Amperage concerns on larger multirotor

[FlagstaffMike]

Hi guys,

I am building my first not-mini multirotor (X8) and I am starting to have some concerns that what I have learned will not hold over to the electrical needs of the larger craft.

For example each of the motors is rated for 32 amps which means I am immediately in the 10 AWG range for routing power. Also this makes me think that I should avoid PDBs in favor of an all wire harness as I'd really have to know the design of the PDB in order to know how much it could handle.

Unfortunately my question is pretty open ended but I am just wondering if I am thinking correctly or if I am perhaps over engineering this. I have looked at some other build threads and videos and it doesn't seem like the setups have been excessively beefy and I don't want to add unnecessary weight.

Thanks in advance,
Mike

 

[LitterBug]

Perhaps start by googling Octocopter PDB and see what options there are. Here's a 280A design I found pretty quick.
http://www.rc-terminal.de/OCTO-PDB-...sbord-Power-Distribution-Board-for-OCTOcopter

Cheers!
LitterBug

 

[Snarls]

Depending on your intended use, a PDB should be no problem. If you are not lifting something super heavy or trying to win races you will probably stay far away from the rating of the motor. If you want to play it safe though, most PDBs will include a current rating in their specs. A quick search yielded this 300A PDB for a nice price. Of course you could do a direct wire harness, but depending on your soldering skills it could get real messy real quick. A PDB will also allow you to easily remove and install ESCs as swell as other equipment like BECs and extra battery leads.

 

[FlagstaffMike]

Thanks guys, I have been looking at those, as you said snarls a harness could get messy, so I already have a link saved to an octo PDB.

I guess the open ended-ness of the question (PDB was one example) is how much upscaling will I have to consider?

Another Example is that with 32A for 8 motors I am looking at pulling 256A, not even an AX150 connector is rated for that. So I think, for example, that I would have to hook two batteries up to a single PDB to allow for this draw. Then the PDB could pull 300A from the batteries and the connector won't be the bottle neck.

But this is based on the really basic stuff I learned from miniquads of taking the amp draw of the motor multiplying it by number of motors etc. I guess what I'm really asking is this:

I am assuming, physics being what they are, that what I have learned on minis will hold true as the craft gets larger, that the math will remain the same. Is that correct? Or is there a different methodology to assembling a 650 vs. a 250 size multi?

 

[Snarls]

You most likely won't be pulling 250A for long periods of time so an AX150 may do the job. You can also check out XT150s which seem to be rated up to 250A. I'm not sure how big this project is, but a lot of the larger multis use at least two batteries in parallel. Like you said that will reduce the stress on the connectors.

With the bigger multis it is a lot more about reliability and safety. The math is all the same of course, but a lightweight 250 falling from the sky is going to hurt a lot less than a fully loaded 650. You'll be wanting to make sure that everything is tightly secured and that your FC has a redundant power source.

 

[LitterBug]

Not sure how far along you are in purchasing parts and you didn't specify what S rating you are planning on running. 3S, 6s? You would be better going with higher voltage to reduce current and in turn reduce weight by using smaller gauge wire.

Watts = Current x Volts

So if you are running 32amps at 3S that would be 11.1v * 32Amps = 355 watts per motor.

If you ran 6S for that same 355 watts, the current draw would only be 16 amps. (355 watts / 22.2 volts = 16 amps)

Obviously you would need ESCs and motors rated for the higher voltage...

CHEERS!
LitterBug

 

[FlagstaffMike]

Thanks!

@snarls Awesome, i have been googling different variations of "high amperage bullet connector" for quite a while and never found anything like that. I was kind of hoping I wouldn't be maxing out the motors all the time and you're right those might do the trick.

Oh, your comment about a redundant power supply to FC is exactly what I am looking for. When watching videos and reading build logs I haven't seen anyone mention that before. Do you mean I should use something like this? http://www.hobbyking.com/hobbyking/...y_Redundant_Dual_8A_UBEC_Rx_Power_System.html

Again, when googling build logs there is just so much out there, is there a place you suggest I go that maybe has more info like this?

Also looking at naza m v2, which from what I have found has some pretty good location and failure features. Granted nothing that keeps gravity from taking hold. (I know I wont be able to use a gimbal with the 8 motors but DJI gimbals are a little insane anyway)

@LitterBug That is a great point, I had thought about this for a while and decided to sort of bite the bullet on it by sticking with 4S (By looking around at other applications it seems like a good voltage that I can apply to some medium sized multis as well as some larger flying wings)... or even some smaller ones