xb36

I am building a 114" span 6 motor B-36. Each of the 6 brushless motors will have an ESC. The batteries and receiver will be in the center of the wing, at the root rib. Will it be better to place each ESC near the motor or near the batteries and receiver? If each ESC is placed near the motor, then the 3 motor wires are short but the ESC power and ESC control wires are long. Obviously I can place all 6 ESC's near the batteries at the root rib but then the motor wires will be long. I am concerned about long leads acting like antennas and picking up noise that might cause radio interference. FYI, I plan to purchase a JR dual receiver radio to reduce radio interference problems.

Any thoughts?

Mike

Glacier Girl

My Feedback: (4)

Mike, I'm sure you will hear from both camps on how it should be done, but I always try and put the esc near the motors, and run the battery leads as long as needed.

Couple of positives in doing it this way, esc's can be located to get airflow over them.
2 wires run instead of 3 for each motor.
2 wires would have less of chance of interference then three, by count alone.
Then there's always the one about if you run the 3 motor wires and they are not the same length, will it affect the phases of the motor.

In your case, you could run the 2 battery wires to the most inboard motor/esc, then jump to each of the next two motors on the wing, for simplicity, VS having to run 9 wires for the motors.

xb36

Good comments.

I agree that having 2 long battery wires is better than having 3 long motor wires. Two wires should induce less radio interference than three. I don't buy in to different lengths of motor wire will affect the phases of the motor, so I can throw that variable out. I too would put that in the "Then there's the one about" category of old wives tales.

You have an interesting thought about running battery wires out the wing and then just tapping in to them as they continue on to the next motor on the wing. I hadn't thought of that. I will have to give that some thought. If I did run 2 battery wires along the wing to power 3 brushless (E-Flight Park 480) motors, will the current be too high for the battery connector?

There is one more consideration though; the 3 wires from the receiver servo output to the input of the ESC. If the ESC's are near the motor, then these control wires would get long too which causes me some concern about radio interference again.

I also agree that it will be easier to cool the ESC if I place it in the motor nacelle.

You're right, there are 2 camps of thought on this.

Thank you for your comments. I look forward to hearing a few more before making a decision.

Mike

-pkh-

My Feedback: (2)

Most ESC manuals caution against changing the motor wire lengths. The ESC powers the motor through these wires, AND senses the rotor position through these wires. Adding length to these wires increases the inductance and probably screws up the rotor sensing at some point.

The other thing to consider is the 3 motor wires conduct switched (AC) currents, so adding inductance to them is more of a problem than adding it to the battery wires which will conduct mostly DC currents. So I believe it's better to add length to your battery wires instead of adding length to your motor wires on your ESC.

xb36

OK, I will admit that I could be wrong about the impact from changing motor wire length. I will email a couple of ESC vendors and post my question to them. If I get a response I will post it here. My gut feel is to not buy in to it but I will go neutral on this one until I get some feedback from an ESC vendor.

I still invite experience, comments, and opinions to be posted here. I would rather learn from others than learn the hard way on my own.

Mike

ron_van_sommeren

I don't belong to a 'camp', but I have a good memory don't lengthen battery leads, and if you have to, add extra capacitors:

Schulze controllers and battery lead length:
http://www.schulze-elektronik-gmbh.de/tips_e.htm

Castle Controllers and battery lead length:
maximum recommended wire length

Bob Boucher (Astrobob, http://www.astroflight.com) on long battery leads:
What's better? Long battery lead or motor lead.

How to add extra capacitors (English and German):
http://www.schulze-elektronik-gmbh.d...e/gfutc-de.pdf

ron_van_sommeren

They caution against shortening the motorwires. Because in brushless motors the motorwires ARE the wires of the windings. (That's why they are stiff and hardly flexible).

Motor inductance is already that high, the inductance of the extra wire is negligable.

