Recently, I have been involved with the electric skateboard community because of my custom ESC. I get many questions about motor kv, gear ratio, current, voltage and efficiency. In this post, I will try to explain how things are connected and how to chose the right setup. I will try to keep things simple and not involve too many equations to provide a good intuition for the DIY community. The assumption in this post is that we are using an 50mm-60mm hobby outrunner motor.
KV
First of all, I will try to make some things clear about the KV rating of these motors. Even if there are several versions of the same motor available with different KV, the properties of the motor are exactly the same but at different voltages/currents. The only difference the KV makes is how to choose an ESC and a battery pack, but I will explain more about that in the ESC section. For the same motor, the low KV versions have more windings with thinner wire, while the high KV versions have less windings with thicker wire. As long as they have the same mass of copper, they are exactly the same in regard to max power output, torque, efficiency, max RPM; but at different currents/voltages. If they don’t have the same mass of copper they are different, but it is always true that the more copper is squeezed into the stator, the better the motor is.
Let’s look at an example: Suppose an 8 turn motor has one ohm winding resistance. The winding resistance is proportional to the wire area times wire length. Making the same motor with 4 turns would allow twice as thick wire. Since that wire also is half as long, the resistance is four times lower: 0.25ohm. Further, since current times turns is proportional to torque, we need twice as high current with 4 turns to produce the same torque as with 8 turns. The copper losses are the voltage across the windings times the current: U*I. The voltage across them is R*I, so the losses are R*I*I. This square relation means that doubling the current will produce four times as much losses, however, since we got four times lower resistance the losses are the same for the 4t motor with double the current as for the 8t motor with half the current. Also note that the 4t motor will spin twice as fast as the 8t motor at the same voltage, so it will have double the kv. Putting this together, the 4t motor is equivalent to the 8t motor, but at half the voltage and double the current.
So, regarding KV: different KV versions of the same motor are fully equivalent. KV only affects the battery and ESC choice. Therefore, while comparing motors, lets talk about torque instead of current because torque is proportional to current / KV and, as explained above, the KV value can be changed freely with the amount of turns and copper thickness.
RPM/Losses/gear reduction
Now we know that copper losses are proportional to the square of the torque produced by the motor, and at low RPM and high load they are dominant. As RPM increases, other losses start to add up exponentially. In my experience, these losses start to get significant around 60k electrical RPM, which for a 14-pole motor is about 8570 mechanical rpm (most 50mm+ outrunners have 14 poles, some unusual ones have 18). Because of the square relation, it is desirable to run at as high speed and low torque as possible as long as we stay below 8.6k RPM. To express the square relation in some numbers, having double the RPM and half the torque at a certain power output will cause four times less losses. The lesson from this is that: make sure the top speed you design the skateboard for is at around 8.6k rpm on the motor if you are using an 50mm-60mm outrunner.
For my longboard with 84mm wheels, where I would like to design for a top speed of about 35km/h with a sing motor, I would need a gear ratio of about: (35 / (0.084 * pi * (8600 / 60) * 3.6) = 0.257 which is 1:(1/0.257) = 1:3.9. Note that this gear ratio is independent of battery voltage and motor kv. Keep in mind that 8.6k rpm is not an exact number, but a guideline that seems to apply quite well to all 50mm-60mm hobby outrunners I have tested so far.
Batteries/ESC/KV
As noted previously, we are designing for 8.6k rpm since we are using 14 pole motors and 60k electrical rpm seems to be a good value according to my experiments.
The resistive losses in the MOSFETs in an ESC are proportional to the square of the current, because the voltage across the MOSFETs is Ron*I and the power, which is U*I, in this case is Ron*I*I where Ron is the ON-resistance of the FETs. This means that for a given FET, doubling the current will produce four times the losses. Remember that current is directly proportional to motor torque. From the ESC perspective, we should run on as high voltage and low current as possible. Now, one interesting fact about MOSFETs is that the lower voltage they are designed for, the lower resistance they tend to have. So if I make an ESC for a lower voltage, the FETs will also have lower resistance. However, the PCB traces always have the same resistance and MOSFET resistance does not seem to decrease as fast as their voltage decreases, meaning that an ESC designed for higher voltage tends to be more efficient in general. When the voltage gets too high, handling it while switching fast starts to become problematic. I have discovered that a good trade-off is at around 60V (quite safe for 10s or 12s lipos), where the efficiency is good and the voltage is not too problematic to handle.
