Motor torque DOES NOT depend on KV and here is why!
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- Опубликовано: 22 май 2022
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A Brushless motor's torque does not depend on the KV rating. Or at least, does not depend directly on KV. Yes, KV and KT motor constants describe torque but do it in relation to the current.
Current depends on voltage and resistance. If you just increase the KV by winding the motors with thinner wire and more turns, KV will increase, but resistance will increase as well. As a result, higher KT is counteracted by higher resistance and torque does not change.
To increase the electric motor's torque, you can do two things:
1 - increase the motor size while keeping the same KV
2 - increase the voltage
RC motors: size, torque, speed, KV, KT - everything you need to know • RC motors: size, torqu... 
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RC motors: size, torque, speed, KV, KT - everything you need to know ruclips.net/video/3CbRw3dWgbc/видео.html
Lower KV motors are usually chosen that are intended to run larger propellers with lower pitch for heavier lifts. The drone will be slower, motors can run hotter but it can lift bigger weights. Still, requires larger size motors. A 2120 motor with 120KV will be absolutely different than a 5120 with 120KV.
Thank you for the thorough discussion on the math! I look forward to your next video discussing it further!
Yup, it's a very interesting topic
Pawel, thank you for this scientific but easy to understand explanation.
my pleasure!
Pretty interesting stuff indeed, Pawel! 😃
Thanks!!!
Stay safe there with your family! 🖖😊
Thank you Pawel! I've had this question for a long time. New Sub!
Another great video! Thank you Pawel!
my pleasure
Great explanation!
thanks you
Thank you for detailed explanation!
hope to suggest 2 things,
1) measuring motor's R, that is may not familiar for everyone, 4wire measurement.
2) multiple thin strand VS single thick strand. At same turn, same Kv, most user prefer single thick strand IMHO. The motor builders in Rcgroups are only preferred to build single strand.
Thanks.
So, by this, you are saying that a single, thicker wire is - less resistance? Smoother continuity? Better overall flow of electrons?
I'm not an engineer, but that feels right, and I would believe it.
Stranded wire is maybe less efficient, due to more, thinner strands of wire (variable resistance?), but stranded wire feels the "right" choice of wire to build with.....
I have never thought about using a solid wire, but maybe you're onto something here......Thank you for leading my mind down a different, albeit very sensible path...... :)
Gotta go do some Googlin' :)
@@sunsetpark_fpv stranded wire vs non stranded: The fact that it's stranded or not doesn't affect resistance, only the sum cross-sectional area. Obviously we aren't talking about motor windings here, they are all unstranded due to size constraint. I.E. unstranded is always a smaller OD for same cross-sectional area due to packing. Stranded gives your cable flexibility at the cost of overall size compared to unstranded for the same nett area/current carrying ability. E.G 6 strands of 0.2mm^2 carries the same amount of current as 1 strand of 1.2mm^2 but the 1.2mm^2 single conductor one will make for a slimmer but much stiffer wire, they weigh the same amount though, same amount of copper per unit length. For stranded wire think of a stack of beer barrels, wasted space between each barrel, fatter wire. The next big innovation in wire will be hexagonal strands with perfect packing, if only it could be made cheaply and if anyone could be bothered. ;)
@@m3chanist Thank you for taking the time to respond! Appreciated!
@@sunsetpark_fpv You are very welcome.
In an rc truck application i tried going up in pinion size and it coggd. So i went lower kv. Still cogged at 3 teeth. 1000kv difference didn’t allow me to gear up any. So the only way i cud get more power was up 1000kv from stock and i swear it made more power all around and also up’d pinion 1 tooth. I dont get how everyone else goes down in kv and WAY up n pinion size. Maybe it bcuz i just cant stand ANY of the stutter/cogg. It doesn’t bother some as much but its not worth the power to me. Thanx for explaining. I guess others just deal with the cogg for power on land bound toys. 😊
Hello Pawel thank you for your tutorials, but I need some help I been trying to to work on Inav
But The far I go it’s flashing my fc and then on Inav it doesn’t give me any other options to start working with my fc only give me the option to flash my fc any idea why is that I have the latest version of Inav and using my laptop top windows 7 ?????
Good video 😊
thanks
Hi Pawel , how can I get intouch with you on Git hub need help looking up on an INAV issue im having
I have some EMAX RS1106 motors of 4500KV, 6000kv and 7500KV, and I want to test them with 4S batteries. Which one should I use?
Hi pawel, thought I understood multirotor ESCs, but now I have discovered the closed loop goverened mode, is this mode used for 5 inch drones?
Governed mode is for collective pitch helis only. No use on multirotor drones
Overall a good video.