The AC component is low frequency, harmless. There are no switching spikes in/on the AC current, the motor side, the motor inductance already takes care of that. Just a light current ripple due to switching. The DC side however, the battery side, has spikes but the input capacitors take care of that. However, when the battery wires are too long, the capacitors have a tough (read: warm, hot and short) life, hence the necessity of extra capacitors in that case.

xb36

RVS,

I can understand the reservation to shorten the stiff motor winding/wire.

I also agree that the motor inductance is high. If it wasn't, the motor wouldn't rotate.

I also agree that the microHenrys or picoHenrys added by the wiring would be insignificant.

The battery acts as a high capacitance capacitor so it should suppress low frequency ac/spikes. When you suggest adding capacitors to a long battery lead, are you suggesting low capacitance capacitors such as a .01 disc capacitor to suppress RF noise?

You suggest: don't lengthen battery leads. Well, I have motors way out on the wing and I have batteries located at the root rib. Something has to be lengthened, the battery leads and ESC controller (servo) leads, OR the motor leads. I am just trying to identify the lesser of two evils.

Thank you very much for the links to the ESC vendors. I will review their comments. Thanks again.

Mike

-pkh-

My Feedback: (2)

I stand corrected. I was sure my ESC manuals said not to shorten the motor wires, but I don't see that in any of the online Castle Creations manuals. In fact, the Great Planes Silver Series ESC manual states that it is preferrable to lengthen the motor wires, not the battery wires (see page 2 of attached).

xb36

PKH,

Well, that looks pretty definitive then.... Although your attached ESC manual doesn't give a reason, it does clearly state that it is best to lengthen the motor wires, not the battery wires.

Mike

Red Scholefield

My Feedback: (9)

This assumes whoever wrote the Great Planes Silver Series ESC manual knows what he is talking about. My experience with Great Planes products leaves some doubt in my mind. I think I'll go with Shultz, AstroFight and Castle Creations, people that have actual design experience in ESCs.

xb36

So let's leave this thread open then. I had thought it was closed so I didn't contact the other vendors. I will send an inquiry out tomorrow.

Mike

ron_van_sommeren

All RC BLDC controllers work the same, no need to contact other manufacturers. With the exception of sinus-commutation controllers (e.g. www.sinusleistungssteller.de )

ron_van_sommeren

But not anymore at the ESC inputs, due the inductance of the wires.

As a rule of thumb, for every 4inch/10cm extra length/distance to ESC, add an extra 220uF capacity (electrolytic condensators, , voltage the same as the capacitors already installed, low ESR type) (Ludwich Retzbach, German e-flight author, the 'R' in LRK).

@Red
Maybe add the four manufactur links I gave in my previous post to your http://www.rcbatteryclinic.com/ site?

Prettig weekend Ron

otrcman

I'm facing this same wire lengthing issue on a twin engine model. So I contacted the ESC/motor vendor(innov8tiveDesigns.com, aka Scorpion motors) and the response was that they had tried it both ways and found no particular problems either way. The person who responded said he was personally running 46" motor wire extensions in a model with no problems.

I haven't actually operated my system yet so my (and your) mileage may vary.

Dick

cyclops2

He is lucky.
Picked to lengthen the motor leads.

Always the motor leads. As the ESC's get more powerfull in size, long battery leads are......NOT..... the right way.


Rich

mjsas

On the input side or battery side there will be large current spikes which are normally controlled by the input capacitors in the ESC. On the output or motor side the motor is already is a very large inductor so the current spikes are very small.
When designing an ESC, the input current spikes can cause very large voltage spikes, for example the inductance of a paper clip would cause a 5 to 10 volt spike given the size of the current spikes. Therefore every ESC has large input capacitors and very short and compact wiring from the input capacitors to the FETs.
If the wiring on the input side has significant inductance the ESC will be toast. That may be a bit of wire or many feet.

otrcman

OK guys, you're got me convinced that I shouldn't run long battery leads to my ESC. On my P-61 I will place each ESC and battery right in the nacelle with its respective motor. Then run a servo extension cord from the receiver to each nacelle. So my next question is:

I'm running a crossover cable between my two batteries to put the batteries in parallel. As long as both batteries are roughly the same voltage, there will be very little current in the crossover cable. But if one battery gets very low before the other (which is the reason or the crossover cable in the first place), then I've got both motors running on a single battery and full current for one motor going through the crossover cable.

