Low KV Experiment

[Logan's_Rcs]

Really considering purchasing a 1350kv TP 4060 and comparing it to my 1520 1650kv in the MT410. Both motors are approximately 4092 size, and I'm curious as to whether less RPM makes the motor more efficient when geared for the same speed. I calculated out the 1350kv's losses compared to other 4060s, and it is actually one of the most efficient motors in the 4060 lineup, even on 6s. My thought is this: if the Iron/copper losses (waste wattage turned into heat) is about the same as the other 4060s (1650kv, 1730kv, 1950kv etc) on 6s, then wouldn't the lower rpm geared for the same speed run cooler? I would imagine that higher rpm motors draw more amps just to spin up. So while having the same amount of losses and dropping the rpm by approximately 10k, then gearing up for the same speed, I would imagine it would make the motor run cooler.

Has anybody experimented with this, two of the same motor but different kv's geared for the same speed?

Thanks!

 

[KnowAir]

I'm running a 1450kv 4274 in a Kraton 6s currently, 20/50 gearing with Mojave Fortress 6s tires. It runs amazingly for me, longest runtimes I've ever had from a 6s 5200 Zeee battery. The motor runs cool too. Nothing scientific about my experience but, the same rig running a 2000 kv with I think a 16t pinion did need a fan, didn't get as much runtime either. I think the 2050kv is a stretch on the Arrma 6s rtr's myself. 1600kv would likely be a better choice.

 

[Diem Turner]

Just for the record, any of TP's motors of the same type and build (4050, 4060, 4070, regular, CM, SCM, Pro, SVM) have the same max RPM, irrespective of the KV rating (i.e. a 4070 1700KV and a 4070 2250KV motor both have an RPM ceiling of 50k) so you don't really have high and low RPM variants of the same motor.

Also consider that by gearing up to compensate for lower RPM on the same given voltage will in turn increase the load so...whether or not that will result in a cooler running motor is a game of chance. I don't think that there is a formula to this. Some winds run hotter, some run cooler and has little (perhaps nothing but I don't know for certain) to do with the KV rating of the motor.

I don't have an answer to your question but I bid you good luck on your quest!

 

[KnowAir]

Arrma is just using a "one size fits all" approach to keep costs down I suspect. No secret a lower kv motor works better for heavy rigs and big tires.

 

[Logan's_Rcs]

Just for the record, any of TP's motors of the same type and build (4050, 4060, 4070, regular, CM, SCM, Pro, SVM) have the same max RPM, irrespective of the KV rating (i.e. a 4070 1700KV and a 4070 2250KV motor both have an RPM ceiling of 50k) so you don't really have high and low RPM variants of the same motor.

Also consider that by gearing up to compensate for lower RPM on the same given voltage will in turn increase the load so...whether or not that will result in a cooler running motor is a game of chance. I don't think that there is a formula to this. Some winds run hotter, some run cooler and has little (perhaps nothing but I don't know for certain) to do with the KV rating of the motor.

I don't have an answer to your question but I bid you good luck on your quest!

I was talking about the rpm on a given voltage. For example, on 6s a 1350kv would spin approximately 30k rpm. 1730 would spin around 38K rpm, 1950kv would be over 40k rpm. So take the 1950kv spinning at 40k rpm, gear it for 60mph and take the 1350kv spinning at 30k rpm, gear it for 60mph, Logically I would conclude that the 1350kv would run cooler. Again, I calculated out the losses of both motors at 3000 watts, (which is the continuous rated wattage for these motors) and they came out to be exactly the same.

 

[Diem Turner]

No need to explain the RPM difference at voltages for different KV motors. Trust me when I say that I totally understand everything that you're saying. What you're missing in your assumption is that gearing up for lower RPMs won't incur an increase in load/strain on the motor (thereby generating more heat).

Just out of curiosity, how did you "calculate out the losses"? What formula are you plugging what values in to accomplish this? If both motors are consuming 3kW, then they should both have to contend with the same amount of heat absorption, don't you agree? 3kW is 3kW no matter what the RPM, KV or time of day is. That's 3kW that will either be converted to rotational or linear force (torque) or heat. There's no six ways around this. I'm still curious what exactly you calculated and how though.