I still find kV confusing reference for async motors as motor RPM not really depended on voltage (V), or current (I). Rather the frequency from the controller (ESC) determines operational RPM. If no load (torque) on motor, the voltage and current can remain relatively low and the motor can run at hight RPM. kV really more a brushed motor term.
Missing in the discussion of RC motors, in general, is reference to motor power (Watts).
kV is more like maximum spec, used for defining maximum operation RPM at maximum power (ie: at maximum voltage (Vmax) * maximum current (Imax), where Vmax defined by s-rating (eg: 2s-6s) and Imax a constrained to amount of heat a motor can dissipate.
What would be real interesting is a discussion of motor power and toque as they relate to operational RPM range of a RC motor.
ESCs we use make an aproximation of a sine wave. They use PWM because it's simple and cheap. Assuming 50% duty cycle and 16V input, motor gets 8V.
And KV is really not how fast motor rotates. It's how much back-emf it generates when it rotates. The other side around
@@FPVUniversity Sounds so wrong. KV is how fast the motor will try to run on 1V . You do know that the signal on the three wires is not PWM but a three phase sinus? ruclips.net/video/4oRT7PoXSS0/видео.html How electric energy is transformed into kinetic energy is rather complex but I can assure you that E=MC^2.
When the load on a 3 phase motor is too big, in our case a propeller that is too big, the motor will not be able to turn the electrical energy into kinetic energy because the fields of the magnets and the coils run out of sync too far. Imagine the coil is in the middle of two magnets so it is pulled forward and backward equally and the only reason the motor keeps turning is the kinetic energy in the turning bell. The motor will start turning less energy into kinetic energy and what is left is dissipated into heat and when it gets too bad the coils will heat up creating more ohms resistance and the coil will heat up more and will burn. The resistance created by the fields which are responsible for the kinetic energy is called impedance.
The created torque depends on the load. So in our case the torque is low when the propeller is too small or too big big for a motor of a certain KV it becomes inefficient. When the prop is too small the thrust stays very low and the motor is not pulling any power from the ESC. When the propeller is too big the motor will try to pull the energy from the ESC according to the KV of the motor and the propeller is to too slow to follow and much of the electric energy is dissipated into heat. And if the circumferential speed of the propeller becomes higher than the speed of sound and the tip of the propeller will be slowed down by the sound barrier a lot of electrical energy is converted into heat.
To get optimal torque you choose the optimal prop for your motor. First you need a well designed model prop.
Without speed of a propeller the moving air is too slow and no thrust is developed and no torque, the prop is too small. When the speed is too high the sound barrier limits the tip speed of the propeller, the prop is too big. To reach optimum torque you calculate the maximum size of the propeller to stay within the circumferential speed which is a product of the LIPO voltage and the motor KV. This will result in a setup that will give your aircraft speed.
If you want your craft to respond quickly you would prefer somewhat smaller props so when the prop is continuously changing speed the power converted into heat is somewhat limited.
So torque can be used to create an aircraft that has a high top speed, can carry a lot of cargo, or respond very quickly. For example a spitfire, cargo plane and a quadcopter. A race quad or freestyle quad.
with FOC and VRTS 0RPM full torgue seting is kinda.. finaly.. make sence..
I have one question I am trying to figure out and I can't find a proper answer. I have castle 2028 800KV. What would be the differnce with using 5:1 gear or 10:1 gear if the power remains the same. Torque on gera shaft and speed is the same. with 5:1 I will have 10 000 rpm on motor with 2Nm torque and with 10:1 I will have 20 000rpm on the motor with 1Nm torque. I think that current will be the same in both because it is still the same power on same voltage. So is there really any difference for the motor? thanks for the answer
Oh, that is much more complicated. The current in the motor winding is proportional to torque, so there will be 2-times difference in current. That instantly leads to 4 times difference in heat generated as any resistant generates heat = R * I ^ 2 But there is a very confusing thing that most people don't think or talk about.
The winding current is NOT equal to the consumption current! Because there is a PWM signal applied to the motor and it consumes current from the battery only at the ON period. But at the OFF period, the current still goes through the winding and creates torque and heat. If you want to know more about this, google "motor slow decay mode". As a result average winding current is higher than the consumption current.
As a result - with a lower gear ratio you will have much more heat generated in the motor (also it's an energy waste). But at the same time with a higher gear ratio, there will be more energy wasted on friction and that could be a major factor too.
Hope I helped a bit.
@@Positive_Altitude thank you very much I appreciate your reply. It's probably not for discussion in the comments, but I'll ask anyway. I understand the answer, but if the output is the same for both, then the current which is directly proportional to the torque makes up the larger part of the total current and thus generates more heat?
