The resistance of the wire winds determine how much heat the motor produces since amount of heat produced = I^(2)R. How much power the motor can produce is determined by how much heat it can take (eg better cooling, larger size and higher quality parts such as heat resistant magnets etc.) since the amount of power going in is proportional to the current input. As current increases, eventually due to the I^(2)R element, the amount of power wasted as heat becomes significant compared with the useful power output. Hence the kV element is irrelevant in this case, except that a higher resistance winding will derate the power rating due to increased heat generation at higher current.
KV vs Gearing?
- Thread starter ProxRB
- Start date
Real threat is heat then, ok I'll do some maths and get these motors 
Hummina Shadeeba
10 MW
How would you mathematically figure the ideal rpm of a motor if you increased the amount of magnets and stator teeth?
The 8600 rpm number is probably specifically for a 12 x 14 pole motor.
Also are escs more or less efficient at higher speeds? How bout sine escs?
The 8600 rpm number is probably specifically for a 12 x 14 pole motor.
Also are escs more or less efficient at higher speeds? How bout sine escs?
Tangential but related question.
Mechanical power is
Pmech = T x w
Where t is torque and w is angular velocity, aka rpm.
T = kt x I
w = kv x V = V / kt
Put it all together
Pmech = kt x I x V / kt = I x V = I x I x R = copper losses
Does that mean that our motors all waste half the power as heat?
Mechanical power is
Pmech = T x w
Where t is torque and w is angular velocity, aka rpm.
T = kt x I
w = kv x V = V / kt
Put it all together
Pmech = kt x I x V / kt = I x V = I x I x R = copper losses
Does that mean that our motors all waste half the power as heat?
Guys I up this thread because I received many questions regarding utility of ultra low Kv motors (based on Vedder's explanations) and I wish clarify this, please feel free to correct/debate what I'll write.
I myself learnt a lot from Vedder's work so that's right he has long experience but what many people don't understand (and he will agree) actually is that efficiency does not make everything in the overall performance of the motor.
Efficiency is by definition the power output curve of the motor based on the amount of power fed to it, the close the power output is from the power fed, the more efficient is your motor. Supposedly in dLRK motors like widely used in RC brushless outrunners, Vedder estimated maximum efficiency around 6000 to 8000 RPM.
Here are the limits of efficiency :
1)Outside of the "magic 6-8000 RPM", you never max it nor get close to it, it is a power curve and not horizontal straight line band for a reason.
2)You will benefit from it only around these numbers and you always vary the speed of your motor during a ride, equals efficiency is great but not the most relevant number when you use your board in urban conditions or (get crazy?) to make stunts. However if you drag race and/or do sprint races with no need to slow down because of obstacles, yes it is fairly relevant. Mountainboards which often slip on the grass/dirt will benefit way more from efficiency than road e-boards.
3)Remember when I said that outside of magic numbers you are out of maximum efficiency curve? As long as you don't get close to magic numbers, differences between a high and low Kv motor in power output for power fed won't be much. Around the top power of each though, highest Kv motor will produce less heat than lowest Kv motor.
Then let's talk Lower KV. Indirectly YES it gives more torque, else nobody would need gearing. Why? Because Torque is the translation of Watt power into mechanical force. You determine your mechanical force by measuring not the power fed but the power output at given xy RPM.
Simple explanation is that when you measure two motors with the same maximum power output but one gives its maximum at 1000RPM and the other gives its maximum at 6000RPM, maximum available power in terms of mechanical force is not diminished (apart from heat losses, yet both high and low Kv motor suffer greatly from heat so no need to add it in the explanation unless somebody wants to compare precise motors models).
You have the same amount of power fighting his way to spin its max, the higher RPM the higher margin to express this power, thus less resulting torque. The lower RPM the lower margin resulting in more brutal torque.
This is the reason why we usually need enough low KV motors to use with gearing reductions as reduction gearings can do much much benefit but not everything ; you won't go anywhere with a 1000Kv motor because you will need such huge reduction that you won't have enough space to fit it (unless you can achieve a variable volume reduction system, which can be explored too).
