High torque motor recommendation?

[fossil]

Hi, newbie to rock crawling and brushless motors here.

I want to pair a custom chassis(1/10 size) with two brushless motors for a robotic project that needs to be able to carry a payload of about half a kilo off road.
I will probably have a two speed gearbox for high torque-low rpm and low torque-high rpm.

Now, I know that high kv and Amp ratings make for lots of torque, but the problem is that I have never used outrunners outside of RC planes, and inrunners... Never.

So I don't know how to compare brushless motors to brushed motors in terms of torque, but I do know that I need two brushless motors that can each deliver at least 12 kgf-cm after gearing down to the high torque gear, ideally with around 80-120 rpm at least at that same gear.


So my question is, is there a simple way to compare the torque of brushless motors against other brushless motors, and brushless against brushed?

And if you have any suggestions/recommendations regarding specific motors or ballpark spec figures, I'm happy to hear them.

(Also, the can size does not really matter much as this will be on a custom chassis which will be designed to fit the chosen motors, regardless of size)

Thanks in advance, and sorry for the long post!
Fossil

[Natedog]

Check out Holmes hobbies and Dewalt drill motors and planetary gearboxes they are bullet proof and have high and low gears.

[fossil]

Thanks for replying.
Those types of motors are a little bit too high end for me though, I'm looking for a under $50 kind of motor.
What I really want to know though, is how to compare brushless motor torque in numerical terms (like x torque is more than y torque).

Assuming my chassis weight is around 2 kilos, and the payload and other stuff brings it up to 3 kilos.
Is there a simple way to know and say: yes, this motor has enough torque (after gearing) to do what I want it to do, with these wheels and terrain, etc. Perfect!

Thanks.

[meatmonkey]

It doesn't sound like your planned project will require much in the way of monster motor torque. Your planned weight for a vehicle with 2 motors is about what many of us run with 1 motor on a scaler.

Why do they have to be brushless? What is the required top speed? If it doesn't have to go any faster than a walking pace then a pair of cheapo Axial 27 turn brushed motors with proper gear reduction will do quite nicely.

[fossil]

Thank you for the additional questions, they help narrow my questions down.

I would like them to be brushless for the lower noise, lower maintenance, higher torque compared to brushed(under certain conditions, I know....). Also, I heard that they last longer.

My speed requirements are high teens at least(on-road), with torque to spare.
And about running pace(7 or so kph) in grass?
This is where a two speed gearbox could be useful.

So what about either Turnigy TrackStar 1/10 12.0T 3300KV Brushless Motor or Trackstar 540 size 4 Pole 4250KV Sensored Motor or something else like that? I'm just looking at these based on Current draw and kv, maybe someone has a better option?

Also, I might go with just one motor, for cost. Two motors, even if they are lower powered, need two esc's, which will cost more than one bigger esc, and is twice as likely to fail?

Thanks.

[new2rocks]

You shouldn't need two motors or even a 2-speed transmission to meet your objectives. I run sensored brushless setups on several rigs that outweigh yours, including an 8+ lb Wraith and a 7+ lb SCX-10. Running on 3S, I get plenty of torque and can easily hit 10 mph (16 kph) in either one. A good 3000-3500kv, sensored,4-pole brushless motor such as a Tekin ROC 412 3100 or HH Puller Pro 3500 on 3s should be more than enough to do the trick. My recommendation: keep it simple (1 motor, single-speed tranny) and buy quality instead of quantity.


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[Natedog]

Originally Posted by new2rocks

You shouldn't need two motors or even a 2-speed transmission to meet your objectives. I run sensored brushless setups on several rigs that outweigh yours, including an 8+ lb Wraith and a 7+ lb SCX-10. Running on 3S, I get plenty of torque and can easily hit 10 mph (16 kph) in either one. A good 3000-3500kv, sensored,4-pole brushless motor such as a Tekin ROC 412 3100 or HH Puller Pro 3500 on 3s should be more than enough to do the trick. My recommendation: keep it simple (1 motor, single-speed tranny) and buy quality instead of quantity.


