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Torque figures for brushless motors ?

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Does anyone out there have any torque figures for brushless motors ? I was thinking about using a 800 size motor in a scratch built model, it produces around 6300g/cm of torque at about 4500 rpm, it's about 3 times the torque of a standard 540 but much less rpm, just wondering how it compares to a low KV brushless motor system ???

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There are a few torque values on the Castle website about their products but I could never find a direct conversion formula for general use. In the low KV range the number of poles makes a difference. If it helps the 2800 kv 4 pole I put in the vanning is comparable to the 1.7 HP nitro at the same RPM range at the wheels. It's a 540 size motor too.

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There are a few torque values on the Castle website about their products but I could never find a direct conversion formula for general use. In the low KV range the number of poles makes a difference. If it helps the 2800 kv 4 pole I put in the vanning is comparable to the 1.7 HP nitro at the same RPM range at the wheels. It's a 540 size motor too.

I'm looking for something with lots of grunt, but not any faster than a 540. look for standard txt-1 / Jugg2 sorts of speed. It will be going in the grapple skidder. I'm thinking about the Ezrun 4274 2000KV motor, but it's 5 times the cost of the brushed motor I have my eye on, and in theory about 4 times the RPM, but I can't find any torque figures for it. The brushed motor I have my eye on has a 70mm diameter can against a 540 sized 36mm, but it's only 60% efficient which is why I want to explore the brushless option. I will have a look at castles website though to see if there's any info I can convert.

Thanks

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Hi Mad -

What sort of speed do you want? And what sort of torque? You mention the capability of the brushed motor, but what are your requirements?

From Hobbyking's homepage they have a motor selector on the right side of the page. Start by picking a size range, then they will provide a listing sorted by kV.

They have everything from this 1104:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=25435

To this 12070:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14427

Once you have the motor selected, then picking an ESC would be the next step.

For my 1/8 scale e-buggy (about 8 lbs fully loaded):

http://www.hobbyking.com/hobbyking/store/__14632__Turnigy_TrackStar_4168_2650KV_1_8th_Brushless_Motor.html

http://www.hobbyking.com/hobbyking/store/__22632__Turnigy_TrackStar_120A_Brushless_Short_Course_Truck_ESC.html

Geared 12:1 with 115mm tires, it has crazy tire shredding power and will go 45 mph easily.

Standard 540 silver can motors are about 2000kV (roughly); you could always source this motor and ESC and run them on 2S LiPo.

http://www.hobbyking.com/hobbyking/store/__22409__Turnigy_TrackStar_1_8th_Sensored_Brushless_Motor_1900KV.html

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=28377

Sensored operation should provide smooth startup...

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For that kind of size motor if your really after torque look at dewalt 18 v drill motors. The torque is savage even compared to the 775 I replaced it with they are cheap and rebuildable too. Like you say though relatively low rpms.

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Hi Speedy,

Thanks for digging that out for me. I'm loving that 12070, definetly my kind of motor, but out of my price bracket. The 1900KV motor you mentioned is very similar to a 2000KV combo offered by ezrun, but surprisingly about half the price (Although the ezrun speedo is good for 4S and is waterproof. However, I don't think I'll be able to fit 4S in the model, so perhaps that is irrelevent.

This is the model I'm putting it in:

http://www.tamiyaclub.com/forum/index.php?showtopic=70984

It's about the same size as the TXT-1, but it's going to be a lot heavier as it made nearly all from steel and aluminium. I wanted it to have the speed as a standard 7.2V powered TXT-1 but on a 3S battery, it's got exactly the same gear ratio as a TXT-1 on 15T pinions. The motor I had intended to fit had roughly 3 times the torque of the 540, but at 8500rpm @12V, so would be a little slower than a normal 540.

That turnigy brushless motor is an 1800W motor max, (assuming that is on 4S, 3S would be around 1350W max), the brushed motor I'm looking at has a power output rating of 80Watts at max efficiency, the only thing that bothers me is I don't want all that brushless power going into RPM rather than torque, and dont want to have to add yet more reduction in the transmission. The model is going to be a miniature forestry machine, and I want to be able to drag at least a small fence post behind it, so torque is far more imprtant than RPM. Question is, would that brushless motor be 3 times the torque or more ???. I must admit I'm very tempted in it at the price, but I have to incorporate it into the model and don't want to have to rebuild scratch built components if I need to fit the brushed motor if the gear ratio is too high for the brushless one.

