Newbie motor questions

[nitrag]

Been doing my research for the last week or so and still can't find any concrete evidence of the following. Sorry in advanced for all the questions, I vow to update the wiki for people following in my footsteps.

What's the difference between a xxxx watt motor that requires a max voltage of 6S vs 10S. Is the S just letting you know "for max efficiency/power use me"? If I buy a motor with a max volt of 10S but only use 6S to the ESC, what am I losing? Do I then have to get a 10S capable ESC and batteries?

Disregarding physics and motor quality and roughly speaking:
Is a single 150kV motor approximately the equivalent power/torque to two 300kV motors?

Assuming top speed >30kmh is not important to me (power/torque is) would choosing a motor that's say 175kV and 230kV make a difference if I can just make up the torque in the gearing? Where's the magical formula to calculate gearing? I'm pulling my hair out with all the combinations of motors and batteries I haven't even looked into how to make gearing work. How fine tuned can you get the gearing, looks like (unless you have a 3d printer or a CNC manufacturing and $$$) you're limited to the few stock ratio/sets of the e-board companies?

What's the difference between dual 130kv and dual 230kv? Max speed will be less on the 130kV but it will climb hills better and accelerate to speed sooner? Is that it? When that 130kV hits it's max speed can the motor burn out if kept there for a long time (assuming gearing isn't terribly setup)?

Assuming same specs, you would think that two motors would split the power/load required between themselves, therefore range would not dimish perhaps even improve if the two motors are running cooler and at a lower duty cycle? But you have all that power for when you need the hills. Am I thinking about this right?

Sensored motors. This means they have Hall sensors built into them? And through the cabling they feed this information back to the ESC which controls of the motors better? Besides ride smoothness, what else will this improve?

How import is airflow to these motors? I plan to make a mountain board and am consider enclosing about 3/4 sides to prevent mud/sand. Is this ok? Or do they need direct airflow for proper cooling? (Again, assuming you're not running the motors at full throttle for an extended time period).

That is all...for now

 

[Rolling_Friction]

for practical purpose think kv# corresponds to the ideal rpm at which the motor is at its best. lower kv motors perform best at lower speeds. for gearing; Mountain-board wheels are generous with the amount of space you have for gear ratios of various sizes, there is bigger space restriction for gearing in long boards, so getting the right motor might be important.
that being said, my 213kv motor powered by 6s lipos (it can do 10s) was no slouch

 

[Hummina Shadeeba]

U can get the same wattage and torque and heat out of a motor regardless of kv or voltage if they're paired right. If not paired right be a bit more inefficient, generating more heat and reducing ur possible max wattage. Wattage numbers by manufacturers are bunk. U will get different continuous or even peak wattage output in the Arctic or the equator.

The volt max of a motor is often not real. The true volt max depends on the rpm the bearings can take and the dielectric ability of the winding insulation. Amps are what the motor runs on and the cause of heat and possible failure. Higher voltages really only keep the esc cooler and a bit more efficient but if u have a esc that can do high amps and low voltage and u pair that with a high kv motor ur just as good on the motor performance and even the esc performance and ability to produce power without getting hot

 

[thepronghorn]

Kv does not correlate to motor power at all. It is the velocity constant of the motor, which is inversely related to the torque constant (Kt) of the motor. The velocity constant is just rpm/V (or whatever units of angular velocity and electrical potential you want to use) and the torque constant is just Nm/A (again whatever torque and flow rate of charge units you want).

Watt ratings for chinese motors are not to be trusted. The best measure of a motor's power is to look at its size (5055 < 5065 < 6355 < 6364 < 6374 < 8085 < 80100) for the common outrunners. When comparing similar size motors, you can also look at price a little bit to determine quality of construction, a higher quality motor will be more powerful and efficient for the same size than a lower quality motor.

To get the most power out of a motor, you want to spin it as fast as you can. Motors are mostly limited in rpm by parasitic (hysteresis and eddy current) losses in the stator iron as well as bearing drag. For common chinese outrunners that number seems to be about 9000 rpm before parasitic losses start getting significant, although gearing down from 9000 rpm to a longboard wheel would be a pretty tough challenge unless you were looking to go very fast.

Two motors running together is generally less efficient than one big motor, although packaging them is sometimes easier. Think more bearing losses, drag from two belts, etc.

Edit: This is a great writeup of some basic motor theory and why different windings make no difference in the power capabilities of a motor.
http://vedder.se/2014/10/chosing-the-right-bldc-motor-and-battery-setup-for-an-electric-skateboard/
Sensors help the controller know where the rotor is at low rpm when the back emf from the motor (which the controller uses to sense rotor position) is small, so sensors are mostly useful for startup.

In real life people sometimes claim that they get more torque out of a lower Kv motor. I haven't experienced this myself, but I would guess that it's due to people not adjusting their operating voltage when they switch motors. If you one motor with 100 Kv and another with 200 Kv, the 100 Kv motor will make more torque per amp so if you have the same batteries and same controller you will get similar amperage into your motor which will be more torque. However, this increased torque comes at a cost since the tradeoff of getting a high torque constant (more torque per amp) is higher winding resistance which means your motor will make a lot more heat as the controller shoves the same amps it sent to the lower winding resistance, higher Kv, lower Kt motor.