 

[Logan's_Rcs]

No need to explain the RPM difference at voltages for different KV motors. Trust me when I say that I totally understand everything that you're saying. What you're missing in your assumption is that gearing up for lower RPMs won't incur an increase in load/strain on the motor (thereby generating more heat).

Just out of curiosity, how did you "calculate out the losses"? What formula are you plugging what values in to accomplish this? If both motors are consuming 3kW, then they should both have to contend with the same amount of heat absorption, don't you agree? 3kW is 3kW no matter what the RPM, KV or time of day is. That's 3kW that will either be converted to rotational or linear force (torque) or heat. There's no six ways around this. I'm still curious what exactly you calculated and how though.

Ok, so first I benchmarked my 1520, and pulled a data log. For the type of driving I like to do, for a full pack, the max amount of watts was 2800. I emailed castle, asked what the continuous wattage for the 1520 was, and they said 1750 watts. So, to run cooler, I need a motor rated continuously for above 2800 watts. I found the 4060, which is rated at 3050 continuous watts, which would work. To calculate losses, what I did was first calculate the amps at around 2800 watts on 6s, which is approximately 130. Then, I squared 130 and got a value of 16,900. I then took this and multiplied it by the Rm value of the 1350kv, which is 0.0065. This gave me a value of 109.85.

Next, I took the no-load current of the 1350kv, which is 2.5 and multiplied it by 6s (22.2) and that gives me 55.5. I then added 55.5 and 109.85 and that gives me approximately 165 or so watts of waste heat. So I then subtracted 165 from 3050 (the watts of the TP 4060) and got 2,885. Then to calculate the efficiency, divided 2885 by 3050, which gives me .94, or 94% efficient.

I did this with other variations of 4060s, and the 1350kv was tied with th 1950kv, both at 94% efficency @ 2800 watts. If both are around the same, I would imagine that the lower kv, less rpm, would run cooler.

To get this, I used the formula found here in this video:

Skip to around 10:15 to see him do the calculation

 

[Hector_Fisher]

Unfortunately, as @Diem Turner has mentioned, the situation is quite a bit more nuanced than just an apples to apples comparison. The best comparison is trying to take two motors of the same manufacturer and same model type and compare that way. Otherwise, you won't be able to draw consistent data, as you'd be comparing, say, oranges to bananas.

Each manufacturer builds their motors slightly differently. Quality of materials, number of winds, thickness of wind wire, type of wind, tightness/quality of winds: all of these are subject to how the manufacturer desire to build it, and each can affect performance and heat. These vary from company to company.

As far as generalities for lower vs higher kv motors: lower kv motors have more winds of thinner wire to produce high torques at low RPM -- higher kv motors generaly have fewer thicker winds to allow high power at high amp draws. Therefore, within the same motor make/model, running two motors a the same voltage and geared speed (one lower kv and one higher kv, would see the lower kv motor running hotter. If both running the same power level, the thinner longer wires of the lower kv will have more electrical resistance than the shorter fatter wire of the higher kv motor.

EDIT: a good example of an apples to apples of lower/higher kv comparison can be seen on castle's page for the 1717, which comes in 1260kv and 1650kv:

 

[Garcbomber]

Only reason to go lower KV IMO would be higher voltage, electric motors have an optimal powerband based on rotor/stator size and running a long/thin can with high/gearing low/RPM usually results in even more heat due to current demands.

Plus unsensored motors geared too heavily/undervolted could create unwanted cogging in transition.

 

[Diem Turner]

Ok, so first I benchmarked my 1520, and pulled a data log. For the type of driving I like to do, for a full pack, the max amount of watts was 2800. I emailed castle, asked what the continuous wattage for the 1520 was, and they said 1750 watts. So, to run cooler, I need a motor rated continuously for above 2800 watts. I found the 4060, which is rated at 3050 continuous watts, which would work. To calculate losses, what I did was first calculate the amps at around 2800 watts on 6s, which is approximately 130. Then, I squared 130 and got a value of 16,900. I then took this and multiplied it by the Rm value of the 1350kv, which is 0.0065. This gave me a value of 109.85.

