ESC mounting position.

[Axbri]

I am making a scratch built hexacopter. I am thinking about where the best position to mount the ESCs is. Is it best to mount them at the end om the booms close to the motors or closer to the center of the frame?

This hexacopter is my second multirotor. On my first (which was an H-quad), I mounted the ESCs close to the motors because I wanted to keep the noise I know they produce away from the rest of the electronics. This worked perfectly. But I have noticed that most people place there ESCs close to the center of there frames. Is there any thought behind this or is it just because it is easier to mount them there?

I assume that it is bad to extend to wires between the motors and the ESCs since the current there is pulse width modulated by the ESCs, and therefore produces a lot of noise. The current in the cables between the Battery and the ESCs should be relatively stable.

Share your thoughts and ideas!

 

[Craftydan]

I pull mine inside to "clean up" the build. Others mount them out on the booms there to take advantage of the prop wash for cooling.

If you're not heavily loading your ESCs they won't need the extra cooling, but it doesn't hurt.

Moving the ESCs out on the edge will also increase the rotational inertia of the quad -- pushing it toward the more-stable/less-agile side of the spectrum -- where moving them in will *slightly* improve your agility.

 

[cranialrectosis]

I have done both.

I like the clean look and the agility of having my ESCs in close.

I also have put the ESCs out on the booms for shorter ESC to motor leads and for the cooling. The only difference I have noted is that (as Dan said) putting them in closer gives me slightly better aerobatics. Putting them out closer to the motors makes for a slightly more stable copter.

I have three copters. My BatBone Tri-copter has the ESCs out on the booms for foldability, cooling and stability. My tiny Knuckle I quad with shortened booms has the ESCs in close for performance. My AnyCopter camera carrying copter also has the ESCs in close (inside the hub) but just for aesthetics.

What really matters is heat. If they get hot, put them in the rotor wash or expose the heat sink by removing some of the heat shrink.

 

[Axbri]

Thanks for your opinions. I have decided to pot mine on the booms to improve cooling and keep them away from the other electronics. I am building a stable 2 kg camera copter, so a little increased rotational inertia is not a problem for me.

My ESCs and motors:

 

[jhitesma]

I can't remember where I read the discussion...I thought it was on here but I can't find it so it was probably over on rcgroups and I doubt I can dig it up over there.

But the gist of it was that lengthening the motor leads is generally better than lengthening the power leads. The reason is longer motor leads are hard on the filtering capacitors on the ESC's and can cause them to fail while longer motor leads have almost no downside (despite what you and I both thought apparently noise isn't a big issue off the motor lines when they're lengthened.)

I forget the details of the discussion and my electrical theory is way rustier than it should be...but after reading it I was in agreement that lengthening things on the power side isn't a good idea unless you also upgrade the capacitors - though doing so is tricky and in generally it's really just best electrically to leave the power leads as short as possible.

 

[Axbri]

After some fast google research I found this: http://www.rcgroups.com/forums/showthread.php?t=952523

I have not read the entire thread, but it wound be interesting to know why it is bad for the capacitors. Is it the moment where the battery is connected and the capacitors are charged, or is it the noise from the ESCs that is bad? I have a quad with four of those ESCs and motors that also have very long power leads, I have flown it for 4-5 hours without any problems.

I can always add some capacitors at the center of the frame where the ESCs are connected to the battery.

 

[Axbri]

I have realized that the problem is caused by the voltage spikes (higher than the battery voltage) generated by the ESC when running the motor at less than max speed. If the power wire length are increased, the voltage of those voltage spikes are increased, and this is bad for the capacitors and they get warm.

My questions now is: How bad is if is a capacitor explodes?

They say "The extra load shortens the life of the original cap and it will eventually fail, probably catastrophically". But is it relay that "catastrophically". Electrolytic capacitors usually fail open ( http://youtu.be/sW0a9d_vWoc ). If this happens the ESC will probably work a few seconds after this happens, and even if it does fail totally and the motor stops, the rest of the electronics and the battery will survive. And if it is a hexacopter with one failed motor, it may even be possible to make an emergency landing/controlled crash?

 

[Cyberdactyl]

I have the ESCs mounted out on the booms. Yes indeed it raises the loss in angular inertia efficiency, but unless you're flying like hallstudio (which I definitely am not) it doesn't matter a heck of a lot. It actually helps a bit in taming the twitchiness.

Also, I highly recommend you mount the ESC sideways. It loses nothing in cooling and reduces the planform under the thrust column, buying you a few seconds more flight time and a few extra grams of lift.

 

[Craftydan]

My questions now is: How bad is if is a capacitor explodes?

There will be a very loud pop and the copter will likely fall from the sky.
Electrolytics typically fail open, instead of fail short, so the battery won't be shorted out, but the power supply may likley go unstable and shut down the ESC. Depending on how confused the ESC gets in it's death throws, it might lock on a tfull throttle, but likely it'll just shut down.

Keep in mind, while technically possible, this is in practice rare -- many people do it and have no issues. It may shorten the ESC lifespan, and may be harder on the low end models, but otherwise the power filtering is there for a reason. It doesn't hurt it to go easy on it, but it should be designed to take on the HF surges and spikes from the inductance the long leads might cause.

 

[jhitesma]

That's not the thread I was reading...but it's a far better reference to the same topic - good find!

It's not so much not being at max speed - it's a matter of changing the power requirements and then the "inertia" of the current in the wires needing somewhere to go. I built a fuel injection controller for my dune buggy (Not the one in my avatar, a nicer one that unfortunately hasn't been driveable for about 7 years thanks to upgrades that got out of hand.) and there's a similar problem that can cause injectors to burn out due to flyback current after they cut off.

