Brushless Motor Ratings

[daveimi]

Hi all, need to ask the stupid question to make sure I have this down.

1. Inrunner means a powered shaft, outrunner means a powered outer can?
2. kv is how many rpm per volt the motor does?
3. Amps is how many amps it will pull at full chat?
4. Wattage is similar to any other motor, in that its volts x amps and therefore the max power it gives off regardless of can size?

Been looking at brushless motors for a planned beetle and whilst some manufacturers give you something like 2830, it's a 28mm can with 30mm shaft length, you can have two 2830's and one be double the power of the other depending on manufacturer. Other manufacturers don't even give you that you just get something like NT17 etc. So...to try and get round this I'm just looking to confirm the above so that if I select motors on Hobbyking etc. by size I can then compare the power outputs of the different models to get to where I wnat to be.

As ever any input with examples of what your using for feather vs beetle applications massively appreciated. (ie. what do the big feather spinners run vs the scary beetle spinners. What sort of wattage and kv do you look for. Pro's cons to inrunners and outrunners.

The info is scattered through a few different threads but want to get it all down in one place so I can refer back to it.

Thanks in advance as ever.

[john_frizell]

You asked:

1. Inrunner means a powered shaft, outrunner means a powered outer can?

Yes, and the powered outer shell drives a shaft.

2. kv is how many rpm per volt the motor does?

Yes

3. Amps is how many amps it will pull at full chat?


Yes.

4. Wattage is similar to any other motor, in that its volts x amps and therefore the max power it gives off regardless of can size?

Yes.

Outrunners usually have a lower kv rating than inrunners.

[typhoon_driver]

Just to add to Johns response,

"Outrunners usually have a lower kv rating than inrunners"

They also generally have a greater torque value for a given size of motor

[daveimi]

Oh OK that's cool. Which brushless do you run in your feather?

[Eventorizon]

If you are talking weapon, it varies depending on the type of weapon but generally you are look something with 2500W+ of power, eg -

NTM 50-60
Scorpion HK4025 or 4035
Turnigy HeliDrive SK3

Discs and drums don't need such big motors as they are fairly aerodynamic. But if you have a beater or bar then you need more power, generally speaking. Binky has a HK 4035-560, max output of 3500W, running on 10S. Conker runs a massive HK 5020 with a max output of 5350W, on 12S.

Note, I am having to put up the maximum power ratings as many companies advertise their motors based on that number rather than the continuous rating. Easiest option is to go with a known motor like the NTM 50-60.

[typhoon_driver]

It's been a few years since I've used boner (heh) but I was running a scorpion 4525 or 4535 (can't remember which). Either way, lots of power and pricey but a lovely motor.

[daveimi]

That's fab thanku loads! Been a big help!

[harry hills]

What are you considering building, Dave?

[daveimi]

Wanted to build a nice drum but with my skills god knows. Will probably end up with the usual tie wrap / gaffa tape creation. It's hard finding a brushless with a metric shaft.

[harry hills]

Where are you looking? Generally unless you're looking at 540 class replacements it's hard to find one without a metric shaft.

[daveimi]

Been looking at a lot of the hobbyking 28mm and 35mm ish sized stuff and a lot of the shaft diameters appear to be something of an inch (pet peeve of mine). However just found a fingertech blade mount I can get with paypal and the guy apparently can drill them to what you need. Want to build something that doesn't need a pully, just mount straight to the motor. I know that's not the best idea but it's a start. Want to use a light blade at high rpm as an undercutter. Something like alex's robot or sam's.

[mrsam]

I'll throw my two cents in seeing as Undercutters seem to be my thing, I can see the draw of wanting to mount the blade directly to the output shaft but for Beetles i would strongly advise against it. The current crop of Beetleweight spinners are truly scary machines, Lynx's single tooth bar has made it through 15mm hdpe and 3mm (Jamie?) steel, to support that it needed a 12mm high tensile shaft and needed the be run via a pulley. Obviously i can only advise, do what you feel like doing but the power to weight ratio of these machines means that your brushless' output shaft will likely Die a very quick death.

Edit: I seem to have confused Beetles with Feathers

I think my point still stands, doubly so if you're planning a Feather

[maddox10]

The smaller motors use 4 or 3.17 (1/8") shafts. That is asking for dead motors after the first hit.

[Eventorizon]

Been looking at a lot of the hobbyking 28mm and 35mm ish sized stuff and a lot of the shaft diameters appear to be something of an inch (pet peeve of mine). However just found a fingertech blade mount I can get with paypal and the guy apparently can drill them to what you need. Want to build something that doesn't need a pully, just mount straight to the motor. I know that's not the best idea but it's a start. Want to use a light blade at high rpm as an undercutter. Something like alex's robot or sam's.

Don't do what I did with that under cutter beetle... bad idea! I did that so I had something besides a pushing box.

Mounting something directly to the motor in a FW is generally a bad idea because the shafts will never be thick enough to take the loadings. I wouldn't get a FW motor with a shaft that is less than 6mm, and even then I might want to swap it out for something thicker. I swapped C3's shaft from 6mm steel to 10mm Ti.

Doing it in a beetle is viable if you are careful. Just make sure you get it right or you will fine it will break very quickly.

Are you trying to build a beetle or a feather btw? You have asked about both so I'm now confused.

[daveimi]

Yeah my ramblings have confused me too. Want to build a beetle And will not mount to motor directly if the consensus is it will muller it.