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X2

Traxxas Titan 21 turn motor is great on 3S lipo, ok on 2S.

[Olle P]

Originally Posted by fossil

... ideally with around 80-120 rpm at least at that same gear. ...

This figure bugs me... Is this your intended motor speed, or the wheel speed?
Below 1,000 rpm the motors get less predictive.

You also mention noise. Brushless motors typically generate more noise because of the ESC's PWM frequency. A brushed motor can be made to run with just a little bearing noise.

Motor life time is a matter of years and how much maintenance you do. Shouldn't come into play for a school project.

[new2rocks]

Originally Posted by Olle P

This figure bugs me... Is this your intended motor speed, or the wheel speed?
Below 1,000 rpm the motors get less predictive.

You also mention noise. Brushless motors typically generate more noise because of the ESC's PWM frequency. A brushed motor can be made to run with just a little bearing noise.

Motor life time is a matter of years and how much maintenance you do. Shouldn't come into play for a school project.

Agreed. The OP's rpm figures make sense if the OP is referring to rpm at the wheel. When you add in the reduction from the transmission and axles, that would put motor rpm in the 30k+ range to achieve the desired vehicle speed.

I agree that it's possible to get a nice combination of torque and wheel speed with a good brushed set up (e.g., Crawlmaster 13T on 3S might work), but it's easier to get there with a good sensored brushless set up.


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[fossil]

Originally Posted by new2rocks

snip
I run sensored brushless setups on several rigs that outweigh yours, including an 8+ lb Wraith and a 7+ lb SCX-10. Running on 3S, I get plenty of torque and can easily hit 10 mph (16 kph) in either one.
snip
My recommendation: keep it simple (1 motor, single-speed tranny) and buy quality instead of quantity.
snip

Thanks for the recommendation, its good to know that most decent motors will work.
I think that I will go with one motor, per your suggestion. It allows me the budget for a better motor and esc.

Originally Posted by Natedog

X2

Traxxas Titan 21 turn motor is great on 3S lipo, ok on 2S.

Thanks for the suggestion. I still want to go with 3s brushless though.

Originally Posted by Olle P

This figure bugs me... Is this your intended motor speed, or the wheel speed?
Below 1,000 rpm the motors get less predictive.

You also mention noise. Brushless motors typically generate more noise because of the ESC's PWM frequency. A brushed motor can be made to run with just a little bearing noise.

Originally Posted by new2rocks

Agreed. The OP's rpm figures make sense if the OP is referring to rpm at the wheel. When you add in the reduction from the transmission and axles, that would put motor rpm in the 30k+ range to achieve the desired vehicle speed.

I agree that it's possible to get a nice combination of torque and wheel speed with a good brushed set up (e.g., Crawlmaster 13T on 3S might work), but it's easier to get there with a good sensored brushless set up.
snip

Yes, new2rocks is absolutely correct, it is wheel rpm. I will use a motor around the 3000kv range.

About the noise, is it made by electronics or mechanics in the esc?
Btw, are brushed motors not PWM controlled too? So do brushed esc's not make noise too?

Also, what is the effect of a higher or lower rpm motor? Can you not gear a higher rpm motor more than a lower rpm motor? Or are you limited by temp?

I will be buying the electronics and motor after the chassis is mostly complete.

Thanks everyone!

[DeviledEggs]

Faster motors will run hotter and make less torque. Lower kv motors will have more torque than higher kv but lower rpm and hence less top end.

[Olle P]

Originally Posted by fossil

About the noise, is it made by electronics or mechanics in the esc?

The audible noise comes from the motor. The motor is fed a pulsed current that cause vibrations in the entire motor. (Also used deliberately to generate the tones heard when the ESC is turned on.)

[QUOTE=fossil;5333206]Btw, are brushed motors not PWM controlled too? So do brushed esc's not make noise too?[QUOTE=fossil;5333206]PWM is used for brushed motors too, but there you have the option to add a capacitor between the motor cables to get a smooth signal with decidedly reduced (voltage) amplitude.
Can't do that with BL, since the currents there also switch directions at relatively high speed.

