A quick question about Brushless Motors, Feat. Spinner Design.

All things antweight

Moderators: petec, administrator, BeligerAnt

patlinnett
Posts: 16
Joined: Tue Mar 21, 2017 1:53 am

A quick question about Brushless Motors, Feat. Spinner Design.

Postby patlinnett » Mon Apr 10, 2017 4:10 pm

Hi there guys,

I'm in the process of building a spinner, and I'm currently at the point of selecting a motor to power the blade. I have been reading this thread , and I am somewhat confused by the recommendation people have given for the motor.

Now I should add, I am completely new to electronics really - I come from a physics background (I know how electromagnetism works on a basic level, but circuitry and actually building electronic systems is beyond my scope). When I have been looking at the ratings for these brushless motors, KV seems to be the cover-all parameter for the power of the motor. However, as I understand it, this parameter measures the maximum angular velocity of the motor for a given voltage (And therefore, indirectly, the torque the motor produces), and the part recommended is not the part with the highest KV to weight ratio on the HobbyKing store - which looks suboptimal to my untrained eye.

One possible explanation for this is that the KV rating does not directly measure torque, but is a measure of some internal limitation that brushless motors have that is currently beyond my sphere of knowledge. Some of the motors on the site also seem to have power outputs, measured in Watts, but not all. This leads me to believe that it is the power of the motor assuming that you are running it at the maximum voltage - Wich I probably won't be, as I'll be running at 7.4V, so doesn't seem relevant.

Can anyone shed any light on this? I've been trawling the internet for reading material about these things but it's proving frustratingly opaque.

Here are some of the motors I have been looking at, I would appreciate it if someone more expert than I would cast their eye over them and give me their thoughts!


Thanks for your time, guys!
Last edited by patlinnett on Thu Apr 27, 2017 11:46 pm, edited 2 times in total.

User avatar
Shakey
Posts: 947
Joined: Sat Mar 13, 2010 8:38 pm
Location: Reading

Re: A quick question about Brushless Motors

Postby Shakey » Mon Apr 10, 2017 6:13 pm

kv is the measure of RPM per volt. So a 7.4v battery and a 1000 kv motor rotates at 7400RPM at full throttle.

The power outputs are as it says on the tin the power output, typically the peak output of the motor. Generally listed at the highest voltage the motor is rated for. Since power is proportional to the square of the voltage: (Listed power x ( intended voltage / max voltage)^2 = output power at intended voltage. (You can substitute cell count for voltage here to make it easier). Do note hobbyking is weird and is not consistent in whether they quote peak or continuous power or which voltage the power is from.

One key thing you have done is the assumption that the highest power to weight is the most desirable. In ants the power to weight ratio is already skewed so high it doesn't really matter to be honest (Unless you're trying to self right from a bar or something). Typically we operate on 30-50w (AT 2s) outrunners (generally a 22m diameter weighing 20-25g) with a kv ranging from 1500-2500. The far more important thing is the motors construction, how robust is it, how will it take hits and also how easy is it to mount to a robot and to mount a blade to it.

From the motors you link at the end of your post:
1 - Too fast, too fragile, terrible blade mounting
2 - Too fast, too fragile, terrible blade mounting
3 - Reasonable candidate but I hesitate on it's ability to take hits and it has awkward mounts.

Generally you don't want to use prop adapters to mount the blade or mount to a shaft, you just want to bolt it to the can of the motor. This is where motors like the tried and trusted DYS 1806 come into play. https://hobbyking.com/en_us/dys-be1806- ... 2s-3s.html

I myself use redcon 2204's which are pretty much identical to the DYS motors but a tad more powerful but way more importantly are hilariously robust, considering how much I have abused these motors yet not one is showing signs of complaint is astounding.
Nuts And Bots - For all your components and ready built antweights!

Alex Shakespeare - Team Shakey / Nuts And Bots:
AWS 44, 45, 49 & 51 Winner - Far too many robots!