Your channel is full of professional information, keep up the good work.
@@matejpetras2016 Thank you! Not exactly a "larger portion". Let's say currently ESC applies battery voltage to the winding with 20% duty cycle. That means that roughly speaking current flows through the battery 20% of time. The other 80% of time the winding is literally shorted, but the current continues to flow through the shorted winding because the winding is an inductor and it accumulates the energy in the form of magnetic field. So roughly speaking the same current flows through the winding 100% of time (just with some ripple in reality). But the same current goes through the battery only 20% of time. So the average winding current will be 5 times more than current consumed from the battery!
Yes it seems kinda wrong that more current flows through the motor than through the battery, but there is no mistake it's easy to prove if we take a look at energy conservation.
Let's say we have an ideal 1000Kv motor that currently rotates at 2000 RPM and 10V battery and 1A consumed from the battery. Power consumption = 1A * 10V = 10W. But where does this power go? Current in motor works against motor back EMF and that's where electrical energy converts to mechanical. Motor back EMF = 2000RPM / 1000Kv = 2V. And if we have 10W in and 10W out, that means that the current in the winding that works against back EMF should be 5A. 5 times more than we take from the battery.
So that's very funny, but really the whole thing works as a step-down converter. It reduces the voltage and increases the current. And if you check the schematics of DC buck (step-down) converter you will see that it is indeed the same thing. Is is just the motor winding is used instead of the inductor and motor back-EMF instead of load.
Sorry if it's hard to read, I tried to give you the proper answer, but BLDC is a freaking complicated thing, I can not do anything about it.
Basically the stator size, magnet quality and the air gap decides how many watts the motor can output. The KV just shifts the operating range to match the battery,
It's slightly more complicated than that, since motor load is also a factor. But yeah, KV does not matter
I'm one of those that tried 2500 Kv and 6s 130c🔥
I mean, it was pretty fast for a few seconds🤦
he he he
i measured kV with vesc and kV is voltage dependent and wikipedia says: kt is the torque produced divided by armature current. It can be calculated from the motor velocity constant kv so i dont know whos right/wrong here
You only look at half of it. Yes, torque depends on KT and KT on KV. But also current changes with KV
Seriously ? Ohm loss is minimal compared to power generated. The only case à higher kv is insteresting is when the prop load is smaller enough to use torque available at the higher hand.
For more torque you need a larger diameter motor
To be more precise, more volume. But yes, bigger motors have more torque
It seems to me that ignoring the inductance and back EMF from the magnets means you are ignoring the primary factors that determine the current. I would expect the static resistance to typically be very low ("negligible"?), and only really limiting the absolute maximum current. While I think its fair to say that torque does not depend *only* on KV, I think its at least misleading to claim that torque does not depend on KV.
I haven't tested it, but I think that if I have two otherwise identical motors, but with different KV, connected to the same ESC and battery, the lower KV motor will be capable of more torque, while the higher KV motor will be capable of higher speed . Is that not true?
Yes, passive resistance is low. And inductive resistatnce applies as well. But if you apply inductive resistance instead passive one, it's the same story. On top of that: higher KV will have lower inductance but higher frequency. Lower KV will have higher inductance but lower frequency. It all cancels each other. And at the end, torque does not depend on KV :)
I re-watched the video to make sure I wasn't missing anything, and I still disagree. First, I continue to assert that the static resistance of the windings is not really a factor - its mention in the video is a red-herring. You do state that the lower KV motor will generate more "torque per amp", so in any situation where the current is limited (perhaps by the ESC, battery or wiring), then at that maximum current, the lower KV motor produces more torque. Sure, in theory, with infinite available current, then the max current is limited only by the static resistance - but that is not the real-world. In the real world, we do have limits on the available current.
I remain convinced that the practical reality is: You always have to balance your need for both torque and speed. In cases where you need more torque than speed, then you choose a lower KV motor. In cases where you need more speed than torque, then you choose the higher KV motor. I look forward to the longer video you mentioned to see if it will change my mind.
As always, thank you for your informative and thought provoking videos!
Lower KV is more efficient?
No, not really. It uses less amps per torque, but needs more voltage to have the same RPM. So it all balances itself out
I would argue that most RC systems are current limited in general so that in practice lower kv does end up meaning more torque. Secondly it's not just the resistance of the motor you have to take into account, connectors, escs, wiring etc all have resistance too and *usually* you don't change this. Lastly current driven heating tends to be another limiting factor so lower current tends to deliver more efficiency overall, more efficient conversion of power = more torque.