So I've got two considerations: Low current in the crossover cable when both batteries are well up, and high current in the cable when one battery goes over the hill before the other. Am I right that low current means low voltage spikes? If I wind up running both motors off a single battery will the (lower) battery suppress the voltage spikes which appear on the crossover cable?

I'm having a bit of trouble wrapping my mind around the interactions of the two parallel batteries. Maybe Red Schofield is the best guy to address this because I'm starting to think that the two batteries will remain at pretty much the same voltage regardless of whether one is a little stronger than the other. Is it possible that with the crossover cable I'll never truly have one battery "drop off line"?

Dick

Red Scholefield

My Feedback: (9)

If you are running both batteries in parallel there is no way one can go down before the other. They will discharge in proportion to their avialable capacity.

ron_van_sommeren

And they don't create input spikes anyway.

Vriendelijke groeten Ron van Sommeren
• int. electric fly-in, Nijmegen, the Netherlands
• brushless motor building tips & tricks
• diy brushless motor discussion group
• Drive Calculator download & discussion group

xb36

So now that we are also talking about multi ESCs for multi motors ......

In my original posting I mentioned that I was building a 114" span B-36 with 6 brushless motors.

I believe that we came to an agreement that long motor wires are better than long battery leads to the ESC. If so, then the ESCs should be placed at the center of the wing, as close as practical to the batteries. There should be no need to run extension cords to the receiver since it will also be near the center of the wing. The motor wires to my outer motors will be around 40 inches long, the middle motor wires will be around 30 inches long, and the inner motor wires will be around 20 inches long. I am still building so these are estimates. I know that these are long lengths, but the alternative is to place the battery and ESC in the engine nacelle and that would cause a very unstable B-36 to get that much weight that far out on the wing.

Perhaps I should start another thread or two, but there has been such good response in this thread that I want to take advantage. Are there any suggestions here regarding wiring? My current plan is to use 6 ESCs for the 6 brushless motors. I don't know if I should run 6 smaller LiPo packs or 3 larger LiPo packs. I just got MotoCalc 8 and I am not sure that I am using it correctly. How do you specify three or six of the 3S1P LiPo packs in MotoCalc 8.... or should I be running 3S2P packs? It is easy to specify 6 ESCs, 6 packs, and 6 motors, but what do I put for cells.... 3S6P? If so, then it must not be possible to specify six of the 3S2P packs.

I haven't been on RC Universe very long, so if it is inappropriate for me to ask these questions in this thread just speak up.

Thanks in advance,

Mike

otrcman

Mike,

I can't answer your electronic questions -- that's why I'm on this thread asking some of my own questions. But I do know a bit about airplane design and conceptual design in general.

First things first: From the powerplant system you want Thrust, adequate, symmetrical, reliable thrust.

1. Adequate: It's a given that you are using six motors. They don't all have to be of the same size, but they have to be efficient enough for good power-to-weight ratio and they have to all add up to enough power to fly your airplane.

2. Symmetrical: With a multi-motor airplane, particularly one with swept wings, you don't want to introduce yawing moments due to unsymmetrical thrust. So you need to assure that at least pairs of motors keep running. That is to say, if an outboard stops or loses power, you want the other outboard to stop or lose power at the same time. This argues for the two outboards (1 & 6) to be run off a common battery or a pair of batteries which are paralleled together. Then the two middle motors (2 & 5) similarly in parallel. Ditto for the two inboards (3 & 4).

