Revo BL Conversion questions

[Sammus]

Originally Posted by Serum

the internal resistance of a lower turn motor is higher than the higher turn one.

Artur (griffinru) made a motor calculator. those motors you mention are in his database.

Why is that? I thought I higher turn motor would have higher resistance? since with more turns the length of wire is longer and thus so is the resistance...

Also I couldn't find GriffinRU's calculator, but I found BrianG's datasheet which includes the specs of those motors, which seems to agree with my understanding.

Or is the internal resistance something different to just the normal resistance that I'm thinking of?


Also, with lower kv motors being higher torque, wouldnt that cancel out the extra current burst because you would have to gear a the lower kv motor higher, so it would be using more torque but at a lower rpm = same power...?

[Serum]

sorry for the confusion;

a higher KV motor (lower turn) has a lower internal resistance than a lower KV (higher turn)

and yes, you are right; the internal resistance of a lower turn (less length of wire with a larger diameter) has a lower internal resistance. That's why it can take more current.

Like i said; it simple takes a larger current burst to spin up a high kv motor, than a low kv motor. gearing would compensate the difference in gearing at the same inertia/speed. but bottom line is that the startup burst will be higher of a higher KV motor.

But that's not all;

One of the reasons why these motors heat up is because they are less ideal for partial load; this seems to be due to eddy currents that disturb the movement of the magnet. with thin sliced magnets (segmented or sintered) the eddy currents on partial load are remarkable lower, and makes it more ideal for partial load. magnetism is a weird thing.

it also takes an ideal gearing per wind of motor to get
into its own sweet-spot (read the story in BrianG's signature ' is my setup too hot ' this would illustrate why you can't simple say 'geared for the same speed with a different KV'

[BrianG]

I can see how it can be confusing for a person newly into brushless.

It comes down to this: it takes X amount of power to launch your vehicle with the acceleration and top speed you want. But you have to take gearing, weight, etc into consideration.

Power is volts X current. Looking at this formula, it's probably easy to see that for a given power, increasing the voltage decreases the current and vice versa. However high power is easier and more efficient to obtain with higher voltage and lower current. Lower currents are drawn when using higher turn motors (higher resistance).

Torque is dependant on motor kv and current. Torque (in ft-lbs) is 1352 / kv X amps / 192. So, decreasing kv or increasing current creates more torque. For the same kv, larger motors will generate more torque compared to a smaller motor. This is because they will draw more current (lower resistance).

For example, lets say you think you need 1500w to make the vehicle perform like you want. Let's say you have a 6s and 4s battery. 6s (22.2v) requires 67A. 4s (14.8v) requires 101A. The 6s option is better to use 6s than 4s because of the lower current. You generally pick the highest voltage your ESC and wallet will support. Then, you pick your motor kv based on that voltage. Some motors have a higher max rpm than others. Cheaper motors lose efficiency at higher rpms. Feigao/Wanderer motors typically operate best at around 35k max rpm. Better motors operate better at higher rpms (like up to around 58k for Neu).

You also have to pick the proper size motor (for torque) based on the size and weight of vehicle. Larger vehicles like buggies, truggies, MTs, etc work better with larger motors like XL series, or Neu 1512 or 1515 series, or an equivalent size motor.

So really, when designing a system, you start by picking the size motor you'll need (for the torque). Then, decide the voltage (higher is always better). Then, pick the motor kv based on that voltage by dividing the max rpm by the voltage you decided on.

[lincpimp]

Well put Brian, I could not have said it better myself. The cheaper motors also seem to prefer lower current draws (lower kv models). Case in point, a feigao 9l on 3s and a 15l on 5s. I have tried both in my 2wd emaxx, both geared for the same speed. The 15l does not get much more than warm, but the 9l runs hot, and requires good batteries to perform.

[BrianG]

Yeah, along with the hard formulas, there IS a certain amount of user experience that helps when making motor selections.

[lincpimp]

Brian do you think that there is any specific reason that the lower kv feigao motors work cooler than the higher kv motors? Is it just the extra torque and lower current demands, or am I just seeing the battery perform better due to less load on it? Setups were almost identical, but I have about 80amps cont on the 5s, and more like 120 amps on the 3s.

[BL-Power]

Hey Linc,

a motor with lower kv is running cooler because the efficiency of electric motors is going down with rising amps. So if you got 120A on 3s, the efficiency will be around only 70%-75%, that means from the around 1100W you battery delivers only around 800W go to the pinion and 300W are going into heat. At 80A on 5s, the efficiency will be at around 85%, so from the 1300W your battery delivers around 1130W will go to the pinion and only 170W go into heat.