Next, I took the no-load current of the 1350kv, which is 2.5 and multiplied it by 6s (22.2) and that gives me 55.5. I then added 55.5 and 109.85 and that gives me approximately 165 or so watts of waste heat. So I then subtracted 165 from 3050 (the watts of the TP 4060) and got 2,885. Then to calculate the efficiency, divided 2885 by 3050, which gives me .94, or 94% efficient.

I did this with other variations of 4060s, and the 1350kv was tied with th 1950kv, both at 94% efficency @ 2800 watts. If both are around the same, I would imagine that the lower kv, less rpm, would run cooler.

To get this, I used the formula found here in this video:

Skip to around 10:15 to see him do the calculation

Ok, I see what you're saying but, I respectfully disagree. I don't believe friction heating from rotation to play any significant role in the temperature of the motor. I could be wrong but, until I see conclusive evidence to the contrary, that's my take. I'll still be curious as to your findings though.

 

[Dan B.]

Subbed, this is a really good discussion.

 

[Logan's_Rcs]

Ok, I see what you're saying but, I respectfully disagree. I don't believe friction heating from rotation to play any significant role in the temperature of the motor. I could be wrong but, until I see conclusive evidence to the contrary, that's my take. I'll still be curious as to your findings though.

I mean, more rpm will indeed create more heat. I've tested this before with a castle 1515 2200kv. I ran on 4s, geared for 57mph, and the motor didn't even need a fan. I went up to 5s, geared down for the same speed, and the motor ran a LOT hotter. The only thing that changed was the rpm. In theory, 5s should be more efficient than 4s due to lower amp draw, yet the motor ran hotter when geared for the same speed. That's what makes me wonder if higher rpm actually affects motors more than we generally think it would.

Only reason to go lower KV IMO would be higher voltage, electric motors have an optimal powerband based on rotor/stator size and running a long/thin can with high/gearing low/RPM usually results in even more heat due to current demands.

Plus unsensored motors geared too heavily/undervolted could create unwanted cogging in transition.

Yeah I get that thinner wire has more resistance, but lower kv motors also have lower no load current, so as long as the difference isn't huge with the kv value, I don't see it making much of a difference. Like I said before, I calculated out the losses of a higher kv 1950kv motor and the 1350kv motor, and they came out to be basically the same because the no load current of the 1950kv was twice as high as the 1350kv, although the 1350kv did indeed have slightly higher IO value

Unfortunately, as @Diem Turner has mentioned, the situation is quite a bit more nuanced than just an apples to apples comparison. The best comparison is trying to take two motors of the same manufacturer and same model type and compare that way. Otherwise, you won't be able to draw consistent data, as you'd be comparing, say, oranges to bananas.

Each manufacturer builds their motors slightly differently. Quality of materials, number of winds, thickness of wind wire, type of wind, tightness/quality of winds: all of these are subject to how the manufacturer desire to build it, and each can affect performance and heat. These vary from company to company.

As far as generalities for lower vs higher kv motors: lower kv motors have more winds of thinner wire to produce high torques at low RPM -- higher kv motors generaly have fewer thicker winds to allow high power at high amp draws. Therefore, within the same motor make/model, running two motors a the same voltage and geared speed (one lower kv and one higher kv, would see the lower kv motor running hotter. If both running the same power level, the thinner longer wires of the lower kv will have more electrical resistance than the shorter fatter wire of the higher kv motor.