3. Reliable: That's why you're using electric motors, right ?


Here are some random ideas for configurations:

You're right about not wanting to place batteries too far out in the wings. If your roll moment of intertia is too high, particularly in proportion to the pitch and yaw inertias, the airplane will be more difficult to fly. So you're pretty much stuck with batteries toward the center and the motors in the nacelles. Now we're worried about voltage drop from extending the motor wires from the center of the airplane all the way out to the outboard motors. One answer is to use larger gauge wiring, but that adds weight. Perhaps a better choice is to use smaller motors for 1 & 6 to reduce motor weight and current draw. By selection of lower KV motors for the outboards, you could run the same diameter props all around but simply not turn the outboards so fast. As a bonus, smaller outboard motors would further reduce your roll inertia.

On the most inboard motors, you have the least worries about wire length, voltage drop, and wire weight. So you could employ either more powerful motors or at least higher Kv motors on the inboards. Overall, the idea would be to draw the most amps on the inboard motors and the least amps on the outboard motors.

Rather than thinking of the B-36 as a six motor airplane, think of it as three sets of twin motors. The only place where all six have to be treated together is the connection from the receiver to the ESC's. I haven't tackled that one yet, but you may need to boost you receiver signal level or suppress noise on all those Y-harnesses. That's something the giant scale folks cope with all the time when they use many servos.

Dick

ron_van_sommeren

Mabye run the three sets of brushless motors of three brushless ESC's? This should not work, from an electronic point of view, but there are many reports it does. But then again, not always. Not a good idea anyway if you would be using gearboxes, startup can by a bit rough an the gears. If and when it works, both motors on a single ESC will run in *perfect* sync. And if the battery or ESC fails, both left and right motors will stop.
If you want to keep wire losses down (and/or wires thin) go for high battery voltage and low Kv motors. That will reduce the current in the wire.

Anyone building a XB-36 has my vote

Vriendelijke groeten Ron van Sommeren
• int. electric fly-in, Nijmegen, the Netherlands
• brushless motor building tips & tricks
• diy brushless motor discussion group
• Drive Calculator download & discussion group

cyclops2

You can build the motors to both stop if 1 of them develops a heavy current demand or short.
Measure the amps. of a single motor at full throttle. Say it is 10 amps. For a pair it would be 20 amps. A fuse of 20 amps, FAST blowing, glass type could be put in the + line of the the 2 motors after they have been joined in parallel but before the battery. Just pair them up as stated above.

I have that on a 4 engined flying boat. I tested it by tossing a large rag into the prop of 1 motor. fuse blew in 10 seconds. No motor or ESC stink or problems. I actually fused mine at the FULL current of both motors. At full current, fuses hang on forever. Fusing each motor is not a good thing.


Wait a minute!!
If you lose 1 motor out of 6, I dont think you are going to have a problem to still fly well enough, to take your time on a nice big sweeping landing.

I say wire the motors to each ESC.

I would go with 1 big battery if at all possible. Try not to use LIPOS in big planes. Wing loading should be very low on a 36.
LIFEPO4's .....A123.......DEWALT packs....... Pick the right motor & prop and you could use a DEWALT Pack whole. Or 2 or 3 in parallel. Even use the stock charger they have. Sweet.

Wake us up with progress pictures

Make sure 1 of the main wing spars is right where the landing gear assy. will need to be mounted.


Rich

ozrcboy

Guys - just a thought on why it doesn't matter...

Speed of electrical impulse through copper is about 2.0 x10^8 m/s if memory serves correctly (roughly 1/10 the speed of light).

Let say your motor is humming round at 20000rpm, so about 333 revs per second, and that each rev requires three switches (1 and 2, 2 and 3, 3 and 1) so around 1000 switches per second. Per switch, the electrical impulse could travel about 200km (140 miles or so). So a 10cm (4" ) motor lead equals 0.00005% of that distance while a 100cm (40" ) motor lead equals 0.0005% of that distance. You could probably have 10 metre long motor leads and it might still work - the main problem would be pushing so much DC current down 100 metres of wire rather than the sensing...

Cheers,
Oz.