[lincpimp]

Thanks BL, makes sense to me!

[Sammus]

I read somewhere (one of the mamba manuals perhaps?) something that basically said that brushless motors will use all the power you can throw at them. This agrees with my experience with brushless systems - it implies that either your batteries or your motor is going to be overloaded and overheating - and thats a full throttle - if your driving around half throttle then your esc will just cook. That follows my experience pretty well. If I put some crappy batteries (3s 3000mah 15C) in my rustler it doesn't go very hard and batteries will cook. If I put some relatively good (3s 4000mah 25C) batteries in, the batteries barely get warm, but the esc and motor cooks.

Does this mean you have to match your batteries with your esc/motor combo as well? and according to people here there is an optimum gearing as well, so then you can't change gearing or something will overheat? that doesnt sound like theres a lot of versatility in these systems? something just doesn't seem to add up right right... I don't want to spend $600 on a brushless conversion to have a truck that I have to stop and check temps and let cool down every 5 minutes.. I crave the power of brushless in my truck, but to me nitro engines (although very messy and expensive to run) seem so much more versatile and reliable (or have been in my experience anyway...)

Basically I guess is there a balance with brushless/lipo combos? If I run quality nimh packs in my rusty then nothing overheats and its got nice controllable power good speed and is fun to drive. If I put lipo's in, either it goes like a dog and the batteries cook, or it absolutely insanely overpowered to the point where you can't give it full throttle without it swapping ends or flipping, which usually results in the esc overheating trying to limit all the power from the batteries, or if u let the power through the esc, the motor cooking from dealing with all the power..

Sorry bout the long post guys :P I hope I'm kind of making sense.

[suicideneil]

There are limits to how many amps a motor can pull, but generally it should be fewer than the battery can supply, and the esc can handle, otherwise thats when things start to cook as you discovered. If you use the most efficient motors like Neus, and a 6s setup, you should get plenty of power yet run at a safe temperature. THe MM motors arent all that efficient -they seem to be similar to feigaos in design and construction- , so that would be why they run so hot, and suck the life out of whatever battery you hookup; they can pull a rather large current for their size, and thats down to the insanely high Kv ratings. The MM esc just gets stuck in the middle, and suffers from the high current being pulled through it.

If you switched the MM motor in your rusty to an S can feigao, or VXL even, you should see much improved temps and run times- use BrianG's calculator to determine your best voltage/motor selection choices; the same rules apply governing gearing/voltage/motor kv as has been previously mentioned.

As for batteries, much depends on your budget, but buy the best you can afford for the specific application (size, weight, capacity); an efficient system wont draw so many amps, so you can get away with a lesser capacity or C rating, but you want atleast 80amps on tap for any setup I believe (4000mah 20c, or 8000mah 10c etc etc)

[Sammus]

OK So if I get a low kv motor (say 11S - 3200kv) that will sit at around 35k on my 3s it should run cooler, even If I gear it up to get some decent speed (25/76 or something in the rusty).

So back to the revo anyway :P If I get that cheapo turnigy 100A HV esc, say run 6s in the revo then I could perhaps find a feigao 11XL somewhere I should be sweet - it would rev out to about 33k, then gearing it say 18/38 I could pull 45+mph. What kind of burst currents would you expect from 6s in a revo? That esc is only rated to 110A burst, so I could fry it...either that or triple my esc expenditure :P

[BrianG]

If you gear it reasonably, I doubt an 11XL will pull anywhere near 110A, even burst. I would expect 10A average (from runtime and batt capacity calculation) and 70-80A bursts. 80A @ 6s is the same power as 120A @ 4s.

[Serum]

I would expect it to be around something like that too.

120A bursts might be possible, but my guess is that it requires a tall gearing for ~70mph or so.

[Sammus]

Well according to the speed calc the 18/38 gearing should top out at about 47mph (in a perfect world...one so perfect that my wild guess at tire ballooning is accurate :P).. hmm should be insteresting to see how this esc works.. only thing that worries me is there doesnt seem to be a settable lipo cutoff. "low" "mid" and "high", 3 settings for a 12s controller? somethin fishy goin on...

[suicideneil]

LVC is based on the single cell voltage (total voltage divided by number of cells), so low might be 2.8v per cell, mid would be 3v, and high 3.2v, at a guess...