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 Finding brushless motor specs

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fogdog



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PostSubject: Finding brushless motor specs   Thu Aug 25, 2011 2:00 am

I am having trouble finding brushless motor specs, especially for torque. Would be nice if all current brushless motor specs were on a chart somewhere. In the past when I used more brushed motors, I was always able to find specs that stated so many oz-in/amp. Seems like all I can find now are the kV, which relates to motor speed.

If the motor lists the watts at a certain voltage, can I use that to find the torque? Was thinking of converting watts to HP, then using HP to find torque, but not sure how or if it is even possible.

We are building a 15# bot and are looking for drive motors.
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rdubard
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PostSubject: Re: Finding brushless motor specs   Thu Aug 25, 2011 6:55 am

I had the same frustration when our teams started using brushless. I think the reason that torque is not listed is because the torque-speed 'curve' for brushless is not a simple linear thing like it is with brushed.

For instance, a brushless motor typically will NOT work properly if it must produce a lot of torque at startup. Instead it will do what's sometimes called 'cogging,' where it kinda jerks a little and makes a sort of grinding noise--do this long enough and something burns up (motor or esc). Bigger motors, and outrunners seem to be less likely to have this problem, but so far I haven't found a way to avoid this when selecting brushless motors. We just buy one that will give us the desired top speed and isn't too large for the space, weight, and current supply limitations. If it cogs, well we try to reduce the startup demands (by reducing friction, loosening belts, changing pulley ratios, etc), play with any settings available on the ESC, and if all else fails put it in a drawer for some other project and buy a bigger motor.

There are ways to estimate the rated torque, but then the question becomes "at what speed does it do that?"
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Stephen
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PostSubject: Re: Finding brushless motor specs   Thu Aug 25, 2011 11:41 am

why are u looking at brush less motors is there a reason because we have always used bane bots motors that are brushed
and they work well

http://banebots.com/index.html
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Evan Steeves
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PostSubject: Re: Finding brushless motor specs   Thu Aug 25, 2011 7:24 pm

dont worry about torque if you plan on using an inrunner as a brushed replacement for a p16 gearbox or banebots using rs540-550 motors.

I highly recommend the castle 1410 motor on 2s-3s it has about 4x the torque of a nice handwound brushed motor but about 45k rpm at 12 volts and 750 watts
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fogdog



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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 12:34 am

Stephen wrote:
why are u looking at brush less motors is there a reason because we have always used bane bots motors that are brushed
and they work well

We are building some new bots and will need more motors. We are looking at the brushless because we keep hearing/reading about how they are more energy efficient. If I am not mistaken, I thought I read that brushed motors are around 40% efficient, whereas brushless are closer to 80%.
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fogdog



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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 12:50 am

Evan Steeves wrote:
dont worry about torque if you plan on using an inrunner as a brushed replacement for a p16 gearbox or banebots using rs540-550 motors.

I highly recommend the castle 1410 motor on 2s-3s it has about 4x the torque of a nice handwound brushed motor but about 45k rpm at 12 volts and 750 watts

Pricey little buggers.....guess that's what happens with brushless. The Castle Creations site says the 1410 has a max of 2S. I am just a bit weary dishing out that kind of coin to find out it's not enough to get us running. Evan - How heavy was your bot that you used the 1410's for? We are in the 15# category.

One of the bots we saw last year that ran well had a Traxxas motor installed onto a Banebot gearbox. Anyone have any experience with Traxxas motors?
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Stephen
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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 2:17 am

We are building some new bots and will need more motors. We are looking at the brushless because we keep hearing/reading about how they are more energy efficient. If I am not mistaken, I thought I read that brushed motors are around 40% efficient, whereas brushless are closer to 80%.[/quote]

yah im not sure like i said thats what we have always used
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rdubard
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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 5:36 am

Actually, I had assumed you needed brushless for a weapon--I have not had success using them for drive, but then I really haven't tried all that hard. If there is a battle-proven brushless drive setup, then it might be worth the money. I know team Jag (rjw is the big poster here for that team) used them, but I thought their bots seemed 'twitchy' after they made the switch. And you are right, brushless are much more efficient--you could use smaller batteries, or have better pushing power and/or top speed.

Someone asked about Traxxas (brushed) motors--both our 120's used the Traxxas 775's for drive motors on the 'chinese cheapie' copy of the banebots gearbox for that size motor. On our 2nd place robot el Cholo, they worked beautifully (of course, we had 6 of them for our 6 wheel drive), they all survived the entire competition and the robot was very fast and a pretty fair pusher. On Super Fluffy, which took First Place, we lost the front two, but I'm pretty sure that's a design problem, cuz it had a 55lb drum so the front two motors were basically moving the entire robot.