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I don't think I have a good answer for you. 1800W vs. 80W seems extreme. Also, as the weight of the model goes up, are the diffs fortified so they don't destroy themselves under stress?

I think an essential quality you'll want is smooth startup from rest. That implies brushed or sensored brushless to me. It might be worth scanning some RC crawler boards to see what they use for smooth movement and extra torque. Somebody should have a large crawler out there and overcome these challenges.

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I think the diffs will be ok. They are from a traxxas revo, which is a big nitro monster truck, and I'm not looking at putting that much speed through it.
I agree the 80W versus 1800W seems extreme too. The figures quoted on the MFA website (The supplier of the brushed motor) quoted the power output at best efficiency, which I think is somewhat misleading. The stall current at 12V is quoted at 61A, which equates to around 735Watts. However, the motor is only about 60% efficient, so actual max power output seems to be around 450Watts. Even taking that into account, the brushless motor still has 3 times the power output of the brushed motor, but funnily enough, around 3 times the RPM at the same voltage. It's looking like the brushless motor will give me the same amount of power output at the same rpm, but have the capability to have a lot more rev in it. I'm sorely tempted bt the brushless system, but can't help thinking that the massive size of the 850 is going to give better torque. Perhaps it's just a case of old versus new tech, and I'm a bit of a dinosaur. If only brushless motor manufacturers would publish torque figures :angry: .

The other alternative would be to fit 4 540s in there. The hull at the front is 96mm wide, so I have just about got room for it.

Arrgghh, I'm going to go and have another look at the hobbyking site, and also check turnigys site to see if they have any torque figures.

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What gear ratio are you planning on for this build?

34:1, the same as is standard for the TXT-1.

I just found this monster in Hobbykings website:

http://www.hobbyking.com/hobbyking/store/__17705__XK5674_800KV_Brushless_Inrunner.html

It's primarily for boats I think, only 800Kv, but according to the comments in the reviews, the torque is between 8000-12000g/cm. I'm just wondering if 800KV is too low, as it only gives 9600rpm for 3S. It also may a bit on the heavy side at 600g, but this is much more torquey than the original motor I was planning on using :). I'm very tempted to go for this, it's going to be a toss up between this and either the turnigy motor mentioned earlier, or an ezrun 4274 2000KV waterproof combo.

I'm wondering if it would suitable in a heavy model when it's been designed for use in a boat, but then it's a 4 pole motor and has bags of power, so I'm thinking it should be alright. Maybe. ????

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Martin I have read all this with great interest . As you know I have a similar problem to resolve with the Tonka . Boats I do know a fair bit about though . That outrunner should be ideal for you . Its designed to turn large props on large scale boats or Aircraft . You would be surprised just how much torque that requires . Think of the rotational mass of a lets say 100mm 3 blade brass prop with the added resistance of having to move that volume of water or a Aircraft prop moving that amount of air .

Its weight should work to your advantage too . The greater the mass of the tow rig the higher the mass it will be capable of towing - basic physics . Providing you have the gearing correct ( seems like you have ) and good tyres this beast will pull up roots never mind the odd fence post :D

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How heavy do you think the final model will be? 34:1 gearing is pretty substantial, so the trick is sizing a motor to do the job but not destroy the drivetrain too much with a lot of lurching and hard starts. Does 34:1 include the pinion, spur, other gears, diff pinion and ring gear (i.e., the complete drivetrain)?

