What has more torque? lower or higher kv?

[CM9000]

before I jack someone elses thread to start an argument... here is my statement and what I believe.

lower kv has more torque than a kigher kv brushless motor. in or out.

so to compare... you have a 770kv outrunner, its gonna have more torgue than a 2200kv outrunner.

a 1900 inrunner is gonna have more torque than a 3100 inrunner.

all say running at their top recommended cell amount.

so running a 1900 kv on 4s has the same amount of torque as a 3800kv on 3s? I think not.

the 1900 has more punch, more torque, and is more efficient than a 3800 on 3s.




take the people who like to do speed runs. who has the jump? the lower kv guy running more cells than the higher kv guy. who's motor is cooler to the touch after the run? the lower kv guy.

I am willing to believe I am right to a certain extent.

if im wrong, go ahead and correct me. and prove your right.

 

[Szczerba]

Some paraphrasing from the Internets and some fun math, that I barely comprehend. Don't quote me on any of this except, "A lower kv motor does have more torque, at start up..... per amp."

A lower kv motor does have more torque, at start up..... per amp. The lower kv motor will make more torque at a given current than a higher kv motor all else being equal (i.e. You are comparing motors that are identical aside from how they are wound).

But this is just starting torque. Once the motor starts spinning, at the lowest PWM duty cycle of the ESC, your torque is going to be (theoretically) equal no matter what the KV of the motor is. Anyone that thinks different is forgetting how an ESC works and what KV means.

Remember how the ESC works, using PWM duty cycle to create a limited voltage on the motor. For instance, a 20% duty cycle means that the voltage is 1/5th but the current will be 5x. 10% duty cycle means 1/10th the voltage and 10x the current. Etc. By 10%, I mean if the ESC provides 100A, 1V at the motor (100W), a 10V battery would see a drain of 10A from the ESC (100W) -- if the ESC were 100% efficient.

As for KV, say your battery provides 10V for the sake of simplicity. Likewise the battery can provide a fixed amount of current and no more. If the motor were spinning at 10000rpm, a 5000KV motor would require 2V (0.2 duty cycle) and the 10000KV motor would require 1V (0.1 duty cycle). Of course the 5000KV motor has twice the turns of wire so it generates 2x the torque per amp as the 10000KV motor. In theory, everything is equal in the end. Once you bring PWM into the picture, you are dealing with power and not current, you are dealing with power.

The only way a lower KV motor would have greater torque is if an ESC was just a simple linear voltage drop-down circuit whose efficiency was like a linear regulator (ie. 10V -> 5V = 50% efficiency, 10V -> 2.5V = 25% efficiency).

But, ultimately the 'max torque' of a motor is determined by its design, the coil core material, the rotor size, the can size, etc. As the RPMs increase, this torque is flat to a point but then starts to drop off. By the time it reaches max RPM, the torque is very close to zero.

Edit. I have slowly started migrating to lower kV motors in my pure crawlers. The low end startup "torque" and feel is like no other. Then just volt up to get some mid range punch back. I am finding my 8-10lbs pure scale crawlers work best at around 1800kv on 3s or 4s. I run 1200kv on 4s with good success as well. I have always ran sub-1500kv motors on 4s on my big rigs over 12lbs, where you need that low end start up torque to get things rolling.

...

 

[CM9000]

thanks shane. so im half wrong. but saying they all have the same amount of torque is just.. ridiculous. I could be at 1/4 throttle with two different kv motors and can tell witch one has more torque at a crawl. driveshaft breaking torque. the low end of the rpm range..

from a racers perspective... if a kit is built out of the box and geared to the mft. per turn of the motor.. a 5.5 turn and a 10.5 turn sensored motor have the same top speed. but they are geared different. gear them the same using the recommended pinion for the 5.5 the 10.5 will be slower. but now it has more torque.

im in disbelief that all brushless motors have the same amount of torque no matter what.

 

[Szczerba]

If you are solely trying to compare different kv motors, gearing is not a related variable. Transmission and axle gearing are of course going to change torque at the wheels.

 

[CM9000]

well in terms of one searching for more low end torque it can be achieved 2 ways in light.

 

[JohnRobHolmes]

Motor Kv and torque are not correlated when all else unchanged. A motor is the same motor regardless of Kv, ignoring extremes that render it unusable.


What IS affected by Kv is the ESC. Lower Kv requires lower phase amps per unit of torque, thus lower Kv motors will cause less voltage drop and thus slightly higher efficiency inside the esc. If we want to maximize torque, we must maximize voltage and motor unloaded speed. In example, a 2200kv motor on 6s will have a slight advantage over 4400kv on 3s. Very slight, and measurable in temperature reduction at the esc.


In the real world where most folks have a ton of 3s packs, we are stuck with just one voltage that fits into the rig. So what now to maximize torque? Maximize Kv within the motor limits and vehicle gearing limits, with the goal of usable wheel speed. So between 4400 and 2200 Kv on 3s, the 4400kv can output almost twice the torque if we aren't getting into saturation. If we can gear it down for 2200 wheel speed, we have almost four times the torque at the wheel. We also snuff out the phase amp differences and lose the low Kv efficiency edge in esc for a given torque load at the wheel.



Because of the real world limitations, maximum usable torque will always occur at the highest voltage or Kv choice that meets your needs. In a nutshell, spin the motor as fast as possible. Volt up and gear down, or Kv up and gear down. My best performing (brushless) trail rig has a 3500kv puller 540 motor. Can't go higher in Kv, because we can't gear down more. My best performing (brushless) "slow" crawler runs 2200 puller 540. The trail rig has gobs more torque, but that sort of wheel speed would not suit the crawler. It doesn't lack torque either, but certainly is a different class of power.