based on my limited knowledge on motors, why are NEOs considered less powerful for swerve and in general when they have a higher kT? when are drive motors going to draw more than 200 amps to take advantage of the falcons greater peak torque?
also, does the Vortex’s lower kT mean that its actually worse than NEOs?
I am basing this off of the recent motor performance report by CTRE
Because kT is not the complete story of the motor. DC motors can be charactierzed by four constants: torque constant kT, backemf constant kb (often given as kV which is 1/kb), resistance R and inductance L.
A motor with a high kT can output very little power if it also has a really high resistance (see pancake motors). While the manufactures don’t post information resistance/inductance in any capacity, the NEO vortexes and Krakens definitly have a much lower resistance which means that for a given amount of backEMF, they can pull more torque than higher resistance motors which means they have more useful output power.
so what your saying is motors with lower resistance need less voltage to produce the same amount of current/torque as higher resistance motors (V/R = I). can this not be solved by just giving it more volts? at the same amount current draw (what we care about presumably/limiting factor), a higher kT means, definitively, means more torque.
i might be completely wrong, but at 12 volts it can draw more current than what its limited to in general use cases so do we really care about resistance? i still dont understand how resistance prevents torque (current) when we can use voltage to overcome resistance.
lets say you are in a pushing match with another robot with the same current limits. if you are hitting this current limit, the voltage is overcoming resistance and thus resistance doesnt matter? then the robot with the higher kT would win?
I think the issue here is that you don’t have a manner of increasing voltage. The motor is going to get whatever your battery voltage is and a variable amount of current.
It’s also possible to estimate a motor’s peak power output knowing only its resistance and the supply voltage.
The resistance of a motor (assuming everything is nicely linear) can be given by Stall Voltage / Stall Current. So for a Kraken, which has a stall current of 374A, this is a resistance of 12V/375A = 0.0321 ohm.
Knowing that a motor’s peak power output is at stall current / 2 (ignoring free current), we can find that the peak power output occurs at 374/2 = 187A. The power input to a motor here is 12V, so the total power into the motor is 12*187 = 2,244W. We lose a bunch of power to the current flowing through the coils, equal to I^2 * R, which is 187^2 * 0.0321 = 1,122W. So the peak output power should be around 2,244 - 1,122 = 1,122W. The Kraken is interpolated with CTRE’s data to an output power of 1,136W. You can do the same thing with the FOC stall current of 476A to estimate a peak output power of 1,428W, which is close to the 1,417W interpolated by CTRE. Note that this is a measure only of the motor, and not the Talon FX attached to it - higher current testing would be needed to see how the Talon FX responds to such massive current loads.
So in general, you can simplify all that down to V^2 / R / 4 = peak output power of a motor. Or Supply Voltage * Stall Current / 4. Now, how do you reduce the resistance to increase the output power? Well, you could reduce the number of windings of a motor and increase the wire diameter. This increases your peak output power (to a point) and increases your kV as well. This could explain why the Vortex, despite having a (presumably) smaller stator volume than the Kraken, is able to get close to the peak output power: the kV is about 11% higher, making more efficient use of the volume.
Note that the effective resistance of a motor is a bit hard to measure. Commutating a motor with 6-step commutation results in a different stall current than commutating sinusoidally or with FOC. For 6-step commutation the resistance is the resistance phase-to-phase of a motor (easy). For FOC it depends on how good your FOC is, but there is a theoretical max… somewhere.
Any motor that you use, you need to apply gear ratios to trade the speed for torque. Power of a motor is the most important metric, as your gear ratio can be designed to maximize performance for any input RPM. I would not want to go back to using 775pros for everything though.
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