You can work out the speed easily enough with:

Motor kV rating * nominal battery voltage = motor RPM

Motor RPM / 60 = motor revs/second

Motor revs/second / gear ratio = axle revs/second

Axle revs/second * tire diameter * pi = distance per second

Distance per second * 3600 seconds = distance per hour

Distance per hour * conversion factor = distance per hour in a regular unit

So,

800 kV * 11.1 V = 8880 rpm

8880 rpm / 60 = 148 revs/second

148 revs/second / 34 = 4.35 axle revs/second

4.35 axle revs/second * 150 mm tires * pi = 2050 mm/second

2050 mm/second * 3600 seconds = 7380000 mm/hour (let's say 7.38 km/hour after dividing by 1000 mm/meter and 1000 meters/km)

7.38 km/hour * 0.621 = 4.58 mph

That's roughly walking pace at full speed. With a 34:1 gear ratio, that says the torque is muliplied by 34, so 10 kg-cm becomes 340 kg-cm at the axle output. That seems like a crazy high axle-snapping torque figure. If the tires dig in, I would expect the axle stubs to snap or the splines to strip.

Here's my 1/8 scale e-buggy setup and resulting figures:

2650 kV * 11.1 V = 29415 rpm

29415 rpm / 60 = 490.25 revs/second

490.25 revs/second / 34 = 14.42 axle revs/second

14.42 axle revs/second * 150 mm tires * pi = 6795 mm/second

6795 mm/second * 3600 seconds = 24462000 mm/hour (let's say 24.46 km/hour after dividing by 1000 mm/meter and 1000 meters/km)

24.46 km/hour * 0.621 = 15.19 mph

This is about the speed of a DT02 Sand Viper or DF02 Gravel Hound with a stock silver can in it. This might be overkill in terms of speed; you might have to reduce the end point on your transmitter throttle channel to have better control over the model. I don't know the torque rating on the 2650 kV 6-pole motor, but I'm sure with 34:1 gear reduction it's plenty. I know based on how quickly I can launch 8 lbs to 45 mph with just 11:1 gearing that there is a ton of torque available.

You could also re-run these calculations with 1900 kV and 2100 kV 1/8 scale motors for less speed; Hobbyking has some grey-can inrunners for 1/8 scale that are about $10 less than the 2650 kV motor. The 1900 kV motor seems pretty nice -- the top speed of the model would be about 10-11 mph.

800 kV motor -- about $50

1900 kV motor -- about $40

2100 kV motor -- about $40

2650 kV motor -- about $50

http://www.hobbyking.com/hobbyking/store/__17705__XK5674_800KV_Brushless_Inrunner.html

http://www.hobbyking.com/hobbyking/store/__17980__Turnigy_1_8th_Scale_4_Pole_Brushless_Motor_1900KV.html

http://www.hobbyking.com/hobbyking/store/__19114__Turnigy_1_8th_Scale_4_Pole_Brushless_Motor_2100KV.html

http://www.hobbyking.com/hobbyking/store/__14632__Turnigy_TrackStar_4168_2650KV_1_8th_Brushless_Motor.html

The Trackstar short course truck ESC is rated for 120A continuous and 760A burst. It also has a nice 6V, 5A BEC for a good steering servo. I like this ESC because it can be programmed with the transmitter; it doesn't require a programming card to set up. This costs about $55.

http://www.hobbyking.com/hobbyking/store/__22632__Turnigy_TrackStar_120A_Brushless_Short_Course_Truck_ESC.html

You'll want to make sure your LiPo is up to the job, especially if you start loading the motor heavily -- just make sure the capacity in amp-hours (milliamp-hours/1000) times the continuous C rating is about 2x the ESC rating. So, a 3S battery with about 4000 mAh rating and 65C continuous / 130C burst rating would be pretty good. Some examples:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=19145

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=19153

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=21941

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=21945

The 6400 mAh pack looks like the best value in terms of capacity, C rating, and cost.

Fun problem to think about... Good luck!

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Hi Speedy,

Wow that's a massive response with a lot of data in. I think the final model will probably be about 6-8kg. On top of the rest of the bodywork, I need to fit a steel logging blade, and add the rear grapple, plus extra motors for the steering, operation of the blade, and lowering. raising the grapple arm.

The gear ratio includes the complete drivetrain, everything,from pinion down to the final axle.

