Brushless Motor Tech: Everything You Need to Know

[Greywolf74]

I showed the nitro side some love by reposting an article I read about glow plugs so I figured Id do the same thing and show the Electric side some love this time

One thing that I would like to add to this is that when they talk about electric motors that are higher RPM the article keeps saying that the tradeoff is shorter battery life. That may be true but the more important trade off that they don't even mention is that of torque. The higher RPM the motor is the less torque it will have. With that in mind Higher RPM doesn't always mean faster either. What I mean by that is this. Lats say you have a 3800kv motor and a 5400kv motor. The 5400kv motor is higher RPM so given that the gearing does not change then yes a 5400kv motor would be faster than a 3800kv motor. However, a 3800kv motor has more torque and because of that you can run a bigger pinion thereby turning that extra torque into speed and sometimes you will actually be better off by choosing this option over simply buying a motor with higher KV ratings (or lower Turn ratings).

Castle size comparison pic.
Going from left to right I believe these are:

2028 - 800kv - 1/5 Scale - 720L can size
1717 - 1650kv - 1/6 Scale - Can size on this motor is odd. Its somewhere between a 700 and a 720 if you're going by the dimensions of it.
1515 - 2200kv - 1/8 Scale - 670 can size
1512 - 1800kv - 1/8 Scale - 670 can size
1415 - 2400kv - 1/10 scale - Another oddball can size but the closest size is a 540XL
1406 - 7700kv - 1/10 scale - 540 can size

View attachment 126582

I've learned a lot since I originally posted this info. Back in 2017, when I posted this, I believed in the myth that lower kV meant higher torque. I think that myth stems from a misunderstanding of the fact that Kt (torque per ampere) is inversely proportional to Kv (rpm per volt). My current understanding is that the maximum torque a motor can output is dictated by the size of the rotor and the stator. So if you had a 1000kV motor and a 5000kV motor that have the same exact size rotor and stator both motors can generate the same amount of torque. The problem is as the kV goes up the amount of amperage required to generate that torque increases. As amp draws increase the more heat is generated which also means capacity is consumed at a greater rate which Bill's post is a perfect example of that. One way you can bring amp draw down while keeping RPM up is to increase voltage but higher kV motors have lower voltage tolerances because of RPM limits. A given motors RPM limit is dictated by its magnetic capacity. The magnetic capacity of the motor's magnetic (iron) circuit is designed to the relationship: voltage/frequency (V/f). If the frequency drops the V/Hz goes up. This means that the motor needs a larger magnetic circuit. Without it, the magnetic circuit can be overloaded. This is called saturation and it leads to a rapid increase in current draw and a corresponding large increase in temperature, a motor's chief enemy.

That is my current understanding anyway and I always welcome civil discourse as that is one of the best ways to learn

 

[lcam88]

Very nice write-up and formatting. Very informative especially the stator turns to KV rating.

I would have to add what I consider an important consideration to understanding KV rating, lower KV does indeed directly mean less RPM, however it also means greater torque at the same power draw. More torque means you can use a larger pinion gear, and lower KV motors also allows for running higher voltages. For example, a HobbyWing 2050KV motor will work with 6S, whereas their 2400KV motor is max 4S. I think the XeRun 2200KV will work with 6S and so will the Castle 2250KV but those are the highest KV ratings manufacturers seem to support 6S.

This enhancement to the KV description clarifies why high RPM is not as important as it seems as it emphasizes other motor characteristics that are equally important.

 

[LibertyMKiii]

Some additions from my notes on Brushless motors

Given a specific motor size like 40x75 if one motor is 2200kv and the other 3800kv the higher KV motor will pull significantly more amperage and generate much more heat also. Heat and motor rpms are the limiting factor. There are guys running the 2200kv motors on 8s, but it can only be done for short 10 second bursts. This only works if traction and gearing keeps the motor rpms in a safe limit.

Second thing to note that often is not mentioned is choosing a Y-wind versus D-wind. The D-winds typically do not like higher timing and have a characteristic of high amperage draw on initial acceleration. Y-winds typically can play with timing some to find that sweet spot and as a general statement pull less amps.