EDIT: a good example of an apples to apples of lower/higher kv comparison can be seen on castle's page for the 1717, which comes in 1260kv and 1650kv:
View attachment 344100

yeah, I've seen this before, but also internal resistance isn't the only factor. Like the video I put up earlier showed, you need to calculate both iron and copper losses to find out how efficient a brushless motor is. If the 1260kv actually runs a lot hotter than the 1650kv, I would assume it is because the copper and iron losses add up to more waste heat, not just because there is more resistance. You have to take into consideration the no load current as well, which would be higher with the 1650kv. Maybe with these castle motors, the internal resistance value is still enough to make the 1260kv run hotter, but in my calculation with the TP motors, both the 1350kv and the 1950kv come out with the same value of losses. If they have the same losses, I would imagine the one running at lower rpm would run cooler.

 

[LibertyMKiii]

In general I have found that higher rpms = more heat.
Yet these motors do not have much torque and higher rpms can help with gearing torque.

In the end it depends on what you are trying to do with the RC and how you drive, but I suspect you will find for bashers that lower KV is the ideal choice.

 

[Logan's_Rcs]

In general I have found that higher rpms = more heat.
Yet these motors do not have much torque and higher rpms can help with gearing torque.

In the end it depends on what you are trying to do with the RC and how you drive, but I suspect you will find for bashers that lower KV is the ideal choice.

My thoughts as well! I really wish I could buy both the 1950kv, and a 1350kv, gear them for the same speed and see what happens. However, the funds don't allow me to spend $300 on motors right now. Maybe if I could sell them off for a decent price, but I wouldn't bet on that.

 

[Diem Turner]

I mean, more rpm will indeed create more heat. I've tested this before with a castle 1515 2200kv. I ran on 4s, geared for 57mph, and the motor didn't even need a fan. I went up to 5s, geared down for the same speed, and the motor ran a LOT hotter. The only thing that changed was the rpm. In theory, 5s should be more efficient than 4s due to lower amp draw, yet the motor ran hotter when geared for the same speed. That's what makes me wonder if higher rpm actually affects motors more than we generally think it would.

If there's a lot of friction, sure. I'm not saying it's impossible that might be the case but I remain unconvinced. There are too many variable that remain unaccounted for and there's a bit too much subjective experience and not enough hard numbers presented here, no offense. I'm not opposed to having my mind changed but I would need the following for this comparison to begin being meaningful in any way: record of ambient temperature, temp gun reading of the motor when you set off and again when you return and (assuming you're using a Castle ESC) the data log for that run to see if there are other factors that could account for any temp differentials (more active drive time, amount of time spent at WOT, etc). I think a look at the actual current draw of the motor under these scenarios would be educational as I'm genuinely curious if my/our assumptions regarding a decrease in current draw at higher voltages is as expected or not. I'm also unsure whether or not this experiment is a substitute for or analogous in any way to a comparison of same sized motors with different windings but it could still provide some interesting insights, irrespective of whether it is or isn't.

As for the efficiency, higher voltages are said to be more efficient as a general statement but, this has been my experience anyways, they never go on to define what they mean by this or provide anything in the way of an explanation that would answer the question, "Compared to what?". Another issue that I think is in play here is that in my experience, at those speeds these motors will be running, at minimum, in the low 90s in terms of efficiency, realistically probably closer to the mid 90s. I don't think it's feasibly possible to push the efficiency much higher than that, no matter how much voltage you throw at it. I'm not dogmatic or married to my thoughts on the subject (I'm just an old stubborn cunning stunt).

 

[Logan's_Rcs]

If there's a lot of friction, sure. I'm not saying it's impossible that might be the case but I remain unconvinced. There are too many variable that remain unaccounted for and there's a bit too much subjective experience and not enough hard numbers presented here, no offense. I'm not opposed to having my mind changed but I would need the following for this comparison to begin being meaningful in any way: record of ambient temperature, temp gun reading of the motor when you set off and again when you return and (assuming you're using a Castle ESC) the data log for that run to see if there are other factors that could account for any temp differentials (more active drive time, amount of time spent at WOT, etc). I think a look at the actual current draw of the motor under these scenarios would be educational as I'm genuinely curious if my/our assumptions regarding a decrease in current draw at higher voltages is as expected or not. I'm also unsure whether or not this experiment is a substitute for or analogous in any way to a comparison of same sized motors with different windings but it could still provide some interesting insights, irrespective of whether it is or isn't.