I recalculated the sped for 165mm tires (which is the Jugg tyre diameter), which came out at about 5mph for the 800KV motor. The torque does seem very agressive, but I like overkill like that. Note sure if you've had a look at the build thread or not, but the transmission is definetly up to the job as far the centre gearbox goes ;) (The final drive gear is mod 1.0 gears with 15mm wide teeth, made from a very tough steel :) ). I've also used 8mm diameter steel drive shafts. I think the weak points will be the ring gear and pinion in the diff housing, but these were the strongest I could find.

I re-ran the numbers for 2000KV, and it came out at about 14mph, probably a bit to quick, looking for 10mph ideally I think (so probably around 1500Kv), but if the torque is adequate I don't have to use full throttle.

I'm also looking seriously at the 1900KV 1/8 motor, just worried it might have overheating issues with all the weight and the extra RPM.

I'm still thinking about the ezrun combo I mentioned earlier, I had a look through TC earlier today and found some threads where people had trouble not being able to get reverse with turnigy speedo's, so whatever brushless I decide to go for I will be using a ezrun ESC with it. I'm also thinking in the back of my head that if the ezrun 4274 2000KV motor is too fast I can always put it in the TXT-1. I am very tempted by this big 800KV job, but the other thing to throw in the mix is that it's currently in the bargain bin at hobbyking, so if I buy it then it won't come with any warranty, and one of the reviews of it said that their motor had unfinished threads, and the bearings were a bit loose, so I'm wondering if there are build quality issues.

Also with regard to the ESC, I don't need to worry about power output for a servo as I'm planning to use a 380 or 540 motor through a reduction box to drive a screwjack for the steering. Overkill is my middle name :)

The 3S battery I'm planning on using is 4000mah, and I think around 25-30C rating, max rating on the 800KV is 78amps, but that is at 6S, so I think the battery should be ok.

I'll hopefully make a decision on this on the weekend, will let you know what I decide, but it has very useful having feedback from you, especially the food for thought on the rpm and torque calculations.

Martin I have read all this with great interest . As you know I have a similar problem to resolve with the Tonka . Boats I do know a fair bit about though . That outrunner should be ideal for you . Its designed to turn large props on large scale boats or Aircraft . You would be surprised just how much torque that requires . Think of the rotational mass of a lets say 100mm 3 blade brass prop with the added resistance of having to move that volume of water or a Aircraft prop moving that amount of air .

Its weight should work to your advantage too . The greater the mass of the tow rig the higher the mass it will be capable of towing - basic physics . Providing you have the gearing correct ( seems like you have ) and good tyres this beast will pull up roots never mind the odd fence post :D

Hi Brom,

Thanks for this. I wasn't sure what sort of size prop this thing would spin, but it's reassuring to know that it should have plenty of pull. I suppose a U-boat engine would go pretty good in a tractor :). If I get it then it should fit nicely directly over the front axle, which should help with the grip. PS, the motor is an inrunner, not an outrunner, and I think it's one of the biggest inrunners available. The diameter is the same as the length of a 540. The 6mm output shaft will fit directly onto a 6mm UJ, which is also handy. Got to have another look around to see if there are any other options at hobbyking, but for the price I think it will be this one or the ezrun combo. I've discounted the brushed motor, as I think the rpm on it will be too slow, and it's a similar size to the 800Kv monster. Will add here what decision I make, and of course the motor will appear in the build thread in due course. (I've nearly got the gearbox finished now, I've been editting the last post in the thread with this weeks updates, so fitting the motor will happen shortly after that)

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one other thought has come to mind . How about using a cordless power screwdriver motor ? I was looking at one I have knocking about . I need to strip it down but it certainly has the torque required .

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one other thought has come to mind . How about using a cordless power screwdriver motor ? I was looking at one I have knocking about . I need to strip it down but it certainly has the torque required .