Of course, 775's are prolly way too big for a 15lber.
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gtPR



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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 8:57 am

Quote :
If there is a battle-proven brushless drive setup, then it might be worth the money.

Agreed!

I say keep it simple, especially if your just starting out. If you have the time and money to experiment with a brushless drive setup then by all means go for it! but having something that you know won't fail during a fight seems like more of a priority to me. Also I think one should aim for a slower and more controllable robot over top speed.
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Evan Steeves
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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 9:22 am

fogdog wrote:
Evan Steeves wrote:
dont worry about torque if you plan on using an inrunner as a brushed replacement for a p16 gearbox or banebots using rs540-550 motors.

I highly recommend the castle 1410 motor on 2s-3s it has about 4x the torque of a nice handwound brushed motor but about 45k rpm at 12 volts and 750 watts

Pricey little buggers.....guess that's what happens with brushless. The Castle Creations site says the 1410 has a max of 2S. I am just a bit weary dishing out that kind of coin to find out it's not enough to get us running. Evan - How heavy was your bot that you used the 1410's for? We are in the 15# category.

One of the bots we saw last year that ran well had a Traxxas motor installed onto a Banebot gearbox. Anyone have any experience with Traxxas motors?


I have used the 1410's in sand drag rc cars geared 3:1 with 4" paddle tires that's the largest load i was able to put on them and with my ice 50 i logged a 130 amp burst load for about 1 second as it spooled up i dont think a 15pound robot with planatery drive will even phase this motor.

If you are considering traxxas motors I warn you to not try it, although they make very user friendly systems they are underrated so far as power handling when I worked at the robot marketplace I had almost 15 burned up traxxas brushless systems come in during my 1 year employment I had only 5 burned up castle motors and 3 were water damage.

personally any 4 pole 540-550 size brushless inrunner is going to crush its brushed equal in every aspect

the 4 pole rotor is what makes it viable for drivetrain use as it dosent need to spool past 1000rpm to make torque as all 2 pole brushless motors do, it can make torque as low as 5-10 rpm as i have tested it in rc rock crawlers using similar ratios such as a banebots 16:1

also they are rated 3s max but i have used them up to 4s just keep it below 75k rpm or else the rotor will explode and violently lock up the motor and break whatever it is mounted to.





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fogdog



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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 9:31 am

rdubard wrote:
Actually, I had assumed you needed brushless for a weapon--I have not had success using them for drive

So is it more common to use brushless for the weapon and brushed for the drive? If so, why? The previous post makes brushless seem like an obvious choice for the drive. I am sure that there will be different opinions on the matter and will come down to what has been battle proven.


Last edited by fogdog on Fri Aug 26, 2011 9:51 am; edited 1 time in total
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Evan Steeves
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PostSubject: Re: Finding brushless motor specs   Fri Aug 26, 2011 9:48 am

fogdog wrote:
rdubard wrote:
Actually, I had assumed you needed brushless for a weapon--I have not had success using them for drive

So do most people use brushless for the weapon and brushed for the drive? If so, why?

Most people use brushless for weapon and brushed for drive. the reason for this is brushless drive has been limited to motor technology and 4 pole inrunners are only a year old as people play more with 4 pole motors they will learn that they are just as controllable as their brushed counterparts but way more powerful in rpm and torque.

The difference is people know brushed works its simple and its cheap.

people that try brushless are delving into a new area because they must tune their system to replicate the brushed system without making disadvantages for themselfs such as cogging and high amp draw from wrong kv selection.

my best advice if you want to go brushless drive is to pick up two mamba max pro's and two 3800 combos sell the sidewinder escs from the 3800 combos and recoup some of that investment.

the mamba max pro can be programmed to make ANY brushless motor close or identical to a brushed motor, PROVIDED you understand what the graph of the torque curve needs to look like as a brushless motor responds differently.

rough setup to clone brushed performance would be 40% soft start, 100% punch, zero degree motor timing advance, and a power graph that resembles a parabola for the first half then linear for the second half, and finally 30% drag brake.
that was my setup for my rockcrawler running a transmission ratio of 14:1 with a 3800 motor and i could not stall the motor on ANYTHING with 5.5" tires i did strip the transmission's first stage 3 times before i sold it.
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