How to choose a motor

[Dan B.]

I have some questions and need clarification on how to make an intelligent decision in choosing a motor for any particular ESC, not necessarily for a particular rig. 6s to 8s application.
When researching motors, I can of course see the amp load and work with that number in regard to the ESC specs. However, which motor to choose? 40 series, 49, 56, short can, long can, etc. and why. Torque, heat, load, etc. Why is one 2000kv motor, for example, better than another configuration of a 2000kv motor? Bashing vs. speed running, suitability for a particular purpose, etc.
I sort of get the basics here, but the nuances are giving me trouble. Not looking for the obvious like physically fitting in a particular space, but instead the physics of pairing an ideal motor with any given ESC for a specific intended purpose.
I would love to more fully understand this.

 

[Smilinbri]

I have some questions and need clarification on how to make an intelligent decision in choosing a motor for any particular ESC, not necessarily for a particular rig. 6s to 8s application.
When researching motors, I can of course see the amp load and work with that number in regard to the ESC specs. However, which motor to choose? 40 series, 49, 56, short can, long can, etc. and why. Torque, heat, load, etc. Why is one 2000kv motor, for example, better than another configuration of a 2000kv motor? Bashing vs. speed running, suitability for a particular purpose, etc.
I sort of get the basics here, but the nuances are giving me trouble. Not looking for the obvious like physically fitting in a particular space, but instead the physics of pairing an ideal motor with any given ESC for a specific intended purpose.
I would love to more fully understand this.

Excellent question and perfectly worded. Exactly what I would like to know too. Thanks for asking!

 

[Dan B.]

Excellent question and perfectly worded. Exactly what I would like to know too. Thanks for asking!

Thanks, and you’re welcome! This forum has been invaluable for relieving me of my ignorance in a wide variety of subjects, and I’ve had the hardest time understanding this one. Looking forward to the experts weighing in.

 

[Sandracer_NL]

AFAIK you can calculate the motor rpm‘s you need to achieve a certain speed, based on the gearing of the diffs, pinion/spur And wheel diameter. One you know the motor rpm’s needed you can divide those by the lipo cell count giving you the kv.

I do it the simpleway (simpe guy): for BASHING I like to have 35000 rpm for the motor. This seems a good amount for durability and efficiency (Heat, runtime). When running 6s this gives 35000rpm/(6s * 3,7v) = 1576kv. Like you said there are more motor sizes giving that number of kv’s (more or less). The larger the the diameter or longer the motor the cooler it can run (more motor area to transfer heat, basically the heatsink). But there is one quite important value that is often overlooked; Watt.
Watt is basically the power (Watt = amps x voltage). The more Watt the motor has the more powerful is (the larger the pinion you can use). The motors with larger diameter or longer can usually have more Watts.
The heavier the car the more Watts I’d like to have. It’s always good to have more power, but at some point all you‘ll be doing is wheelies and straining your drivetrain.

The speedrunners usually go way higher in rpm’s at 50000-60000rpm.

Like I said this is my basic understanding.

 

[Dan B.]

AFAIK you can calculate the motor rpm‘s you need to achieve a certain speed, based on the gearing of the diffs, pinion/spur And wheel diameter. One you know the motor rpm’s needed you can divide those by the lipo cell count giving you the kv.

I do it the simpleway (simpe guy): for BASHING I like to have 35000 rpm for the motor. This seems a good amount for durability and efficiency (Heat, runtime). When running 6s this gives 35000rpm/(6s * 3,7v) = 1576kv. Like you said there are more motor sizes giving that number of kv’s (more or less). The larger the the diameter or longer the motor the cooler it can run (more motor area to transfer heat, basically the heatsink). But there is one quite important value that is often overlooked; Watt.
Watt is basically the power (Watt = amps x voltage). The more Watt the motor has the more powerful is (the larger the pinion you can use). The motors with larger diameter or longer can usually have more Watts.
The heavier the car the more Watts I’d like to have. It’s always good to have more power, but at some point all you‘ll be doing is wheelies and straining your drivetrain.

The speedrunners usually go way higher in rpm’s at 50000-60000rpm.

Like I said this is my basic understanding.

I never thought of a bigger can providing more of a heat sink, this helps. Would a bigger can of the same kv rating provide higher amp spikes? A good start, thanks!

 

[morrjr71]

As an example, Castle has two 1650kv motor in different sized cans. The smaller 1520 size motor runs hotter than the 1717 size motor, not only due to the can size but due to the size of the rotor. The 1520 has to work harder to get the same kv output.

It's harder to compare the same kv motors between different manufacturers because the can size may not be the only difference between motors. The size of the rotor, and how it and the stator are wound, have a significant effect on how well a motor runs. The quality of the bearings also has an effect.

Here's a good web page to answer some of your questions.

https://www.youtube.com/c/RCexplained/search?query=motor kv

 

[Dan B.]