User avatar
peterwaller
Posts: 3493
Joined: Fri Feb 15, 2002 12:00 am
Location: Aylesbury Bucks
Contact:

Re: A quick question about Brushless Motors

Postby peterwaller » Mon Apr 10, 2017 6:21 pm

KV is just a measure of the speed of the motor in revs per minute per Volt applied with no load on the motor.
So if the motor is rated at 2000 KV and you apply 8 V it should run at 16000 rpm with no load but this drops off as you increase the load.
It is no indication of power in fact very small motors often have much higher KV rating than larger ones.
To get an idea of the power you need to look at the wattage most of my antweight spinners have motors between 40 and 80 watts.
Looks like Alex has answered this while I was typing. :oops:

patlinnett
Posts: 16
Joined: Tue Mar 21, 2017 1:53 am

Re: A quick question about Brushless Motors

Postby patlinnett » Mon Apr 10, 2017 11:56 pm

Thanks for the responses guys! The voltage to power equation was super helpful. I also hadn't considered the robustness of the motors.

I'm having trouble understanding how you can have a spinner that's too fast, though. Isn't the whole objective to store as much kinetic energy as possible? Is it an issue with the impact being too unpredictable, and the spinner pinging off and pitting itself?

Is this the motor you're referring to, Shakey? If my calculations are correct, then it produces 59W at 7.4V. The power seems a little low to me, but reliability is important. I might pick one up!

I found a more powerful version of the Viking, that produces 128W at 7.4V, and is a bit bigger (And hopefully more robust) and has a lower KV. What do you think of that? I'm thinking of picking one of these up as well unless there are any glaring issues with it.

On a slightly unrelated note, do people use brushless motors for drive systems at all? They just seem better than brushed motors to me. Is it a reliability issue?

GeorgeR
Posts: 57
Joined: Sun Aug 28, 2016 9:53 pm
Location: Bath, Somerset

Re: A quick question about Brushless Motors

Postby GeorgeR » Tue Apr 11, 2017 6:24 am

There's a few issues that Shakey is referring too with motors being too fast;
1) If the weapon itself spins extremely quickly you can struggle to get good engagement on the hits, so despite the high energy of the blade only a small amount is transferred per hit.
2) For a given motor output power the kv is inversely proportional to the torque, so a high kv motor will take much longer to spin up.
3) As you mentioned the problem with the robot launching itself out of the arena.

59W is plenty of power for an ant spinner, for comparison that's roughly double the power to weight ratio of the heavyweight spinners like Carbide, PP3D, etc...

That motor does lack holes to mount the blade directly, which makes life harder, and with the body of the motor being 20mm tall it might be awkward to fit it in the bot (depending on your design). Also the higher current it draws will require a larger esc and battery.

I've not seen brushless drive on an ant, the standard n20 gearmotors have enough power to wheelspin currently, so the only possible advantage would be a reduced weight, but with the n20's being only 9g inc gearbox, i don't think there are any brushlesses small enough.
Team Zero
Zero - rambot - - Vampire - horizontal spinner
Axiom - axebot - - Valkyrie - drum spinner (under construction)

Occashokka
Posts: 430
Joined: Sun Nov 30, 2014 9:34 pm
Location: Stroud,Gloucestershire

Re: A quick question about Brushless Motors

Postby Occashokka » Tue Apr 11, 2017 7:17 am

There are definitely quite a few people myself included who have looked into brushless drive but the reliability of it is always very low as turning 10000+rpm into a controllable drive speed is so difficult because it all has to be self designed, I actually 3d printed one side of a brushless drive but within 20seconds the speed of the first gear onto the second started melting the plastic gear and I tried a belt and that got worn to nothing instantly, I believe some people have had success with it but its not an easy task
Team Badger
Has a 3d printer now yay
-£4.82+VAT (intact)
-cool modulated printed thingy
-not yet built nasty mean spinnt thingy

I'm gonna build something huge and stupid, try and stop me :P

User avatar
Shakey
Posts: 947
Joined: Sat Mar 13, 2010 8:38 pm
Location: Reading

Re: A quick question about Brushless Motors

Postby Shakey » Tue Apr 11, 2017 7:33 am

patlinnett wrote:Thanks for the responses guys! The voltage to power equation was super helpful. I also hadn't considered the robustness of the motors.

1 - I'm having trouble understanding how you can have a spinner that's too fast, though. Isn't the whole objective to store as much kinetic energy as possible? Is it an issue with the impact being too unpredictable, and the spinner pinging off and pitting itself?