I don't have any power screwdrivers to dismantle, the power drills I've taken to pieces all have had 550 motors in them. If I was going to go brushed, I would have gone for the 850, as I need much more grunt than 1 540. I'm just going round and round in circles at the moment. I like the idea of the 800KV brushless motor, it will be like having a merlin engine in a fiesta, but it's twice the price of the brushed 850, and the 850 has around 6000g/cm of torque, but the rpm is even less than the 800KV motor. So for the 850 brushed I'd be looking at about 3mph, which I think will be too slow. I think having written this I'm going to ditch the brushed motor as an option. At the other end of the spectrum is the 2000KV motor, which will do about 14mph, which i think will be a bit too quick. 8-10mph top speed will be ideal I think, which equates to around 1500-1600KV. I have seen motors in this range, but they are slightly smaller than the 1/8 1900KV motor speedy suggested, and the 2000KV ezrun I thought of, which means the torque may not be as much, and they don't have built in heatsinks. Arrghhh, decisions, decisions. The boat motor will go to 6S, which would give me 10mph, but I don't have room for a 6S battery. I think I will go for a brushless system, just got to pick the motor that's right for me, at a reasonable price...

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Well, I just bit the bullet and pulled the trigger on the 5674 800KV boat motor. Lets hope it's not too slow, and works ok when it arrives. Got to pull my finger out now and get the gearbox finished, and the drive shafts connected up and ready to go for when it arrives.

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Your grapple skidder is just all ate up with motor! (adapted from Darrell Waltrip explaining his 1977 NASCAR success) :lol:

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Dewalt 12 14.4 and 18 volts are 800 size range with rebuild able brushes. I have a single 12v running 18v geared very high in my emaxx

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Does anyone out there have any torque figures for brushless motors ? I was thinking about using a 800 size motor in a scratch built model, it produces around 6300g/cm of torque at about 4500 rpm, it's about 3 times the torque of a standard 540 but much less rpm, just wondering how it compares to a low KV brushless motor system ???

Been years since college when I actually knew how to calculate this stuff, and I have to admit that I'm not sure how well the math holds up against reality, but there are some interesting formulas on this page http://www.micromo.com/motor-calculations.aspx

With some algebra, it looks like a quick and dirty number would be:

(oz-in torque) = ((battery Voltage)*(ESC Amp rating))/((kV rating)*(battery Voltage)*0.00074)

(Someone please check my math on this before spending too much time cranking numbers through it)

Might have to substitute the battery C rating for the ESC Amp number depending on which is lower.

I'm probably overlooking something important but skimming it over it looks like this could give a theoretical torque for a given ESC+motor+battery setup. Of course this does ignore some important things like the resistance of the entire system.

If someone has the time to run some numbers through this it would be interesting to see what comes out and compare it against any manufacturer data (or actual test data) we can find.

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Been years since college when I actually knew how to calculate this stuff, and I have to admit that I'm not sure how well the math holds up against reality, but there are some interesting formulas on this page http://www.micromo.com/motor-calculations.aspx

With some algebra, it looks like a quick and dirty number would be:

(oz-in torque) = ((battery Voltage)*(ESC Amp rating))/((kV rating)*(battery Voltage)*0.00074)

(Someone please check my math on this before spending too much time cranking numbers through it)

Might have to substitute the battery C rating for the ESC Amp number depending on which is lower.

I'm probably overlooking something important but skimming it over it looks like this could give a theoretical torque for a given ESC+motor+battery setup. Of course this does ignore some important things like the resistance of the entire system.

If someone has the time to run some numbers through this it would be interesting to see what comes out and compare it against any manufacturer data (or actual test data) we can find.

I think you've overlooked something, as the battery voltage on the top and bottom of the equation cancel each other out ....

Also, it doesn't take into account things like whether or not the motor is a 4 pole or 2 pole. I don't think the ESC rating is relevent unless you're running it right on the limit, if you could work out torque, I think it would be based upon the max watts of the motor, the KV rating, and the actual voltage applied, but physical size must come into it as well. The motor I have ordered is 56mm diameter x 76mm long can, a normal 540 is 36mm diameter x 56mm long. The larger the motor, the greater the torque I would think, even if only from the fact that the magnets are larger, and have more leverage being further from the centre of rotation.

BTW, converting the theoretical torque at the wheels of 340KG/cm in other units gives 3.4Kgm, 33.34Nm, 4722ozin, 24.6lbft, and 295lbin. Should have plenty of pulling power.

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