As an example, Castle has two 1650kv motor in different sized cans. The smaller 1520 size motor runs hotter than the 1717 size motor, not only due to the can size but due to the size of the rotor. The 1520 has to work harder to get the same kv output.

It's harder to compare the same kv motors between different manufacturers because the can size may not be the only difference between motors. The size of the rotor, and how it and the stator are wound, have a significant effect on how well a motor runs. The quality of the bearings also has an effect.

So I guess I don’t quite understand the “basics” as well as I thought. I was just comparing the two Castles, both 1650kv, 1520 vs. 1717. Both 3s to 8s. Why is there a 4000rpm disparity between them?

 

[Dan B.]

Also, CC has precious few specs listed compared to other motor brands. Doesn’t make it easier to compare others.

 

[Sandracer_NL]

So I guess I don’t quite understand the “basics” as well as I thought. I was just comparing the two Castles, both 1650kv, 1520 vs. 1717. Both 3s to 8s. Why is there a 4000rpm disparity between them?

Kv = rpm per volt. 1650kv makes the same rpm whether it’s a 1520 motor or 1717 motor. So with the same pinion (gearing) both have the top speed if you don’t use a too large pinion or too small spur gear (overgeared). Additional weight of the heavier 1717 not taken into account.
The 1717 has more Watts so it has more power. It will accelerate more aggressive and you can use a higher gearing compared to the 1520 which has less Watt (less powerful).

Amps of the motor are usually give in the specification of the motor (at least the more serious manufactures give them).
Watts = Amps * voltage, so if you know the Amp of the motor and many cells your lipo is you can calculate the Amps. Take into account the ’good’ motors have an efficiency of around 90%, (90% is converted into power, the rest into heat).

 

[Dan B.]

Kv = rpm per volt. 1650kv makes the same rpm whether it’s a 1520 motor or 1717 motor.)

That’s what I thought I thought, but…. Castle says the 1520 has max rpm of 60,000, and they list the 1717 as having a max rpm of 56,000. Both are 8s capable 1650kv motors. So….???

 

[Diem Turner]

KV only provides you with one metric of the motor, namely how many revolutions per volt it will do. Size is a considerable factor when it comes to brushless motors. Heat dissipation has already been mentioned but size also translates into torque/power. Two motors, all other things being equal, the larger motor will have more power and, from what I've seen, it basically scales relative to weight. So there will be a sweet spot for the weight of your vehicle relative to the size/weight of the motor. Castle does a good job of giving you weight limits for the motors they offer.

That’s what I thought I thought, but…. Castle says the 1520 has max rpm of 60,000, and they list the 1717 as having a max rpm of 56,000. Both are 8s capable 1650kv motors. So….???

It's probably down to the size of the rotor. Smaller rotors can (typically) handle more RPM because they have a lower rotational mass. It will also depend on how the casing of the rotor is constructed (Kevlar and Stainless are the two types most often found on rotors). At the end of the day, both the 1717 and 1520 will spin at the same rates because they have the same KV.

 

[SrC]

AFAIK you can calculate the motor rpm‘s you need to achieve a certain speed, based on the gearing of the diffs, pinion/spur And wheel diameter. One you know the motor rpm’s needed you can divide those by the lipo cell count giving you the kv.

I do it the simpleway (simpe guy): for BASHING I like to have 35000 rpm for the motor. This seems a good amount for durability and efficiency (Heat, runtime). When running 6s this gives 35000rpm/(6s * 3,7v) = 1576kv. Like you said there are more motor sizes giving that number of kv’s (more or less). The larger the the diameter or longer the motor the cooler it can run (more motor area to transfer heat, basically the heatsink). But there is one quite important value that is often overlooked; Watt.
Watt is basically the power (Watt = amps x voltage). The more Watt the motor has the more powerful is (the larger the pinion you can use). The motors with larger diameter or longer can usually have more Watts.
The heavier the car the more Watts I’d like to have. It’s always good to have more power, but at some point all you‘ll be doing is wheelies and straining your drivetrain.

The speedrunners usually go way higher in rpm’s at 50000-60000rpm.

Like I said this is my basic understanding.

+1
Pretty much how I do it also.
Remember that if you want to use both 6s and 8s for instance alternatively, need to consider a motor with the best operational RPM range. Gearing takes care of the rest.
1600kv give or take seems to be the safest sweet spot for both 6s and 8s . Generally speaking. But again there are many other variables to consider. And which rig and whether speed running or just bashing.
I wouldn't bash with a 2250 kv motor for instance. I have tried in the past. Just not efficient.
I could be wrong, but I always felt the larger the size and heavier the motor is , the higher the amp draw there is.

 

[Dan B.]

So wouldn’t the 1650kv 1717 pull more amps than the 1650kv 1520 due to higher rotational mass?

 

[Diem Turner]

So wouldn’t the 1650kv 1717 pull more amps than the 1650kv 1520 due to higher rotational mass?