Also, what is the effect of a higher or lower rpm motor? Can you not gear a higher rpm motor more than a lower rpm motor? Or are you limited by temp?

"RPM" is a very shady unit to use here...
The unit Kv is better. That's defined as "RPM per volt at no load".

Two BL motors with the same can and windings but different rotor size (such as for example Novak Ballistic 17.5T "crawler" vs "stock" editions) have different Kv but the same power. Lower Kv there means more torque at the same voltage. At a given power in- and output they will produce the same heat.

Normally the rotors are the same though, and the difference in Kv stem from different windings in the coils...

Originally Posted by DeviledEggs

Faster motors will run hotter and make less torque. Lower kv motors will have more torque than higher kv but lower rpm and hence less top end.

That's the common misconception!

With less windings (higher Kv) the motor will produce less torque given that the current is kept constant. Given that the winding resistance is greatly reduced with less windings that means the voltage must be kept very low and the generated heat is thus also reduced.

If, which is far more common, the voltage is kept constant the following apply:
Higher Kv (fewer windings) => More power. Both speed AND torque increase.
To get the extra power from the motor more current is drawn from the battery, and the power loss in the motor stay at about the same percentage. If you double the input and output powers the difference, given off as heat, is also doubled.

The most common way to fight the extra heat is to simply reduce the load by gearing down. That way less power is delivered by the motor, most of the time, and thus less power is dumped off as heat. One can also find that higher Kv motors have their peak efficiency at lower loads than low Kv motors.

For a more scientific explanation of it all see my motor theory thread.

[dirtman71]

You could run a sensored motor and esc that is programable like a Tekin(if you want to spend the money) or a Brushless Hobbies 120a. Run a 17.5t to 25.5 to keep bottom end torque with lower gearing, very smooth with a sensored motor. Then add a little timming/boost/turbo in the esc to add top end wheel speed equal to a 10.5 or 3500kv motor

brushlesshobbies esc is like $70(had mine for over 3 years and its been flawless and ran hard)
a good sensored motor will run almost $100 but can find cheaper ones that that will last for $20-50

[fossil]

Originally Posted by DeviledEggs

Faster motors will run hotter and make less torque. Lower kv motors will have more torque than higher kv but lower rpm and hence less top end.

Ok, thank you for answering that.

Originally Posted by Olle P

...snip.....
"RPM" is a very shady unit to use here...
The unit Kv is better. That's defined as "RPM per volt at no load".

Two BL motors with the same can and windings but different rotor size (such as for example Novak Ballistic 17.5T "crawler" vs "stock" editions) have different Kv but the same power. Lower Kv there means more torque at the same voltage. At a given power in- and output they will produce the same heat.

Normally the rotors are the same though, and the difference in Kv stem from different windings in the coils...
That's the common misconception!

With less windings (higher Kv) the motor will produce less torque given that the current is kept constant. Given that the winding resistance is greatly reduced with less windings that means the voltage must be kept very low and the generated heat is thus also reduced.

If, which is far more common, the voltage is kept constant the following apply:
Higher Kv (fewer windings) => More power. Both speed AND torque increase.
To get the extra power from the motor more current is drawn from the battery, and the power loss in the motor stay at about the same percentage. If you double the input and output powers the difference, given off as heat, is also doubled.

The most common way to fight the extra heat is to simply reduce the load by gearing down. That way less power is delivered by the motor, most of the time, and thus less power is dumped off as heat. One can also find that higher Kv motors have their peak efficiency at lower loads than low Kv motors.

For a more scientific explanation of it all see my motor theory thread.

Sorry about the rpm-kv mix up, that was a mistake on my part.
Thanks for answering my questions on noise and for clarifying some things about the motors.
The link to your thread is very interesting, although a little hard for me to fully understand.

Originally Posted by dirtman71

...Snip...
Run a 17.5t to 25.5 to keep bottom end torque with lower gearing, very smooth with a sensored motor. Then add a little timming/boost/turbo in the esc to add top end wheel speed equal to a 10.5 or 3500kv motor

Snip
...a good sensored motor will run almost $100 but can find cheaper ones that that will last for $20-50

Ok, thanks for the advice.

