Brushless Motor Tech: Everything You Need to Know

[RC MAN]

Thank you very much for the helpful reply.

That’s alright always trying to help when i can.

 

[ahr43]

Hey guys. Posting here on the help desk to find out if this will work before splitting open the packages.

HW sensored brushless motor matched to a HW non-sensored ESC? Bandit 13.5T and MAX10 ESC powered up with a 3S x 4500 LiHV.

It wasn't until reviewing the order after shipping that the mis-match combo caught my eye. Focus was on finding a 3S rated ESC. I overlooked the mis-match. Yeah, rookie error. Sam at HW Tech replied by email that it should work o.k. But there were caveats that sent me here seeking second opinions.

Try it or send it back? Appreciate hearing your thoughts. Cheers. 'AC'

 

[Greywolf74]

Hey guys. Posting here on the help desk to find out if this will work before splitting open the packages.

HW sensored brushless motor matched to a HW non-sensored ESC? Bandit 13.5T and MAX10 ESC powered up with a 3S x 4500 LiHV.

It wasn't until reviewing the order after shipping that the mis-match combo caught my eye. Focus was on finding a 3S rated ESC. I overlooked the mis-match. Yeah, rookie error. Sam at HW Tech replied by email that it should work o.k. But there were caveats that sent me here seeking second opinions.

Try it or send it back? Appreciate hearing your thoughts. Cheers. 'AC'

If you have the money and the time Id send back the sensorless ESC and get the sensored one just because sensored is the bees knees, That being said the motor should operate in either mode.

 

[Nefarious]

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.

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.

He mentioned 2 out of the 3 things you stated. He mentioned the torque. He mentioned the Pinion gear.

 

[PoppinCaps]

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

I know this thread is old. I do appreciate the info. Thanks.

 

[bill_delong]

Something to add on motor selection is Voltage and Efficiency.

I used to race 4WD SCT and ROAR has a 2S restriction where most folks would go with a 4000KV motor on average. Only problem is that my run times where around 10 min and both the motor and battery would generate a lot of heat where excessive heat equates to loss of efficiency.

The club in my area didn't care about the 2S ROAR restriction and I started to experiment with 3S and 2500KV which yielded 15 min run times and significantly lower temps proving that improved efficiency was achieved with higher voltage:
https://www.teknoforums.com/threads/build-review-sct410-3.1389/post-14872

I would later go a step farther and run 4S with 1800KV which offered 20 min run times and even lower temps:
https://www.teknoforums.com/threads/build-review-sct410-3.1389/post-22242



I have since taken this information and applied it to my 1/8 eBuggy where the most popular motor at the track in my area has been the HobbyWing 2200KV motor which is arguably the fastest/lightest motor on the market. Unfortunately I have been damaging the sensor boards on my HW motors making it impractical to support the brand anymore because they don't sell replacement boards, you have to replace the entire motor I have since switched to the Trinity XF8 but they only offer it in 2000KV. What I did was increase the mechanical timing from 20° default to 30° and have found that produces the same exact speed as the HW 2200KV motor which is still installed in my son's eBuggy with identical setup and only difference is motor while drag racing each other down the front straight at our local track to confirm both motors run the same speed. What I found is that the temps on the XF8 are avereaging 15°F less than the HW motor proving that lower KV with higher mechanical timing can be equally as powerful and more efficient!

 

[tudordewolf]

I think there's a little bit of a misunderstanding around KV and torque, in that lower KV doesn't necessarily mean the motor produces more torque, but that it draws less current per unit of torque produced.

I'm open to having my grasp improved even further. Here's how I understand it:

If you stall a 1000kv and a 2000kv motor of the same size, they'll be applying similar amounts of torque, but the 2000kv motor will be drawing twice as many amps and heating up twice as fast.

Look at this table for Surpass's brushless motors: All the 3660's are rated for 1200 watts and 50,000 rpm - the difference is in how they achieve that (theoretical) performance. Their 4200kv motor recommends 12V max, 3s, 100 amps. But the 1400kv motor can take up to 36 volts (9S) & 33A. If you were to run them both at max power at their respective voltages, they'd be turning at similar speeds and producing similar torque, but the 1400kv motor would be drawing 1/3rd the current, reducing the power wasted in the rest of the system 9x.

Edit: I will amend my statement to mention that for the first 4 motors in the chart, the lower kv = more torque relationship totally applies - they have decreasing rpm's and increased power ratings, which would mean that their max torque has to have gone up. However, this does highlight the fact that a lower kv enables you to use more power in general if you design for it.