As for the efficiency, higher voltages are said to be more efficient as a general statement but, this has been my experience anyways, they never go on to define what they mean by this or provide anything in the way of an explanation that would answer the question, "Compared to what?". Another issue that I think is in play here is that in my experience, at those speeds these motors will be running, at minimum, in the low 90s in terms of efficiency, realistically probably closer to the mid 90s. I don't think it's feasibly possible to push the efficiency much higher than that, no matter how much voltage you throw at it. I'm not dogmatic or married to my thoughts on the subject (I'm just an old stubborn cunning stunt).

I agree that there isnt much objective evidence for this, but thats why i really want to test it! I just really don't want to spend $130 on a new motor for it to fail, and run hotter than what I have now LOL. But then again, if it worked, it would be well worth the money.

 

[SrC]

Really considering purchasing a 1350kv TP 4060 and comparing it to my 1520 1650kv in the MT410. Both motors are approximately 4092 size, and I'm curious as to whether less RPM makes the motor more efficient when geared for the same speed. I calculated out the 1350kv's losses compared to other 4060s, and it is actually one of the most efficient motors in the 4060 lineup, even on 6s. My thought is this: if the Iron/copper losses (waste wattage turned into heat) is about the same as the other 4060s (1650kv, 1730kv, 1950kv etc) on 6s, then wouldn't the lower rpm geared for the same speed run cooler? I would imagine that higher rpm motors draw more amps just to spin up. So while having the same amount of losses and dropping the rpm by approximately 10k, then gearing up for the same speed, I would imagine it would make the motor run cooler.

Has anybody experimented with this, two of the same motor but different kv's geared for the same speed?

Thanks!

A good rough way to judge their power differences is to weigh both of them. The heavier motor usually has more HP.

 

[Hector_Fisher]

I mean, more rpm will indeed create more heat. I've tested this before with a castle 1515 2200kv. I ran on 4s, geared for 57mph, and the motor didn't even need a fan. I went up to 5s, geared down for the same speed, and the motor ran a LOT hotter. The only thing that changed was the rpm. In theory, 5s should be more efficient than 4s due to lower amp draw, yet the motor ran hotter when geared for the same speed. That's what makes me wonder if higher rpm actually affects motors more than we generally think it would.

Yeah I get that thinner wire has more resistance, but lower kv motors also have lower no load current, so as long as the difference isn't huge with the kv value, I don't see it making much of a difference. Like I said before, I calculated out the losses of a higher kv 1950kv motor and the 1350kv motor, and they came out to be basically the same because the no load current of the 1950kv was twice as high as the 1350kv, although the 1350kv did indeed have slightly higher IO value

yeah, I've seen this before, but also internal resistance isn't the only factor. Like the video I put up earlier showed, you need to calculate both iron and copper losses to find out how efficient a brushless motor is. If the 1260kv actually runs a lot hotter than the 1650kv, I would assume it is because the copper and iron losses add up to more waste heat, not just because there is more resistance. You have to take into consideration the no load current as well, which would be higher with the 1650kv. Maybe with these castle motors, the internal resistance value is still enough to make the 1260kv run hotter, but in my calculation with the TP motors, both the 1350kv and the 1950kv come out with the same value of losses. If they have the same losses, I would imagine the one running at lower rpm would run cooler.

So if internal resistance goes up from thinner wire/long winding, no load current goes down? Something doesn't seem to add up with that.

How do motor efficiency curves play into what your stating? Some of these curves are found only be testing and often kept secret by the manufacturer. So if a motor is more efficient at one RPM vs another, could that also play into heat differences you're seeing.

Another thing to consider is that by changing gearing and running on 5s vs 4s, you can potentially affect acceleration. Even if top speed remains the same, faster acceleration (even slightly more so) requires more power for a given input voltage, thus more amps (heat) at the motor.