-Liberty

 

[olds97_lss]

Anyone have a chart showing many of the various brushed/brushless motors side by side?

rcjuice has a pretty lengthy motor page:
https://www.rcjuice.com/motors

Just wish they showed some of the castle motors in there for reference and the brushed equivalents.

I think the 550 is a 3660 and the 540 is a 3650-54. Think I might need a 3665 for my stampede... Has a 3660 now, but may be a bit hot when it warms up out.

 

[LibertyMKiii]

Just wish they showed some of the castle motors in there for reference and the brushed equivalents.

I made a video a while back with a few motors I had laying around might help some

I have seen especially in Traxxas world that Castle motors are part of the reason people get so confused by KV ratings.
The problem is that they offer very different motors each with different KV ratings, rather than a few motor sizes with different KV ratings.
Example
Castle 3800kv (1410) is a 36x52.7 mm can (really a 2s motor but could run short bursts on 3s for light vehicles under 7lbs)
Castle 2200kv (1515) is a 40x75 mm can (good for 3-6s and vehicles in the 6 to 14 lbs range (lower cell counts when on the heavier side))
Castle 2650kv (1512) is a 40x67 mm can (good for 2-4s on a 6 to 11 lbs range vehicle)

The can size is never the same size with Castle and people associate the KV rating as the motor, when they are very important and unique factors to consider for heat dissipation, torque, and other factors.
Other companies like Neu Motors, (many designs for Castle) TP Power, or Leopard for example have a set motor size like a 40x75mm can and then offer several kv options for it so you can pick the right motor for your desired battery cell count. Also with both motors being the same size the higher KV motor will pull more amps and generate more heat.

While there is a "normal" power range for a certain KV rating, much of that is to keep the motor RPMs in the desired range as well has heat output. Moving to a larger can will greatly help reduce that heat.

-Liberty

 

[RcDCO2W]

Very nice write-up and formatting. Very informative especially the stator turns to KV rating.

I would have to add what I consider an important consideration to understanding KV rating, lower KV does indeed directly mean less RPM, however it also means greater torque at the same power draw. More torque means you can use a larger pinion gear, and lower KV motors also allows for running higher voltages. For example, a HobbyWing 2050KV motor will work with 6S, whereas their 2400KV motor is max 4S. I think the XeRun 2200KV will work with 6S and so will the Castle 2250KV but those are the highest KV ratings manufacturers seem to support 6S.

This enhancement to the KV description clarifies why high RPM is not as important as it seems as it emphasizes other motor characteristics that are equally important.

What’s the 6s reference, sorry I’m new ?

 

[2Slow2Win]

I'm getting ready to build a traxxas xo-1 supercar. I'm not buying 1 that's already together. I wanna know what I have and I wanna know its all about. Can anyone tell me what is the best brushless motor and what gears would be the perfect setup. I'm not making it AWD, it's going be bilt for drag racing only.

 

[2Slow2Win]

Who in this forum is close or near Rock Hill SC?

 

[D3MON]

sorry, I'm not all that knowledgeable on BL motors or the XO-1. I'm sure someone might be able to help you get it figured out though.
and live quite aways away over in MT :/

 

[2Slow2Win]

sorry, I'm not all that knowledgeable on BL motors or the XO-1. I'm sure someone might be able to help you get it figured out though.
and live quite aways away over in MT :/

Thanks anyway brother. Never hurts to ask

 

[olds97_lss]

I'm getting ready to build a traxxas xo-1 supercar. I'm not buying 1 that's already together. I wanna know what I have and I wanna know its all about. Can anyone tell me what is the best brushless motor and what gears would be the perfect setup. I'm not making it AWD, it's going be bilt for drag racing only.

You might want to try the traxxas.com forum. Might be able to find a bit more info there that is x-01 specific.

 

[LibertyMKiii]

I'm getting ready to build a traxxas xo-1 supercar. I'm not buying 1 that's already together. I wanna know what I have and I wanna know its all about. Can anyone tell me what is the best brushless motor and what gears would be the perfect setup. I'm not making it AWD, it's going be bilt for drag racing only.

Are you planning to run in a no-prep class or anything like that? (XO-1 wouldn't be allowed in that class)
I can help you pick a motor, but would need to know what voltage you plan to run.
In drag often keeping the rig lighter is going to help acceleration, so many guys stick with 2s or 3s power.