But unless you're really extending the wire a LONG way and running some really cheap ESC's I doubt you'd run into any kind of problem. You'll probably shorten the life of the caps - but I suspect in most cases the ESC's will outclassed and in the bottom of a box somewhere before you see a catastrophic failure unless you had a marginal cap to begin with.

So if you've already made the choice I wouldn't sweat it. But if you're still debating it's one small reason to favor short power leads over short motor leads.

 

[Cyberdactyl]

Yes sir, the distances we're talking about as being a "long run" for 14-16g wire . . . 15-20" is inconsequential. I have used the HK 20A for many hours of flight time with around 14" battery leads with no sign of deterioration. Not that deterioration is apparent for me to observe.

[edit] changed insignificant to inconsequential.

 

[Axbri]

Cyberdactyl: The booms on my copter are a lot wider than yore. My ESCs will only stick out about 2 mm on each side of the booms when mounted on top. But I agree, I a had smaller booms, I would definitely mount them on the side.

To conclude this, I understand that long power wires is bad for the ESCs, and may shorten there life. But if the wires are not extremely long, or the ESCs very poor quality, the shortened lifespan will likely not even be noticeable.

Thank you all for your thoughts.

 

[lonewolf7717]

Can always add a cap on battery lead if concerned about spikes due to length. But for copter design the issue is negligible at best.

 

[lonewolf7717]

Can always add a cap on battery lead if concerned about spikes due to length. But for copter design the issue is negligible at best.

I retract this statement, have seen this topic get very heated amongst fixed wing guys and after further thought.....I would really rather not step off into this.

 

[Cyberdactyl]

I just can't see how motor and/or battery wire of 14 to 16g range is going to be detrimental to a 'normal' (25A) ESC life if the amps are kept around 18 and 10 second bursts are kept around 30A if the lengths don't exceed 20".

Unless you're like Warthox, giving WOT 70% of the time, or lifting a load where the you're giving it sustained 80% or more for long periods, you'll be fine.

 

[xuzme720]

This is starting to sound like the debate between caps/no caps on high power mobile audio! lol
I have to agree with Cyber. The lead lengths aren't long enough at that low amperage to warrant caps. Higher individual draws at the ESC might but you'd have to be in the constant 25-30 amp range...At that point you're using higher gauge wire anyway, so even that is questionable.

 

[jhitesma]

I just can't see how motor and/or battery wire of 14 to 16g range is going to be detrimental to a 'normal' (25A) ESC life if the amps are kept around 18 and 10 second bursts are kept around 30A if the lengths don't exceed 20".

Unless you're like Warthox, giving WOT 70% of the time, or lifting a load where the you're giving it sustained 80% or more for long periods, you'll be fine.

I still agree that with lengths under 24" you're probably not going to see a difference.

But it's still not a matter of wire gauge and amperage - at least not directly. It's a matter of electricity having "inertia" and not being able to switch on and off instantly - and when it does get switched off where does that flow go until it does shut off. Which is why the caps are there on your ESC's...to absorb that extra "flow" that's still coming down the wires. Think of electricity as water as they really do behave similarly. When you shut off your hose there's still pressure in the line if the line is capped. If you have a hose with a sprayer nozzle on it you can see this by closing the nozzle, shutting off the water - and then picking up the nozzle and opening it back up. You'll be able to keep spraying for at least a few seconds.

Electricity is very similar. When your ESC cuts throttle the current that was flowing in the wires has to go somewhere. I can't just complete it's cycle back because the route has been cut off by closing the throttle. The battery will eventually re-absorb it but until then it's still trying to push it's way through. And that's where the caps on the input come in and buffer that rebound. By lengthening the wires you increase the amount of power that can be left with no where to go and as a result you can burn out the caps.

Again this isn't a "your copter is going to suddenly fall out of the sky" kind of thing. This is a over time those caps will weaken until finally one day one will fail and your ESC just won't work.

It's not at all like running to small of a wire (say switching from 14ga to 20ga) where you run the risk of actually melting down the wire because you're forcing more current than it can handle through it so it will overheat.

I wouldn't believe it either if I hadn't burned out drivers on my homemade fuel injection computer on my car due to a very similar situation so when it was explained to me it all made perfect sense as I'd run into it before.

But still at the powers and lengths commonly seen in multis I don't see it being a big issue. Now if you're running 6s packs on 24" booms...you may burn out some caps sooner than expected. But I don't see many people running that kind of setup!

 

[Cyberdactyl]

Exactly. Setups that large don't usually get quick throttle increases OR drops. As a matter of fact, multirotors of that size are almost always treated very cautiously during all phases of their flight unless they are demoing them just to show the capability if extreme measures are warranted.

 

[SteveRoby]

To conclude this, I understand that long power wires is bad for the ESCs, and may shorten there life. But if the wires are not extremely long, or the ESCs very poor quality, the shortened lifespan will likely not even be noticeable.

Thank you all for your thoughts.

In general, on the internet....
There is ALOT of opinion put across as fact by those who really dont have a clue or havent tested/proven their 'claims'
Not that long ago there where debates buy guys who honestly thought that adding a capacitor to the esc would give them more power .... and then them claimed to notice this power increase.

Its a non issue in the real world of RC .
Just using an esc will shorten its life, so will heat, crashes, extreme cold, poor soldering, out of spec & faked components etc etc