Apart from "I know I need to use gearing reduction because it works and prevents my motor from going PUFFfff", reduction is needed to multiply torque and not fry RC outrunners which are otherwise not made for our usages from the start.
Gearing down is the same than lowering Kv, slowing the mechanical rotating movement at the output point with the same output power in order to obtain proportionnally more torque. A low KV motor would need proportionnally more power to spin at same speed than its higher kv sibling, yes that's true too. But what if you don't need to spin faster?
It's the rotational speed of the wheel against the ground which is relevant in the end, be it with low Kv or high Kv, as long as you set your board good based on the cruising speed and max speed you want to reach.
So stop saying that Low KV motors don't give more torque because indirectly, yes it does. It would be way more correct to say that it doesn't spin as fast or doesn't deliver as much max power or that it might be less efficient, and yet depending on your needs it might be the best choice when you seek more torque.
Again I'm open to corrections on what I wrote, though I tried my best to verify and calculate this before writing.
I myself learnt a lot from Vedder's work so that's right he has long experience but what many people don't understand (and he will agree) actually is that efficiency does not make everything in the overall performance of the motor.
Efficiency is by definition the power output curve of the motor based on the amount of power fed to it, the close the power output is from the power fed, the more efficient is your motor. Supposedly in dLRK motors like widely used in RC brushless outrunners, Vedder estimated maximum efficiency around 6000 to 8000 RPM.
Here are the limits of efficiency :
1)Outside of the "magic 6-8000 RPM", you never max it nor get close to it, it is a power curve and not horizontal straight line band for a reason.
2)You will benefit from it only around these numbers and you always vary the speed of your motor during a ride, equals efficiency is great but not the most relevant number when you use your board in urban conditions or (get crazy?) to make stunts. However if you drag race and/or do sprint races with no need to slow down because of obstacles, yes it is fairly relevant. Mountainboards which often slip on the grass/dirt will benefit way more from efficiency than road e-boards.
3)Remember when I said that outside of magic numbers you are out of maximum efficiency curve? As long as you don't get close to magic numbers, differences between a high and low Kv motor in power output for power fed won't be much. Around the top power of each though, highest Kv motor will produce less heat than lowest Kv motor.
Then let's talk Lower KV. Indirectly YES it gives more torque, else nobody would need gearing. Why? Because Torque is the translation of Watt power into mechanical force. You determine your mechanical force by measuring not the power fed but the power output at given xy RPM.
Simple explanation is that when you measure two motors with the same maximum power output but one gives its maximum at 1000RPM and the other gives its maximum at 6000RPM, maximum available power in terms of mechanical force is not diminished (apart from heat losses, yet both high and low Kv motor suffer greatly from heat so no need to add it in the explanation unless somebody wants to compare precise motors models).
You have the same amount of power fighting his way to spin its max, the higher RPM the higher margin to express this power, thus less resulting torque. The lower RPM the lower margin resulting in more brutal torque.
This is the reason why we usually need enough low KV motors to use with gearing reductions as reduction gearings can do much much benefit but not everything ; you won't go anywhere with a 1000Kv motor because you will need such huge reduction that you won't have enough space to fit it (unless you can achieve a variable volume reduction system, which can be explored too).
Apart from "I know I need to use gearing reduction because it works and prevents my motor from going PUFFfff", reduction is needed to multiply torque and not fry RC outrunners which are otherwise not made for our usages from the start.
Gearing down is the same than lowering Kv, slowing the mechanical rotating movement at the output point with the same output power in order to obtain proportionnally more torque. A low KV motor would need proportionnally more power to spin at same speed than its higher kv sibling, yes that's true too. But what if you don't need to spin faster?
It's the rotational speed of the wheel against the ground which is relevant in the end, be it with low Kv or high Kv, as long as you set your board good based on the cruising speed and max speed you want to reach.
So stop saying that Low KV motors don't give more torque because indirectly, yes it does. It would be way more correct to say that it doesn't spin as fast or doesn't deliver as much max power or that it might be less efficient, and yet depending on your needs it might be the best choice when you seek more torque.