I can't say with certainty as I've never been able to do a head-to-head between the two (I bought a 1717 after accidentally frying my 1520) but, from my understanding, I don't see any reason why it would. Maybe a little bit because the motor is, in general, a little bigger than the 1520? Could be.

 

[Sandracer_NL]

That’s what I thought I thought, but…. Castle says the 1520 has max rpm of 60,000, and they list the 1717 as having a max rpm of 56,000. Both are 8s capable 1650kv motors. So….???
At max voltage, 4.2v cell each cell and with 100% motor efficiency on 8s you would get 8x4.2x1650kv = 55400rpm.

At 8s and with each cell fully charged theoretically with 100% efficiency the motor could do 8s*4.2v*1650kv = 55400 rpm. Probably the 1520 has 60000rpm spec’d bearings, or Castle takes into account their 1520 motors can have a 10% kv deviation.

 

[Diem Turner]

At 8s and with each cell fully charged theoretically with 100% efficiency the motor could do 8s*4.2v*1650kv = 55400 rpm. Probably the 1520 has 60000rpm spec’d bearings, or Castle takes into account their 1520 motors can have a 10% kv deviation.

My initial thought was that the rotor has less rotational mass and, as such, can spin a little faster before flying apart. Maybe they made some changes to the rotor since exploding rotors seem to be a somewhat common phenomenon on the 1717 to give it a little more headroom. I'm just spit balling here though.

 

[Dan B.]

What about the difference in windings? D-wound, Y, 2Y, 3Y, etc.?

At 8s and with each cell fully charged theoretically with 100% efficiency the motor could do 8s*4.2v*1650kv = 55400 rpm. Probably the 1520 has 60000rpm spec’d bearings, or Castle takes into account their 1520 motors can have a 10% kv deviation.

So given the math (which I appreciate) either would do 55,400 rpm. Maybe Castle isn’t listing what they will spin, but rather at what rpm they could fly apart?

 

[Diem Turner]

What about the difference in windings? D-wound, Y, 2Y, 3Y, etc.?

So given the math (which I appreciate) either would do 55,400 rpm. Maybe Castle isn’t listing what they will spin, but rather at what rpm they could fly apart?

That is my reading of it as well. Both are within listed RPM maximums, but taking into consideration that these are calculations with no load, it's safe to say that in real world application you're never going to get anywhere near the RPM limit of the motor. As we've seen on numerous occasions, however, this doesn't appear to mitigate the possibility of failure of the rotor wrap.

 

[Dan B.]

Well, I’m starting to understand why I’m so confused by the technical minutiae. Look at this HobbyStar spec sheet for their 4092’s. They list the same RPM’s for motors ranging from 1050 to 2200 kv. Changes in input voltage do not all equal 50k. Getting a headache…

Product	Watts	Max Voltage	Max Amps	Rotor Poles	IO	Resistance	KV (RPM/Volt)	Max RPM	Length (mm)	Diameter (mm)	Weight (g)	Shaft Diameter (mm)	Shaft Length (mm)	Connectors
4092/3Y	4200	<32	120A	4	2.2A	0.0418	1050	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/4D	4200	<30	140A	4	3.2A	0.016	1300	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/2Y	4200	<28	150A	4	2.4A	0.0096	1480	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/3D	4200	<26	160A	4	4.8A	0.0058	1730	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/2.5Y	4200	<18	165A	4	4.0A	0.0043	2200	50,000	92	40/42 with fins	530	5	18	6.5mm gold

 

[Diem Turner]

Well, I’m starting to understand why I’m so confused by the technical minutiae. Look at this HobbyStar spec sheet for their 4092’s. They list the same RPM’s for motors ranging from 1050 to 2200 kv. Changes in input voltage do not all equal 50k. Getting a headache…

Product	Watts	Max Voltage	Max Amps	Rotor Poles	IO	Resistance	KV (RPM/Volt)	Max RPM	Length (mm)	Diameter (mm)	Weight (g)	Shaft Diameter (mm)	Shaft Length (mm)	Connectors
4092/3Y	4200	<32	120A	4	2.2A	0.0418	1050	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/4D	4200	<30	140A	4	3.2A	0.016	1300	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/2Y	4200	<28	150A	4	2.4A	0.0096	1480	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/3D	4200	<26	160A	4	4.8A	0.0058	1730	50,000	92	40/42 with fins	530	5	18	6.5mm gold
4092/2.5Y	4200	<18	165A	4	4.0A	0.0043	2200	50,000	92	40/42 with fins	530	5	18	6.5mm gold

Correct. The max RPM remains the same because they all use the same rotor. What changes are the windings which are on the inside of the can. You're going to have some variation in what the rotors actual rotational speed is as you'd probably get some weird fractional winding numbers if you wanted them all to have the same RPM. I think to the manufacturer it's just important that the listed max RPM is higher than the rotor can turn without load under the given max voltage specs.