2 - Is this the motor you're referring to, Shakey? If my calculations are correct, then it produces 59W at 7.4V. The power seems a little low to me, but reliability is important. I might pick one up!

3 - I found a more powerful version of the Viking, that produces 128W at 7.4V, and is a bit bigger (And hopefully more robust) and has a lower KV. What do you think of that? I'm thinking of picking one of these up as well unless there are any glaring issues with it.

4 - On a slightly unrelated note, do people use brushless motors for drive systems at all? They just seem better than brushed motors to me. Is it a reliability issue?


1 - GeorgeR covered but I'll explain further, even with 2300kv motors spinners start struggling to engage an opponent at high throttle. It's no use having a higher KV motor (which inherently have less torque) if you have to run it at half the speed to hit things. Engagement is how far the discs tooth is overlapping your target when you go to hit, a small engagement is more likely to nudge the target away than actually hit. It's about how far your robot can move into your opponent in the time it takes for the tooth of the weapon to pass the target. Too fast and it's not enough, you sit there grinding the opponent rather than hitting. This is why single tooth spinners are a thing, because it doubles engagement without changing the energy or speed of the robot.

2 - Yes that looks like it, absolutely unparalleled robustness and the easiest mounting points ever. 60w is more than enough in an ant, remember these are only 150g robots and you'll only be spinning something weighing 10-30g. It'll spin up in a second or so which is definitely fast enough.

3 - The blade mounting point is very awkward, you're having to use the prop adapter with a screw on nut, not to say it won't work. But it's not a great way to hold the blade on, conversely look at the 4 bolt holes on the face of the Redcon or DYS.

4 - Brushless drive does not offer any advantages in antweights compared to brushed. While the motors can weigh slightly less you then deal with super high RPM, you also need 2 ESC's one per side to power them. This compares to Brushed where you can get something like a NanoTwo which is smaller and lighter than 2 brushless ESC's offsetting that weight difference. In return you get less responsive drive, less controllability and more system complexity. The N20 gearmotors have more than enough power to wheel spin, any power past the point of losing traction is useless. I built this robot which uses 2 low powered N20 gearmotors to power 6 wheel drive and it still has more power than it can put down: http://i.imgur.com/b5DbABj.jpg

You seem to be a little bit caught up in power of the motors you're using for your spinner, the power of the motor means nothing about the kinetic energy of the weapon. All it'll tell you is how fast the weapon will spin up.
Nuts And Bots - For all your components and ready built antweights!

Alex Shakespeare - Team Shakey / Nuts And Bots:
AWS 44, 45, 49 & 51 Winner - Far too many robots!

patlinnett
Posts: 16
Joined: Tue Mar 21, 2017 1:53 am

Re: A quick question about Brushless Motors

Postby patlinnett » Tue Apr 11, 2017 2:54 pm

Thanks again guys. I hadn't considered the actual impact dynamics - I was just thinking of energy values!

Can you bolt the blade to those holes? Oh wow, I thought they were to attach the motor to the bodywork of the quadcopter! Does that mean that the whole motor rotates...? I can't see from the picture clearly!

In terms of ESCs, is there any particular go-to? I've been looking at these three:


    The Flyduino KISS 30A ESC (2~6S) seems very good. I'm not sure how much I care about regenerative charging, I should be able to last the 3 minutes without it I think. It weighs in at half the weight of the Littlebee, though, although that may simply be the lack of wires, I'm not sure.

    The Turnigy MultiStar 32bit 30A Race Spec ESC 2~4S (OPTO) also looks good - I just can't tell whether the 6g listed is with or without wires. It depicts it with wires attached, but in the description, it says it comes 'pre-tinned,
    solder ready', implying that the wires aren't attached.

I should be able to power either motor with any of these ESCs, right? I would like to try and find a way to make the more powerful one work!