Going back to my longboard example with 84mm wheels and a top speed of 35 km/h at 8.6k RPM with a 1:3.9 gear reduction, let’s look at a good motor KV. At 12s, which seems good from the ESC perspective, and a moderate charge level, we have 3.8 * 12 = 45.6 volts. Since we want to run the motor at 8.6k RPM, we need a KV of 8600 / 45.6 = 188. Now, that is quite low. Since there are none or few 50mm outrunners with that KV available, we can do two things:
1. Run at 10s. Then we need a KV of 8600 / (10 * 3.8) = 226. Luckily, there happen to be 225 KV motors available on hobbyking 🙂
2. Change the KV of the motor. Yes, this can be done quite easily without rewinding the motor. Almost all outrunners are connected using a delta-connection. By removing the heatshrink of the motor wires, they can be split up and reconnected in a star-connection. This reduces the KV by a factor of sqrt(3) = 1.73. For this modification, we can use a motor with a KV of 188 * sqrt(3) = 325. There are several hobby motors available close to that KV, so that is no problem. I have tested this modification on several motors and it works really well.
Thanks for sharing this really useful post. Just what I need to give my skateboard an edge.
Would you mind pointing out that 225KV motor on HobbyKing? I’m not able to find it … the motors are scattered across several categories.Thanks!
I noticed that my 225kv motor isn’t from hobbyking. I have a 215kv motor from hobbyking, but they don’t seem to have it in stock any more. There are some 245kv motors with the same size though.
Hi.
Thanks for sharing this. How did you calculate optimal RPM with respect to number of poles? I’d like to know the optimal RPM for a 12 pole motor; could it be calculated as 60 000 / (12/2) = 10 000 RPM ?
The optimal RPM depends on a many factors, so this is by no means the mathematically correct way to find the optimal RPM. However, 60000 / (12/2) will probably work well.
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Hi Vedder,
I have bought a motor and alien mount kit second hand from a guy who abandonned his projet. The problem is that the motor is a EMP N6364 230KV and the pulleys are 32T/12T.
From your formulae, the max speed at 8600rpm would be huge (>60km/h). I thought I should buy a 12T motor pulley to reduce the gear ratio. Is that the correct way to go ?
Cheers
Akira
Hi Akira,
If you never intend to go as fast as 60 km/h, you should get a smaller motor pulley. Then you will have more torque and the motor will run cooler.
I think the turnigy 6374 motor has 12 poles. Should I shoot for 8600 mechanical rpm, or is 10000 rpm more accurate?
Kurt, the turnigy 6374 has has 12 slots, but 14 poles.
wow! i just read that someone already asked you that. sorry, palm to my face…well it’s ok, for 10k rpm, correct?
Hi Benjamin, such great great info here on your site; thanks for sharing your knowledge!
I am trying to understand brushless motors a bit. I (think) I understand the basic relationships between Torque and RPM, but was hoping you could help and elaborate a bit more on the RPM vs losses part. How much do you think the 60k electrical RPM, which you find to be the optimal balance between copper and non-copper losses in 14-pole 50-60 mm diameter outrunners, can be generalized to other motor sizes/designs? Are there any general principles or rules-of-thumb that would apply to any given (hobby-type) brushless motor? What factors would mainly affect that optimum RPM?
I am specifically interested in 40-56 mm diameter 4/6-pole inrunners (RC car guy, hope to be trying your VESC in one soon :)).
Tanks!
JT
I don’t have much experience with RC car inrunners, but in general they should work fine with the VESC if their electrical speed is less than 100k ERPM. Motors designed for higher speed tend to have very low inductance which make current measurement difficult. So far all 4-pole motors I have tested work fine, but I had issues with an 6-pole motor (I only have one).
You state: “they are exactly the same in regard to max power output, torque, efficiency, max RPM; but at different currents/voltages”
My question is on torque. Is the torque linear between different KV motors? Basically, if using a 10s battery, at what point does a 150kv motor have the same torque as a 300kv motor?
For example (using 10s) would a 150kv motor (no gearing) have the same torque on a wheel at 1000rpm as a 300kv motor at 2000rpm (geared at 1:2)?
A 150kv motor with 1:1 gearing would have the same torque per amp as a 300kv motor with 1:2 gearing. Also, the speed on the wheel would be the same for the same voltage. However, the efficiency and losses would be completely different. A 150kv motor of a given size has 4 times higher resistance than a 300kv motor of the same size. That is because the 150kv motor has twice as long and half as thick windings. This means that the resistive losses for the 150kv motor with 1:1 gearing will be four times higher for a given wheel torque compared to the 300kv motor with 1:2 gearing. So, as I wrote, the motors have the same properties just at different voltages/currents. It is important to understand this.
hi Benjamin, i am highly impressed with your blog.
After reading your blog, i started one controller project for my bicycle, i am noob in it, have few quation for you.
is your controller work on 2000 W BLDC motor ? (for bicycle)
i dont have any idea about controller, have to read your controller section today.
Hi Benjamin,
I have a follow-up question regarding D vs Y-winds: is there a general difference in performance/Current characteristics between the 2 winds when comparing same motor design (same brand, size, type) with similar kv?
