your logic Mark, I should never use a receiver nbattery because I had one fail once.

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