User avatar
Shakey
Posts: 947
Joined: Sat Mar 13, 2010 8:38 pm
Location: Reading

Re: A quick question about Brushless Motors

Postby Shakey » Tue Apr 11, 2017 3:10 pm

Yes the outside can of the motor rotates, You're in the realm of brushless outrunners here! You have the base with the wires coming out which is stationary, then the rest of the bell or casing rotates. This should show you how these work: https://youtu.be/jf97OvYGV3c?t=17s

The normal go to is the turnigy plush 12A, I like the Afro 12A but you have to tell them to lower their quite aggressive voltage cutoff. I use the multistar ones in my quad (7A) and they've been good ESC's.

Bear in mind the 'more powerful motor' is around the same speed as the Redcon. That extra power is not going to translate to more kinetic energy, only increase in headaches to do with peak currents and voltage drop. Remember Motor power and maximum kinetic energy of a spinner are not related*.

*Adding this in case someone gets pedantic, yes a more powerful motor will reach higher speeds IF air resistance is any kind of significant factor. Which it isn't in terms of bars and discs and 40w is already enough for ants even with drums or beaters.
Nuts And Bots - For all your components and ready built antweights!

Alex Shakespeare - Team Shakey / Nuts And Bots:
AWS 44, 45, 49 & 51 Winner - Far too many robots!

patlinnett
Posts: 16
Joined: Tue Mar 21, 2017 1:53 am

Re: A quick question about Brushless Motors

Postby patlinnett » Tue Apr 11, 2017 5:27 pm

Thanks for all the help guys, my parts are on the way now. That video was quite helpful, I thought it was only the axle that rotated!

Now all I have to do is design the actual blade...

patlinnett
Posts: 16
Joined: Tue Mar 21, 2017 1:53 am

Re: A quick question about Brushless Motors

Postby patlinnett » Tue Apr 25, 2017 4:47 pm

After careful consideration, I went for the Redcon 2204s as recommended by Shakey. I'm in CAD as we speak, designing a blade. However, I have no way of precisely taking the measurements for the mounting onto the motor - does anyone have any advice on how to handle this problem? I can't find the dimensions of the mounts on the motor anywhere online, either. I'm designing a single-toothed blade, and I'm very conscious of the balance - the mounting has to be just right, or all the work I'm doing balancing the thing will be for nothing.

Thanks in advance again, guys!

User avatar
Shakey
Posts: 947
Joined: Sat Mar 13, 2010 8:38 pm
Location: Reading

Re: A quick question about Brushless Motors

Postby Shakey » Tue Apr 25, 2017 5:13 pm

https://www.aliexpress.com/store/produc ... 79679.html

The aliexpress listing has some more information.
Nuts And Bots - For all your components and ready built antweights!

Alex Shakespeare - Team Shakey / Nuts And Bots:
AWS 44, 45, 49 & 51 Winner - Far too many robots!

patlinnett
Posts: 16
Joined: Tue Mar 21, 2017 1:53 am

Re: A quick question about Brushless Motors

Postby patlinnett » Tue Apr 25, 2017 5:33 pm

You, sir, are a scholar and a gentleman. Thank you!

patlinnett
Posts: 16
Joined: Tue Mar 21, 2017 1:53 am

Re: A quick question about Brushless Motors, Feat. Spinner Design.

Postby patlinnett » Thu Apr 27, 2017 11:50 pm

Right, so I'm at the point of designing the actual blade, and after slaving away at AutoCAD I have come up with something that looks reasonable.

It's based on a spiral and the dimensions are in mm. Does this seem reasonable? If I've not overlooked anything I'll order some! The only issue I can think of with it really is the fact that it's a spiral - could that affect engagement? Should I make the spiral more aggressive?

Image

Thanks in advance again guys!

Andrew_Hibberd
Posts: 1287
Joined: Tue Jan 20, 2004 12:00 am
Location: London
Contact:

Re: A quick question about Brushless Motors, Feat. Spinner Design.

Postby Andrew_Hibberd » Thu May 04, 2017 8:05 am

From my experience when the disc is up to speed it's less than 2mm of the tooth that engages. So your tooth needs to stick out at least that from the rest. If you are getting it made by someone else it's worth getting a couple of different designs made and try them out.

Also it doesn't look like the cog is centred. Normally the counter weight is opposite to the tooth. Pulsar the heavy weight is a good example of the counterweight direction from the tooth.
TEAM GEEK!


Return to “Antweights”

Who is online

Users browsing this forum: No registered users and 3 guests