If I go with BL, I will use sensored.

I think that my first BL setup will be cheap.
Do you or someone else here have experience with sensored turnigy trackstar motors?
Also are there any other BL motors in the $40 price range(like the trackstar) that are considered good?

I have my doubts about the quality of $40 BL motors (as opposed to $40 brushed) and hobbyking cs reputation, but I don't really have the budget for what you would consider a decent BL setup.

So as I understand, a $20-$50 BL motor is not as good (in terms of reliability and lifetime) as a brushed motor of the same price for the most part?

I think that, for now, I will plan to use cheap brushed motors in my chassis, like the axial 27t (in #2 post), and get BL when I am satisfied with my chassis

Thanks again everyone!

[dirtman71]

For crawling there is nothing wrong with a brushed set up...plus at least the motor is waterproof. And a AE2 and 27t motor can be had for around $60

[Olle P]

Originally Posted by fossil

The link to your thread is very interesting, although a little hard for me to fully understand.

The in-depth tech parts is meant for engineers, so that they can verify my reasoning and maths.
For most readers the summaries should suffice.

Originally Posted by fossil

If I go with BL, I will use sensored.
I think that my first BL setup will be cheap.
Do you or someone else here have experience with sensored turnigy trackstar motors?

The combination Crawler - Brushless - Cheap rarely go well together.
It's half decent provided that the crawler has worm drive axles, otherwise it's crap.

As Dirtman pointed out: Brushed motor is the way to go with a crawler. It will give good to very good performance at a low cost. To do better with brushless expect to spend >200 USD.

I used to run a Turnigy TrackStar 17.5T in my crawler, since my crawler has worm drives. The top speed on 4S LiPo was okay, but the low speed control I want isn't there, partially because the sensor has a minimum of 15 degrees timing built in.
Don't expect much in terms of drag brake either. That's a non issue with worm drives, but otherwise important for a crawler.
Now I have a HH Revolver S Stubby.

[fossil]

Originally Posted by dirtman71

For crawling there is nothing wrong with a brushed set up...plus at least the motor is waterproof. And a AE2 and 27t motor can be had for around $60

Thanks, I'll go with brushed then.

Originally Posted by Olle P

The in-depth tech parts is meant for engineers, so that they can verify my reasoning and maths.
For most readers the summaries should suffice.

The combination Crawler - Brushless - Cheap rarely go well together.
It's half decent provided that the crawler has worm drive axles, otherwise it's crap.

As Dirtman pointed out: Brushed motor is the way to go with a crawler. It will give good to very good performance at a low cost. To do better with brushless expect to spend >200 USD.

...Snip....
Don't expect much in terms of drag brake either. That's a non issue with worm drives, but otherwise important for a crawler.
Now I have a HH Revolver S Stubby.

Ok, so I will not use cheap BL then.

Worm drives? Probably not the best solution for me, because I've decided that crawlers are not optimal for me, as below.

I'm still going to try study some of the equations, to better understand BL motors.

Also, my requirements have changed slightly. Because I think that my project will be better achieved with a truggy, due to the terrain around here(mostly flat off and on road, smooth slopes).
Plus, truggies are fast(for me).

Still going brushed then.

Thanks!

[Olle P]

The main question is: Do you want to go fast and/or slow?

Going slow (mostly): Cheap brushed system.
Going fast (mostly), with low demand for low speed control: Cheap sensored brushless system.
Demand for both low speed control and high average speed: More expensive system, possibly brushless.

[fossil]

Thanks! That's the sort of answer that I was looking for.

I want to go mostly fast, in the 30-40 kph range.

So you think a cheap brushless system will do? I have no experience with inrunners and I'm worried that a cheap system will fail fast.
At least brushed motors are more dependable, thanks being around a long time?

[new2rocks]

If you want that kind of speed and don't care about low-speed control, get a sensorless, brushless system (like one made for short-course trucks). If you get one with a 4-pole motor, you'll have plenty of power plus plenty of top end for your needs. If you care about precise low-speed control in addition to the top end, sensored brushless would be the way to go.


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