 

[Greywolf74]

Something to add on motor selection is Voltage and Efficiency.

I used to race 4WD SCT and ROAR has a 2S restriction where most folks would go with a 4000KV motor on average. Only problem is that my run times where around 10 min and both the motor and battery would generate a lot of heat where excessive heat equates to loss of efficiency.

The club in my area didn't care about the 2S ROAR restriction and I started to experiment with 3S and 2500KV which yielded 15 min run times and significantly lower temps proving that improved efficiency was achieved with higher voltage:
https://www.teknoforums.com/threads/build-review-sct410-3.1389/post-14872

I would later go a step farther and run 4S with 1800KV which offered 20 min run times and even lower temps:
https://www.teknoforums.com/threads/build-review-sct410-3.1389/post-22242



I have since taken this information and applied it to my 1/8 eBuggy where the most popular motor at the track in my area has been the HobbyWing 2200KV motor which is arguably the fastest/lightest motor on the market. Unfortunately I have been damaging the sensor boards on my HW motors making it impractical to support the brand anymore because they don't sell replacement boards, you have to replace the entire motor I have since switched to the Trinity XF8 but they only offer it in 2000KV. What I did was increase the mechanical timing from 20° default to 30° and have found that produces the same exact speed as the HW 2200KV motor which is still installed in my son's eBuggy with identical setup and only difference is motor while drag racing each other down the front straight at our local track to confirm both motors run the same speed. What I found is that the temps on the XF8 are avereaging 15°F less than the HW motor proving that lower KV with higher mechanical timing can be equally as powerful and more efficient!

I think there's a little bit of a misunderstanding around KV and torque, in that lower KV doesn't necessarily mean the motor produces more torque, but that it draws less current per unit of torque produced.

I'm open to having my grasp improved even further. Here's how I understand it:

If you stall a 1000kv and a 2000kv motor of the same size, they'll be applying similar amounts of torque, but the 2000kv motor will be drawing twice as many amps and heating up twice as fast.

Look at this table for Surpass's brushless motors: All the 3660's are rated for 1200 watts and 50,000 rpm - the difference is in how they achieve that (theoretical) performance. Their 4200kv motor recommends 12V max, 3s, 100 amps. But the 1400kv motor can take up to 36 volts (9S) & 33A. If you were to run them both at max power at their respective voltages, they'd be turning at similar speeds and producing similar torque, but the 1400kv motor would be drawing 1/3rd the current, reducing the power wasted in the rest of the system 9x.

Edit: I will amend my statement to mention that for the first 4 motors in the chart, the lower kv = more torque relationship totally applies - they have decreasing rpm's and increased power ratings, which would mean that their max torque has to have gone up. However, this does highlight the fact that a lower kv enables you to use more power in general if you design for it.

View attachment 157813

Good info here guys!

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

I wish I could go back and edit the first part of my OP.

 

[Bogda89]

Good info here guys!

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

I wish I could go back and edit the first part of my OP.

Yes, I agree with this. Let's say two identical motors, one 2000 kv and one 4000 kv just an example. Running 2000 kv motor on 4s would produce identical torque and power, just half the amps drawn compared to 4000 kv.

Running 2000 kv with twice bigger pinion and using the same 2s will not compensate and be equal performance like 4000kv 2s smaller pinion. Most likely it will be overgeared and run outside efficiency range and overheat compared to 4000kv smaller gearing

 

[airslither1982]

Finally burned up the motor in my Kraton 6s ...in the market for a new one...A GOOD ONE...any suggestions??

 

[Greywolf74]

Finally burned up the motor in my Kraton 6s ...in the market for a new one...A GOOD ONE...any suggestions??

Castle 1515 2200Kv

 

[RustyUs]

Finally burned up the motor in my Kraton 6s ...in the market for a new one...A GOOD ONE...any suggestions??

Hobbywing EZRun Max4985SL-1650KV, or Castle Creations 1520-1650KV

 

[LibertyMKiii]

Finally burned up the motor in my Kraton 6s ...in the market for a new one...A GOOD ONE...any suggestions??

Arrma forum would be a better source of info, but what I have seen time and time again is that 2200kv motors run too hot on 6s in the heavy Arrma rig.
Best ones seem to be in the 1600 -1800 kv range for handling the heat.

The max 6 combo is a great option.