On paper, given the published numbers by various sources (REV themselves, CTRE, Recalc) the NEO v1.1 seems to be superior in different metrics such as kT (Nm/A) and efficiency, and even producing more torque at low current than a Kraken.
So what’s the story behind the Vortex? Are these sacrifices made to achieve its form factor? To achieve a higher RPM? Some combination?
The poor efficiency is mainly a result of friction. Motor peak efficiency is largely a function viscous and dry friction, where motors are dissipating minimal power through the internal resistance. An ideal motor would have 100% efficiency at free speed, dissipating no power through the resistance, which means friction significantly hinders efficiency as the extra torque required to spin is both not useful output and causing the resistance to dissipate more power. Using free current times kT as a proxy for friction, the neo vortex has much more friction, explaining the poor efficiency.
Lower kT is not necessarily a bad thing in a vacuum, because it is almost 1:1 correlated with kB because motors are gyrators, with kB determining free speed. In fact, idealized motor models assume both quantities to be equal. Because the vortex is a lower kB motor, it has a higher free speed, and must necessarily have a lower kT as a result. However, kT being lower then kB is a result of electromagnetic inefficiencies and would indicate design issues.
Keep in mind that torque isn’t really a useful metric for comparing motors per se. As soon as you put a gearbox on it that goes out the window.
If what you care about is the Nm/A for the output of the gearbox you can basically dial that number to whatever you want by adjusting the gear ratio.
As Alex_Y said, peak efficiency is largely dominated by the viscous friction of the bearings in the motor (and other friction/drag sources) so that isn’t a particularly useful metric for comparison either. The Vortex has much larger bearings than other motors and so has a lower peak efficiency number. Under any kind of load it will perform similarly to other motors though.
A better metric for comparing motors is the mechanical power produced by the motor. Peak power is useful but with modern FRC motors the peak power point usually has a large enough current draw that it’s not sustainable on an FRC robot. Mechanical power at a given current (e.g. 40A, 60A, 80A) is a pretty good way to compare. You have to be careful to look at the correct current range that you care about, which isn’t going to be the same for every mechanism on your robot.
Efficiency definitely matters a lot for a motor. Not necessarily because you get more power per watt with a more efficient motor (as much as 10% in this case), but also because lower efficiency means more heat. More heat means your motor will cook a lot faster for the same output power.
An 80% efficient motor vs a 70% efficient motor is a 50% increase in heat.
Wouldn’t you want to maximise the torque the motor produces per unit of current drawn, independent of any gear reduction?
kV * kT is a global constant for these motors. Winding a motor for more torque per amp proportionally reduces speed per volt. The difference is that if you wind for a lot of torque, you’re using a lot more thinner wire, so resistance goes up. When resistance goes up, stall current goes down, and your peak power also decreases proportionally.
This is why historically the 775pro was so powerful despite being just another brushed motor. High speed meant less wire length, less resistance, and higher stall current. Of course, modern brushless motors are even more powerful, but that’s just the modern reality.
Fun fact: knowing only the resistance of the windings of a motor and the supply voltage, you can find its approximate peak output power by using the equation V^2 / R / 4. Stall current is equal to V/R to make this even easier, just V * I / 4.
Kraken stall torque is 366A. Using 12 * 366 / 4 we find a power of 1,098W. The official spec measures it at 1,108W - within 1%!
The advantage of the Vortex is that it is a through bore motor so allows designs that are not possible with any other motor.
Higher speed can definitely be an advantage when powering COTS mechanisms like swerve modules, where you cannot change gear ratios freely. If you want to maximize drive base speed on a COTS swerve you may need to use the fastest motor. For custom mechanisms you probably are looking for peak efficiency at the speed and torque you are using, and the through bore design certainly has its upsides.
How is it that the Kraken has a lower resistance than the Vortex, and a higher stall torque?
And what makes kV * kT constant?
I don’t know what makes kV * kT constant, it’s just how motors work. I’m sure someone else knows.
Even though the Kraken has a lower kT, because of that lower resistance, you can just pump more amps through it - and so, more stall torque.
So was REV’s theory of the case that kT doesn’t matter as much because most use cases use a gearbox, so maximizing kV is more important? Also to solve various assembly/design pain points via the introduction of the through bore design?
And improving power output over the NEO v1.1?
Almost.