Regarding no-load, using the same source (which I have used many times before myself), a brushless motor experiences losses just running. And these change dependent on motor load: "losses caused from hysteresis, eddy current in the iron core as well as mechanical losses found in the bearings".

https://www.radiocontrolinfo.com/brushless-motor-efficiency/brushless-motor-no-load-current/

To sum my points: because they come from different manufacturers and will have different operating specs and efficiency curves, I don't think it will be a very accurate comparison trying to draw conclucsions using two motors from different manufacturers. And generally speaking, lower kv motors are used to run higher voltages, not higher amperages. Thus I don't think you'll see improved thermal results gearing higher on a lower kv motor.

I could easily ramble much longer about motors manufacturers and the "specs" they state and how they are all relative to the specific test conditions they were achieved under, but i'll save that annoyance to everyone here for another day!

 

[LibertyMKiii]

So if internal resistance goes up from thinner wire/long winding, no load current goes down? Something doesn't seem to add up with that.

How do motor efficiency curves play into what your stating? Some of these curves are found only be testing and often kept secret by the manufacturer. So if a motor is more efficient at one RPM vs another, could that also play into heat differences you're seeing.

Another thing to consider is that by changing gearing and running on 5s vs 4s, you can potentially affect acceleration. Even if top speed remains the same, faster acceleration (even slightly more so) requires more power for a given input voltage, thus more amps (heat) at the motor.

Regarding no-load, using the same source (which I have used many times before myself), a brushless motor experiences losses just running. And these change dependent on motor load: "losses caused from hysteresis, eddy current in the iron core as well as mechanical losses found in the bearings".

https://www.radiocontrolinfo.com/brushless-motor-efficiency/brushless-motor-no-load-current/

To sum my points: because they come from different manufacturers and will have different operating specs and efficiency curves, I don't think it will be a very accurate comparison trying to draw conclucsions using two motors from different manufacturers. And generally speaking, lower kv motors are used to run higher voltages, not higher amperages. Thus I don't think you'll see improved thermal results gearing higher on a lower kv motor.

I could easily ramble much longer about motors manufacturers and the "specs" they state and how they are all relative to the specific test conditions they were achieved under, but i'll save that annoyance to everyone here for another day!

I 100% agree when it comes to the math and science.... talking about real quality testing where the scenarios and loads are a real fair comparison.

In the real world observing people having issues with heat. Lower KV fixes the issue time and time again while maintaining the speed/power they want. I believe they will ultimately have a less responsive car but maybe they don't notice.

 

[Dan B.]

If there's a lot of friction, sure. I'm not saying it's impossible that might be the case but I remain unconvinced. There are too many variable that remain unaccounted for and there's a bit too much subjective experience and not enough hard numbers presented here, no offense. I'm not opposed to having my mind changed but I would need the following for this comparison to begin being meaningful in any way: record of ambient temperature, temp gun reading of the motor when you set off and again when you return and (assuming you're using a Castle ESC) the data log for that run to see if there are other factors that could account for any temp differentials (more active drive time, amount of time spent at WOT, etc). I think a look at the actual current draw of the motor under these scenarios would be educational as I'm genuinely curious if my/our assumptions regarding a decrease in current draw at higher voltages is as expected or not. I'm also unsure whether or not this experiment is a substitute for or analogous in any way to a comparison of same sized motors with different windings but it could still provide some interesting insights, irrespective of whether it is or isn't.

As for the efficiency, higher voltages are said to be more efficient as a general statement but, this has been my experience anyways, they never go on to define what they mean by this or provide anything in the way of an explanation that would answer the question, "Compared to what?". Another issue that I think is in play here is that in my experience, at those speeds these motors will be running, at minimum, in the low 90s in terms of efficiency, realistically probably closer to the mid 90s. I don't think it's feasibly possible to push the efficiency much higher than that, no matter how much voltage you throw at it. I'm not dogmatic or married to my thoughts on the subject (I'm just an old stubborn cunning stunt).

Do you ever sleep? What time is it in Wienerslider, anyway? And don’t come back with, “it’s Wienerslider time!!!” Just sayin’…