 

[Hummina Shadeeba]

I wouldn't say kv doesn't relate to motor power at all it tells the rpm at which most power can be
Produced (half max rpm) and what rpm is most efficient(max rpm)

 

[thepronghorn]

I wouldn't say kv doesn't relate to motor power at all it tells the rpm at which most power can be
Produced (half max rpm) and what rpm is most efficient(max rpm)

Well to get rpm you need voltage and Kv. If I have a 30Kv motor how do you know whether it's a 1kW or 10000000000kW motor? How do you know whether max power is at 100rpm or 100000000000rpm? If I run my 30Kv motor at 100V, then max power is at 1500rpm but it could still be a 1kW motor or a 10000000000kW motor...

Also motors are most efficient when copper losses equal parasitic losses which is somewhere below max rpm.

 

[nitrag]

Great feedback guys, thanks.

@thepronghorn, thanks for the vedder link. I think I stumbled upon this too early on and didn't understand it, but now it makes more sense. Now I get it, forget kV and just compare motor sizes as you mentioned before. I'm now leaning towards a single 6374.

In my experience, these losses start to get significant around 60k electrical RPM, which for a 14-pole motor is about 8570 mechanical rpm (most 50mm+ outrunners have 14 poles, some unusual ones have 18)

I don't understand where he is calculating the 8600rpm from. Most of the motors I'm seeing can't even reach 8000rpm until you're up in the 12S+ range. How is he getting this? Can you not just use the manufacturers specs and multiply kV*V to get Max RPM?

I'm trying to put a theoretical spreadsheet estimator together to help me visualize the high level calculations, any feedback would be great, feel free to make improvements.
https://docs.google.com/spreadsheets/d/1iE_BvkimCekVcrlBexwdNtvULhceRXTYWjCehIOyV18/edit?usp=sharing

 

[Hummina Shadeeba]

I think it's electrical rpm. It's 6 or 7 tiimes the regular rpm with 12 magnets. Think it's six times.

Prong I don't u understand what ur last saying and if those are real questions ur asking

 

[nitrag]

Okay I might be coming around to understanding this now.

Is the "Turns" listed on HobbyKing the same as "Poles" vedder is describing on his blog?

Edit: Wait no...now I'm confused again:

At 12s, which seems good from the ESC perspective, and a moderate charge level, we have 3.8 * 12 = 45.6 volts. Since we want to run the motor at 8.6k RPM, we need a KV of 8600 / 45.6 = 188. Now, that is quite low. Since there are none or few 50mm outrunners with that KV available, we can do two things:

1. Run at 10s. Then we need a KV of 8600 / (10 * 3.8) = 226. Luckily, there happen to be 225 KV motors available on hobbyking

If I apply the same formula to a 6S battery of 22.8V I get a 377 kV motor...which you guys say to stick below ~280/250...

 

[thepronghorn]

Great feedback guys, thanks.

@thepronghorn, thanks for the vedder link. I think I stumbled upon this too early on and didn't understand it, but now it makes more sense. Now I get it, forget kV and just compare motor sizes as you mentioned before. I'm now leaning towards a single 6374.

In my experience, these losses start to get significant around 60k electrical RPM, which for a 14-pole motor is about 8570 mechanical rpm (most 50mm+ outrunners have 14 poles, some unusual ones have 18)

I don't understand where he is calculating the 8600rpm from. Most of the motors I'm seeing can't even reach 8000rpm until you're up in the 12S+ range. How is he getting this? Can you not just use the manufacturers specs and multiply kV*V to get Max RPM?

I'm trying to put a theoretical spreadsheet estimator together to help me visualize the high level calculations, any feedback would be great, feel free to make improvements.
https://docs.google.com/spreadsheets/d/1iE_BvkimCekVcrlBexwdNtvULhceRXTYWjCehIOyV18/edit?usp=sharing

RPM is just from Kv*V

electrical RPM is from RPM*pole pairs
poles are magnets, most hobby outrunners have 14 poles, so 7 pole pairs

I think it's electrical rpm. It's 6 or 7 tiimes the regular rpm with 12 magnets. Think it's six times.

Prong I don't u understand what ur last saying and if those are real questions ur asking

Lol they're not real questions. I'm trying to show how Kv means nothing without other parameters. The questions don't have a correct answer...

Okay I might be coming around to understanding this now.

Is the "Turns" listed on HobbyKing the same as "Poles" vedder is describing on his blog?

Edit: Wait no...now I'm confused again:

At 12s, which seems good from the ESC perspective, and a moderate charge level, we have 3.8 * 12 = 45.6 volts. Since we want to run the motor at 8.6k RPM, we need a KV of 8600 / 45.6 = 188. Now, that is quite low. Since there are none or few 50mm outrunners with that KV available, we can do two things:

1. Run at 10s. Then we need a KV of 8600 / (10 * 3.8) = 226. Luckily, there happen to be 225 KV motors available on hobbyking

If I apply the same formula to a 6S battery of 22.8V I get a 377 kV motor...which you guys say to stick below ~280/250...

That's because the motors we are using are designed to be run on higher than 6S. You can run them on less, you just sacrifice a bit of the max power potential of the motor.