Ideally you want to plug in the numbers for the car and motor into a calculator and gear for the theoretical 80mph knowing that it will reach 75% of that number due to efficiencies etc. Basically you are shooting for your car's gearing to peak out at 60-70mph at the end of the drag strip.

 

[RC Guy]

Very nice write-up and formatting. Very informative especially the stator turns to KV rating.

I would have to add what I consider an important consideration to understanding KV rating, lower KV does indeed directly mean less RPM, however it also means greater torque at the same power draw. More torque means you can use a larger pinion gear, and lower KV motors also allows for running higher voltages. For example, a HobbyWing 2050KV motor will work with 6S, whereas their 2400KV motor is max 4S. I think the XeRun 2200KV will work with 6S and so will the Castle 2250KV but those are the highest KV ratings manufacturers seem to support 6S.

This enhancement to the KV description clarifies why high RPM is not as important as it seems as it emphasizes other motor characteristics that are equally important.

 

[RC Guy]

Great post, Brushless RC is the way forward. You can get rediculous speed and power when running on 3s Lipo Battery. Take a look at my Kyosho Sandmaster running on 3S Battery.

Cheers

RC Guy

 

[Losiman56]

I showed the nitro side some love by reposting an article I read about glow plugs so I figured Id do the same thing and show the Electric side some love this time

One thing that I would like to add to this is that when they talk about electric motors that are higher RPM the article keeps saying that the tradeoff is shorter battery life. That may be true but the more important trade off that they don't even mention is that of torque. The higher RPM the motor is the less torque it will have. With that in mind Higher RPM doesn't always mean faster either. What I mean by that is this. Lats say you have a 3800kv motor and a 5400kv motor. The 5400kv motor is higher RPM so given that the gearing does not change then yes a 5400kv motor would be faster than a 3800kv motor. However, a 3800kv motor has more torque and because of that you can run a bigger pinion thereby turning that extra torque into speed and sometimes you will actually be better off by choosing this option over simply buying a motor with higher KV ratings (or lower Turn ratings).

View attachment 19483 View attachment 19484 View attachment 19485 View attachment 19486 View attachment 19487 View attachment 19488

Nice article. Very well written. That was alot of learning for sure. Good job posting this.

 

[Greywolf74]

Anyone have a chart showing many of the various brushed/brushless motors side by side?

rcjuice has a pretty lengthy motor page:
https://www.rcjuice.com/motors

Just wish they showed some of the castle motors in there for reference and the brushed equivalents.

I couldnt find anything showing brushed and brushless side by side but I did find a Castle size comparison pic.
Going from left to right I believe these are:

2028 - 800kv - 1/5 Scale - 720L can size
1717 - 1650kv - 1/6 Scale - Can size on this motor is odd. Its somewhere between a 700 and a 720 if you're going by the dimensions of it.
1515 - 2200kv - 1/8 Scale - 670 can size
1512 - 1800kv - 1/8 Scale - 670 can size
1415 - 2400kv - 1/10 scale - Another oddball can size but the closest size is a 540XL
1406 - 7700kv - 1/10 scale - 540 can size

 

[Papabear_1972]

I am comparing two RC vehicles. Both are 1/5 scale. They have two different motors in them. Which of these two motors would provide higher speeds:
Spektrum Firma 4-Pole 780Kv Brushless Motor
Spektrum™ Firma™ 1250Kv Brushless Motor

Thank you in advance for any advice or information.

 

[Papabear_1972]

Thank you for the article - most of my questions were answered.

 

[RC MAN]

Thanks for the article. very helpful

but if you are looking for the fastest and best motor you would...

pick the biggest can size you can fit in your RC car

Then choose the number of cells you want to run on

then you would choose the highest kv possible for that voltage and can size of motor.

so usually the higher the kv the lower the cell count you can run on but the lower kv the higher the cell count you can run on. But if you were to run, let’s say a 2000kv motor vs a 4000kv motor that were the same size, same voltage and the same gearing the 4000kv one would have the exact same amount of torque as the 2000kv and the 4000kv one would smash the lower 2000kv one in max rpm, so speed. So that’s why higher kv doesn’t all ways mean more torque.

 

[Papabear_1972]

Thank you very much for the helpful reply.