I understand from what you wrote above that a D-wind needs sqrt(3) times more turns to have the same kv as a Y-wind, which would translate into higher winding resistance and thus copper losses for the D-wind, correct? I checked with some specs of popular RC motors and it seems D-winds do have higher winding resistance. In addition, from the specs I checked, it also seems that the D-winds have higher no-load Currents than the Y-winds (again, comparing same motor, similar kv), which indicates higher non-copper losses as well. Does this mean a D-wind is by definition less efficient than a Y wind at comparable kv or am I missing something? Is there a general rule for which types of applications are better suited for Y or D winds?
Thanks!
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You don’t say anything about efficiency. I see calculators that put weighted rpm load at only 70% no load. Doesn’t that mean in use we should shoot for <= 8600 RPM, after reducing the kv to 70% in out calculations?
If you loose 30% speed on a given voltage you are most likely overloading the motor a lot. A typical resistance for a 170kv outrunner is 0.03 ohm, so if you loose 30% of 45V you would run over 400A in the motor. At 70A load, that motor would loose about 5% in speed due to the winding resistance. Most of the voltage drop is most likely in the batteries, and it also depends on the charge level. In my calculations I was using less than the full charge voltage of the batteries, so a good point to start would be 3.7V per cell or so at 8600rpm. Again, these are not exact numbers, everything depends on the rest of your setup.
Hi Ben,
I’d like to try a steup of dual VESC with two tacoon 110 motor (dual rear mounted).
I a worried by the high KV of 295 for a 12S voltage.
Then I read about your trick of lowering the KV by rewiring from delta to star connection. Have you tried it on this motor ? Do you think it should work ?
Thanks again for all the great work !!
Hi,
The D to Y change should work on that motor. Otherwise 295kv is too high for 12s.
/Benjamin
When making this calculation:
Run at 10s. Then we need a KV of 8600 / (10 * 3.8) = 226.
You assume that each cell will be at 3.8 volts? Can you explain why you use this value and not 4.2 volts, the max the cell can be charged?
If you where to assume the battery was fully charged, you would have 8600 / (10 * 4.2) = 205 kv.
Thank you for this very detailed article.
If you WANT to go at say 35km/h then it is not nice if you only can go that fast if the battery is fully charged at 4.2V.
If, for example, you have to comply with a law stating that your equipment is not allowed to be ablet to go faster than 35km/h, you should use the 4.2V.
Thanks Ben.
I ll try that and report on the outcome.
Sorry to bother you again with these KV issues.
A friend of mine is planning to use the VESC with a motor of 270KV.
Do you think he can power it with an 8S battery ?
It is OK with such a high KV motor ?
Hi Vedder. I’m still stuck wondering what the value of volts is. Amps x volts equals watts…but can u explain how voltage makes the motor go. They don’t produce torque like amps.
Volts dictate the maximum speed your motor will able to turn at. Through the gear ratio and the wheels (size) that will translate to maximum speed of your board.
Hi Benjamin
I’m trying to set up my first electric skateboard, I’ve been reading lots of blogs about it, seems like everyone has a different opinion regarding and since I’m not really into mathematics equations, that was my only source of knowledge.
Bottom line I’ve been spending lots of money on wrong equipment and some of them I wont even able to return it.
Any way, long story short I’ve got 90 mm wheels, Tacon bigfoot 160 6364 245 kv 2700W motor, so any suggestions about what type of battery pack should I get?
(I was planning to get the Enertion 10s pack, riding on flat, not looking for crazy high speed).
Also I’d like to know is the vesc would be compatible with that set up?
Any suggestions for motor and wheel pulley?
Please any advise is welcome.
Thank you so much
Best to go to endless-sphere or electric-skateboard.builders. There are many people there who have the answers u need. U can get any battery pack under 10s I believe. U could use any pulleys if they fit. If u want to find the speed ull hit there’s a calculator somewhere.
From what you write above it seems as long as e motor kv is low enough and the erpm is not an issue then 60 volts is possible. Is this the case? I’ve got an 80kv motor
You do not want to come too close to the 60V theoretical limit. There are a bunch of effects that cause parts to be stressed above the battery voltage. Some of these chips are rated 60V and will die closer to 61 than 70. Very little margin.
So… best to stay below the voltage for 12S: 50.4V.
hi Benjamin,
this page has been really helpful.
You say torque and motor turns and resistance are proportional but I cant figure out any formula to help me calculate my motor’s specs.
Thank you
benjamin i read and re read this information over and over and i still dont get it im putting together a single drive 6374 149kv on 12S (im on the heavy side 200lbs if this helps as of why i picked the 149kv) what can you tell me about this set up? is 12s optimal for this? should i downgrade to 10s? i know you are busy but maybe some of the guys that gets the concept can tell me a little more about this set up
ohand my gearing is 16/36