If you ask the question: "If I use a lower kv motor on this quad/plane/car, will I get more torque?" the answer will almost always be yes in most practical examples.
Well, there is something to it, but it's more complex than that. Yes, if you lower the KV then you will increase efficincy slightly. But to keep the same power you need more voltage. So in terms of energy, it stay more less the same. But if you compute how much you gain in terms of waste (less waste) power due to resistance, you will notice the numbers are very low
it is true, if you have infinite current. which it feels like we have, with our 100+C batteries XD
wait, low KV have tighter windings????!!! i am very confused.
Low Kv can take *more* current because of the *lower* resistance, right? therefore it can take more current and therefore produce more torque?
low KV motor, assuming the same stator size, will have more turns of thiner wire. So will generate more torque per amp, but pull less amp since winding is longer and thiner, thus have higher winding resistance.
Low and High KV motors will have roughly similar torque on the same voltage. But because magnetic field can raise much faster (lower inductance) it can spin faster. Lower inductance also means there is less inductive load shifted 90deg and active power is higher.
Also higher RPM mean higher load on the same prop and more torque is consumed to keep current RPM.
In general there is difference between max torque and power and current load and power
but that's not all. We talk about maximum torque! not average torque during the phase cycle. Coil can be energized only for as long as it can interact with a magnet. When you rotate faster, phase can be energized for a shorter period of time. And due the inductive nature of a coil, and the fact that voltage and current will be out of phase, it takes time for the magnetic field to build up and then fade out. Longer for low KV, shorter for high KV.
When we reach max RPM, the power integral goes down and average torque available goes down as well. So motor has no extra torque to accelerate anymore
@@FPVUniversity i see, other videos have suggested otherwise, but this makes more sense now, lower inductance means higher speed, more coils does not mean faster accumulation of magnetic flux, it means higher strength magnetic flux but takes longer to saturate.
nothing to do with torque of course but certainly related with the circumference of the wheell so ebikers ?
KV x voltage = RPM
Size of motor gives you torque, if I’m not mistaken
Nope, KV is not how fast motor rotates per volt. It's the back-emf in volts when it's rotating.
1:25 "the major problem with this way of thinking is that you are not the one who decides on the current".
Unless, of course, you ARE. This video assumes you don't pay attention to power usage when you choose motors. If you choose between two motors that use 10 amps (or 160 watts @ 4S) the one with the lower KV with have more torque.
If you do NOT pay attention to amperage (or wattage) and you choose motors only based on the physical size, then what Pawel said applies.
The think is, that it does not work like that. Current will depends on: voltage, resistance and load. Idle current is not the same as load current. Same motor with the same size, voltage and KV will have different current depending on the load. After all, mechanical power has to be equad electrical power minus losses. energy has to be conserved. And finally, because of the load, the voltage in this equation is not the battery voltage. It's the effective voltage applied by the ESC (that is PWM modulated). So no, torque does not really depend on the current as throttle position is not driving the current but effective voltage. Same voltage with different resistance will cause different curret
@@FPVUniversity Respectfully, I think that last bit may be talking in a circle.
Power = RPM X Torque. Period. That's the formula for power of a rotating motor.
For the same power, you MUST have higher torque if you have lower rpm, and vice versa. That doesn't depend on any other factors. As you said, conservation of energy is a law of physics. You can only have conversation of energy by increasing the torque, if you reduce the rpm at a given power level.
Talking about load or resistance or inductance or ice cream is a distraction from this basic fact from Physics 101.
Please remember why we started this discussion. You do not decide on the current that will go though the coils! At least not directly! You decide on the voltage and only then motor will decide on the current it wants to pull.
@@FPVUniversity That's true. Unless you do choose motors based on tested current / power. If you DO choose motors based on amps, then you are choosing the amps. Which is the first sentence of my first message. If you DO choose your motors based on power, then you trade torque for RPM.
I'm sorry to say that this story is just a fraction of what is really going in and is overall bullshit. To create thrust the air movement coming from the propeller needs a certain amount of speed. If the propeller is turning too slow no thrust is developed and R will stay high and I low and no torque is produced. To create torque you need to balance V, KV and prop size.
The thickness of the wire has nothing to do with this.With a certain amount of I a certain thickness of wire is required or the wire will overheat and start glowing. You will often see that motor coils have parallel wired coils. Instead of using thicker wire using multiple thinner wires in parallel will do.
I swear, he doesn’t give good information. He just throws equations up and says what’s wrong with everyone else’s thinking but doesn’t actually help us find any answers. He just seems to like to say why other people are wrong and hope people think he’s smart.