but youre just looking for motor “life”. motors should last a very long time as long as they dont get too hot to ruin the magnets. magnets lose their magnetism very slowly over time. you can replace bearings and the insulation on the magnet wire is rated to probably thousands of hours at whatever temp you would put them to.
your gearing will likely have a much shorter lifespan than the motor. maybe get steel pulleys and belts
Thank you, this is for a jet surfboard, full throttle would be 14,500 RPM then cruising around 10,000 RPM.
Its direct drive so we cannot use gears. So I was wondering if higher KV would have thicker copper wires and handle the current better than a low kv with thinner wires?
it takes double the amps to get the torque with the kv being double and ultimately a motor with 200kv and 10amps vs a motor with 100kv and 50amps will pretty much have the same heat and same performance
You mean 100kv and 20A I take it?
So it would really come down to the lower KV having a better/slower/smoother throttle response vs higher kv that would jump to full speek fast then, and heat/performance is about the same no matter kv?
you should plug some options into the grin motor simulator. you can search for it. the efficiency will go down if youre not riding close to the max speed of the motor based on the voltage and kv. so you could go with a super high kv motor and limit the rpm but it will be inefficient. get the kv and voltage that will get you to just the top speed you want to go but no further.
and when you compare the results on there and it shows maybe 90 percent eff with one motor vs another at maybe 92 percent, that seems small but its a 20percent increase and that’s a lot
the speed you will want to spin the motor you mean and what would be ideal? i guess you’d just have to try it and hard to guess at that with a prop and water and all. maybe its a very different situation with that vehicle as youre not really NEEDING the torque as you do on a board and it could spin up slowly or something. i have no idea.
figure what you can and then write it up here and maybe i could be of help. i think the best help i could be would be to run you through the grin motor simulator so you can visualize what im saying. what im saying is pretty one dimensional and your vehicle is multifaceted in terms of what it would maybe need to take into consideration. but if you pm me and get on the grin simulator. . or call me while youre on it id help you with that.
you’d have to explain more the loads and speeds and stuff.
Re-reading this, I think I get it, and knowing my running RPMS with sensor This give me a good idea of the KV motor to use I think.
I will take you up on that offer once we do more testing over the next week and your walking me through it would be amazing and greatly appreciated Hummie! Thanks for the offer, ill circle back after more testing with new parts that arrive Monday and we can attack this in a 1-2 weeks. Cheers!
If you gear both motors to the same top speed (they will also produce the same torque), the higher kv motor will have lower internal resistance and generate less heat, and probably more durable. However, due to the higher motor rpm, it may wear out motor bearings faster (maybe…), it may exceed VESC erpm limits, and you may have difficulty fitting the gearing without hitting the ground
I agree the higher kv will have less electrical resistance and with gearing it would produce less heat in the windings but at some point spinning faster wont be a benefit as the faster the motor spins the more lost energy and heat is produced in the iron or other conductive materials through eddy currents and hysteresis. since he’s doing a direct drive on a jet surfboard he will be able to spin the motor much faster than us as he doesnt have the physical constraints of gearing so he can spin at an optimum speed that finds the balance between the copper losses and the iron losses, unlike what we do, and that will be the most efficient speed the motor can run at and therefore that will be the most powerful speed. but you cant always be at that speed on whatever vehicle and there’s loads at different speeds that need to be thought of. for example if he does a super high kv and optimized for efficiency at some high speed…it might take huge current to get up to that speed and overall not ideal.
its hard to figure the iron losses and probably best to just do unloaded testing at high speeds to see what the temp increase is. but if youre on the water and will have water cooling it doesnt matter how inefficient you are if you can rid the motor of the heat. and maybe youd just have a slightly decreased range as the drawback
since you are using a propeller with no gearing at 14,500rpm, you will need the same amount of torque out of both motors to sustain this rpm…
if they are the same KM or size constant, the 600kv has 1/4th as much electrical resistance as the 300kv, because the 600kv winding wires are half the length but twice the cross section area.
to solve this let’s assume we want to reach & sustain 14,780 motor rpm on 2 electric skateboards (300kv vs 600kv), both with the same 2.05:1 gear ratio & 83mm tires… this is 70mph in the graphs below.
the 600kv has half the torque per amp compared to the 300kv motor, so the 600kv needs twice as much motor current to produce the same torque, but since the 600kv motor has 1/4th the electrical resistance of the 300kv motor, it turns out that both motors produce exactly the same amount of ohmic heating (blue line, bottom middle chart – ohmic heating watts, 1 motor).
since both motors are generating the same amount of heat, both motors should last equally as long. and in fact, both motors draw the same amount of amps from the battery to produce the necessary torque to sustain 14,780 motor rpm (red line, bottom right chart – battery amps).
but in the controller it’s a different story… take a look at the motor current (blue line, top left chart, motor amps). it takes 290a motor current to sustain 14,780rpm with the 600kv motor vs 145a motor current to sustain 14,780rpm with the 300kv motor.
even though the 600kv motor has 1/4th the electrical resistance compared to the 300kv motor and therefore produces the same motor heating for the same torque at the same rpm, the motor controller’s electrical resistance will be the same regardless of which motor you choose. you have to send twice as much motor current through the controller to sustain 14,780rpm with the 600kv motor, and twice as much current through the 600kv controller will heat the 600kv controller 4 times faster according to the copper loss equation I^2R=W.
So in summary while both 300kv and 600kv motors should last equally as long at 14,780rpm, you’ll be heating up the controller 4 times more quickly with the 600kv motor.
75v battery, 2.05:1 gear ratio, 83mm tire, (300kv, 0.004ohm, 145a motor current limit) vs (600kv, 0.001ohm, 290a motor current limit)
Fantastic! Thank you!
Yes, the controller heat is something that must be managed and knowing the higher kv setup jacks up the heat more for this setup is one of the golden nuggets I was hoping to learn/figure out.
@thetechtrader-- but as @Jmding pointed out if you double the gear reduction ratio on the higher kv motor, and let the 600kv spin twice as fast – 29,560rpm with the 600kv instead of 14,780rpm with the 300kv, and use the same 145a motor current on both… then the controller generates the same heat for both options, the 600kv motor generates 1/4th as much heat, and consumes less amps from the battery for the same final propeller power and rpm. in this case the 600kv motor’s bearings might wear out more quickly, while less heat is produced in the 600kv windings.
Interesting, thank again. Our focus is a on a direct drive system to avoid the added weight, reduction gears, cost etc… the reduction gears for our situation are about $400 USD. There is trade off, lower cost, less to breakdown, less weight, shorter length, but we will lose some efficiency. I just want to be sure we select the best motor option for durability, ESC and wire heat.
I’m guessing the 14500 number came from the max prop speed before cavitation occurs?
I’m sure you understand what’s your best option, but to summarize the solution to anyone reading all this technical stuff:
You want the lowest KV motor you can achieve while still reaching your calculated max RPM target. KV will not affect motor heat given the same torque requirements, but it will affect motor controller heating. Lower KV is good for sustaining higher power draw for longer periods because it requires less motor amps, which will heat up the esc less. This only applies to a situation where both motors have the same gearing / direct drive.