Again I'm open to corrections on what I wrote, though I tried my best to verify and calculate this before writing.
maxchilton
10 W
- Joined
- Jul 15, 2015
- Messages
- 92
everyone seems to be forgetting that a high kv motor and high gear reduction = massive drag when off power and coasting, which is bad for skateboarding.
Hummina Shadeeba
10 MW
Given the same 2 motors with different windings and therefore different kv, they will BOTH PRODUCE THE SAME AMOUNT OF HEAT FOR THE SAME TORQUE. So a motor with 10 turns and 10kv can produce the same amount of torque as a 1 turn motor with 1000kv. The low kv motor will produce that torque more efficiently as it requires less amps to get the torque and that's the benefit of low Kv. With the drawback of lower speed and reduced overall possible power
A low kv motor is more likely to burn up than a high kv due to the lower amount of amps that can be put in because of the thinner windings
A motor's ultimate ability to produce torque is determined by the other motor parts and mainly its size and airgap inches squared.
Either motor would work fine for a skateboard, 100kv or 1000kv, if the batteries and controller were feeding the right mix of volts/amps. A 1000kv motor would need maybe to be run on 1 volt and more amps and the 100kv run on 30 or something volts. Everyone likes to run at lower amps as they need thicker connections to not be wasteful, but running on high voltage causes higher switching in the esc and motor which cause losses and inefficiency.
So i read
A low kv motor is more likely to burn up than a high kv due to the lower amount of amps that can be put in because of the thinner windings
A motor's ultimate ability to produce torque is determined by the other motor parts and mainly its size and airgap inches squared.
Either motor would work fine for a skateboard, 100kv or 1000kv, if the batteries and controller were feeding the right mix of volts/amps. A 1000kv motor would need maybe to be run on 1 volt and more amps and the 100kv run on 30 or something volts. Everyone likes to run at lower amps as they need thicker connections to not be wasteful, but running on high voltage causes higher switching in the esc and motor which cause losses and inefficiency.
So i read
maxchilton
10 W
- Joined
- Jul 15, 2015
- Messages
- 92
Hummina Shadeeba said:
everything you here posted is completely wrong.
read this if you haven't already.
http://vedder.se/2014/10/chosing-the-right-bldc-motor-and-battery-setup-for-an-electric-skateboard/
Hummina Shadeeba
10 MW
maxchilton said:
wow. I pretty much pulled all of what I wrote here from the motor section where it was just yelled at me " BOTH PRODUCE THE SAME AMOUNT OF HEAT FOR THE SAME TORQUE" by "john in cr" who has like 10,000 posts here and definitely knows his stuff.
I'll check out vedder's blurb but and report back.
dont know what in particular you see that I wrote that contradicts what vedder wrote. He tells of voltage as high as 12s not being a problem with the fast switching for the esc which I didn't know it was that high..
maybe you can tell me what is the everything that is completely wrong. ?
Hummina Shadeeba
10 MW
He does emphasize the 8000, I think it was number, as an ideal rpm. I wonder if that's a balance between copper and iron losses. have to look that up again. thanks for the link. seen it before but lost it.
If same heat for the same amount of torque regardless of kv.. why would anyone get a low kv motor. maybe the need for more amps to get the watts
If same heat for the same amount of torque regardless of kv.. why would anyone get a low kv motor. maybe the need for more amps to get the watts
That is correct and very important to understand. You can run a 1 turn 2000kv motor just fine on a longboard if the electronics and cables can deal with that.
I don't know exactly what you mean by higher switching, but in general this statement is wrong. From the ESC side, making it more efficient is easier on high voltage and low current than the other way around. That's why the VESC can run without heatsinks on a 12s single motor setup and take any hill at 30 km/h. On 6s this would never work without heatsinks even if I choose different MOSFETs made for lower voltage.