You don’t want kV maximized above all else. At some point adding more gearing adds too much weight and lowers efficiency. This is especially the case if you start crossing into territory where the speed of the motor requires adding additional stages of reduction.
Even though kT * kV is constant there is a reason to play with the balance between the two. Increasing kV is done by reducing the number of wraps in the winding. With less wraps, you can use a larger cross sectional area for the conductor which reduces the winding resistance.
For example, if you halve the wraps, you halve the kT and the winding resistance. With half the winding resistance you get double the stall current. That puts the stall torque right back where you started, albeit at double the current. The key thing here is that you kept the stall torque the same but doubled the speed, which effectively doubles your peak power.
Basically for a given voltage and stator design, peak power is proportional to kV.
This is STRONGLY limited by thermal considerations. That along with the limitations of the FRC power system in delivering huge currents and my above comments about not wanting to add a huge amount of gearing to your subsystems means the kV needs to stay within reasonable bounds.
The resistance appears to be higher than the Kraken however. Is this due to the constraints of the chosen form factor of the Vortex?
I mean, the more practical reason to limit KV is because gearboxes are usually designed for the 5000-6500rpm input range. Going above that makes it harder to use the motor on COTS stuff. I would bet doubling the kV of the Vortex would still make a more powerful motor, it would just lose the utility of the passthrough hex and compatibility with most drive modules.
The Kraken and Vortex don’t have the same stator so it’s not a 1:1 comparison. The Kraken has lower internal resistance but there’s no reason to expect that it would be the same as the Vortex (given the different stator designs).
Makes sense. Just trying to understand REV’s case for the Vortex, given the current choices available in the FRC market, and publicly available test data.
Conservation of energy.
I’m gonna explain this using the back-emf constant, kB, which is by definition 1/kV, but makes much more sense from a theoretical perspective.
Lets assume you have a motor with a kB of 1 V-s/rad and a kT of 1 N-m/A with a resistance of 1 ohm. At 12V, you’d have a peak power draw of 36W which occurs at a voltage differential of 6V. On the output, you’d be outputing 6 N-m of torque at 6 rad/s, also resulting in 36W.
Now lets say you have a magic motor that has a kT of 2 N-m/A. Under the same electrical draw, you’d be outputing 12N-m of torque at 6 rad/s, meaning 72w of output power, at 36w of input power, which obviously can’t happen.
If energy is perfectly conserved, kT and kB are necessarily equal, which is what is taught in intro dynamics classes and commonly posted on spec sheets, including brushed FRC motors from the OEMs. However, in the real world, you will often find motors with a lower kT than kB, meaning that there’s electromagnetic inefficiencies or inefficiencies in the control method (trapezoidal or imperfect FOC implementations).
Also on a side note: it annoys me that FRC manufactures don’t post the fundamental motor constants directly, unlike pretty much the rest of the world that isn’t selling cheapo motors. Its not only easier to explain motor performance using the fundamental constants (rather than stall and free speed characteristics as a proxy), but those same constants directly show up in the models used to simulate and control things involving motors.
Is it reasonable to change a motor’s backEMF constant without physically changing the windings of the motor? I’ve always thought of motor’s peak power mainly being a function of resistance (assuming no thermal issues), where changing the kB/kT is just changing what that peak power looks like.
Conservation of energy.
More specifically,If kV and kT increases simultaneously, you get more torque and more velocity for the same input voltage and current.
This statement doesn’t make sense to me. Peak power for the Vortex occurs at around 25A and 250W (mechanical output), which I would count as “any kind of load”. And as others point out, while the power difference is pretty minimal, the heat is somewhat significant.
That said, I don’t know how much of a difference this makes in real world situation. A fair comparison would include a powertrain attached to a mechanism. The flexibility of the Vortex, including the 7T gear shaft, seems useful for having fewer reduction stages and therefore higher efficiency.
Also, efficiency isn’t everything; compared to NEOs you need fewer Vortexes to achieve the same power if both motors are geared for peak efficiency. Compared to Krakens, I think Vortex’s numbers are worse, but I don’t think the difference is big enough to sway that decision.
This argument’s never seemed significant to me. In almost all FRC use cases (i.e. everything except for a tightly packaged shooter) we are gearing the CIM-class motors down so lower free speed/higher torque is better. That said, as mentioned above, Vortex has the 7T shaft which is useful in this regard. SDS modules have gear ratio options that are more than fast enough even for NEOs.