This is not an exact number, everything depends on everything. For example, if the rider is very light and the desired top speed is low (say 20km/h or less), this number is lower. If the motor has higher diameter and more pole pairs this number also becomes lower. If the motor has less pole pairs this number is higher. If you run at high power levels for a small motor with extra cooling this number becomes higher. So everything depends. What I am saying is that if you have motors similar to what most eboarders use, your weight is about 80kg, the gearing is for 30-40km/h and you have about the same terrain as I have (hills here and there) 8000 RPM is a quite good number. For the record, on single motor setups this number is a bit higher than for dual motor setups if everything else is the same. Now, if you make a board with anything between 6000 and 10000 RPM at top speed you are most likely fine. However, if you for example gear such that you get top speed at 3000RPM and use a low kv motor with low voltage, you should consider changing your setup.
ewinters
1 W
I'm not sure if this was clearly covered before, but another compromise with gears is the rpm range.
2:1 gear means half the load, thus half the heat. It also means half the rpm range.
Of course this will only be a problem for some motors / applications.
Gears work like going arround the block to avoid a steep hill.
2:1 gear means half the load, thus half the heat. It also means half the rpm range.
Of course this will only be a problem for some motors / applications.
Gears work like going arround the block to avoid a steep hill.
Hummina Shadeeba
10 MW
vedder 3000rpm or less is typical for hub motors. What kv would u recommend a hub motor have?
ridethelightning
1 MW
- Joined
- Jul 21, 2013
- Messages
- 2,010
bookmark
Hummina Shadeeba said:
That is the problem with hub motors. While a single 6364 motor with gearing can take you up any hill, the same single 6364 motor can never do that as a hub motor. You need at least two hub motors to climb some hills and even then they can overheat. So if you aim to make the lightest possible longboard with a given performance, hub motors are not a good option. Compared to motors with gearing, you need more and/or larger hub motors to achieve the same performance.
Regarding the KV for hub motors: again, this is to fit the motor to the electrical part of the drive train. Based on the RPM that hub motors run on and what KVs are available, the lower kv you can get the better. Below 100KV is a good starting point. You can also reconnect motors from D to Y to decrease the KV with a factor of sqrt(3).
ewinters said:
Again, everything depends. You can gear for a theoretical top speed of 100km/h and you will probably be able to reach that downhill, but then you have a bad setup for general terrain and you cannot go uphill. If you only plan to go very fast downhill then this is a perfect setup.
This is wrong. On half the load you get one quarter of the heat. Doubling the load will give four times more heat. A setup with a hub motor will generate 4 times more heat uphill than the same setup with 1:2 gearing. There is a square relation and this is important to understand.
made_in_the_alps_legacy
10 kW
ProxRB said:
nice brain teaser... the link below was quite interresting :
http://www.ultimaterc.com/forums/showthread.php?t=225280&page=5
vedder said:
Not contradicting what you said but challenge accepted!
I got some badass hills around my town and I will try them... can do some more stress and make them try me! Though actually hub motor can be the lightest choice if made properly, just there are not many brushless outrunners actually made FOR our use. You can get crazy power levels nowadays with tiny cans, but until ultra low kv motors are economically proven on the market as profitable, there won't be any. Sure enough gearing will maintain an advantage in performance but still there are many factors to account for (comfort and feedback are important to me for example, it's the same than driving both a Dodge Charger and a Mazda MX5, the first gives you first rate adrenaline but second one links you to the road like it's second to none ; you can truly appreciate a ride when you are fully confident in what you drive or use).
ewinters
1 W
Tks @vedder you're right.
Considering the square relation, there is probably a certain degree of steepness above which DD hubs aren't suitable for a given weight. Would be good to have a straightforward method or parameters to determine this.
Considering the square relation, there is probably a certain degree of steepness above which DD hubs aren't suitable for a given weight. Would be good to have a straightforward method or parameters to determine this.
mccloed
100 W
From the numbers I've entered in the calculator as they pertain to my boards, this calculator is pretty accurate. The more "real" numbers would be the "weighted" numbers.
polynikesx
1 µW
- Joined
- Apr 10, 2016
- Messages
- 3
I know this is a bit dated, but from the above post regarding the calculator, how does the efficiency of 70% change in a two motor set up? Would it be more efficient say like 80% or no change?
