madin88 said:
teslanv said:
John in CR said:
Teslanv, you really don't want to bring up that there's a lower cost to run a slow wind motor. That topic is best just dropped.
With a Low Kv (High Kt) motor, you can use a lower current (Less FETs) controller and smaller battery & phase leads, than a comparable High Kv motor, which requires MORE current to provide the same torque as the low Kv winding.
The battery for either could be the same total watt-hour capacity, but with higher voltage and lower capacity on the slow wind and lower voltage and higher capacity on the fast wind. So yes, the battery cost is more or less irrelevant in the comparison, unless you want to argue about the cost of a 10S BMS vs a 16S BMS.
John, why is it so hard for you to admit that a slow-wind motor is a more appropriate winding choice for certain applications?
I'm totally with you teslanv. IMO John got stuck in his believs and is not able to look outside the box
Another example would be mxus 3T at 60V vs. 4T at 80V. Latter would be the better choice for performance per costs and system efficiency (less losses over phase wires). And it is not the same if a controller works at 50% PWM (high kV with limited speed) or 100% (without speed limit). Consumption during acceleration (top end) is different, the 100% PWM system has higher efficiency because no additional step down losses.
It is not always about maximum hot rodding. the motors we are talking are "only" 3kW cont, 6kW peak rated for ebike / e-moped and no 10kW+ motorcylce motor like Hubmonster is, so for most applications and together with at the moment highest power dense 100V controllers the fastest motor rarely would be the best choice.
sorry mate you got halfway through being right, then lost it again...
" Latter would be the better choice for performance per costs and system efficiency (less losses over phase wires)"
assuming you change battery voltage and controller accordingly, and all other wiring remains the same, yes... but the difference will be pretty minor in practice.
" And it is not the same if a controller works at 50% PWM (high kV with limited speed) or 100% (without speed limit). Consumption during acceleration (top end) is different, the 100% PWM system has higher efficiency because no additional step down losses."
Nope. sorry, but if you vary the voltage and current by the same ratio as the change in KV, then the duty cycle at a given speed/power demand will remain the same, as both motors will be spinning at the same % of their no load speed, with the same back EMF (as a % of pack voltage) same (relative) resistance and inductance. if your just comparing a high kv and low kv motor with the same battery/controller combo for each then yes, it'll have a small difference. again however you're unlikely to notice it unless the controller is struggling with the high kv motor's R and L.
"Consumption during acceleration (top end) is different, the 100% PWM system has higher efficiency because no additional step down losses."
Nope, it'll be the same, as each controller is stepping down the same relative voltage, to the same relative phase voltage, in order to generate the same phase current, and thus the same motor torque (and same motor heat). Again however if your using the same controller and battery for each motor there will be a small difference, but again, this will be
small.
"It is not always about maximum hot rodding. the motors we are talking are "only" 3kW cont, 6kW peak rated for ebike / e-moped and no 10kW+ motorcylce motor like Hubmonster is, so for most applications and together with at the moment highest power dense 100V controllers the fastest motor rarely would be the best choice."
really its only a choice based on what your controller can handle. if your controller is capable of running low resistance and inductance motors at high loads and low duty cycles then there's really very little difference between motor KV's. As has been stated repeatedly, there's no difference in heat generation between a low and high kv motor for a given torque output. Basically, choose a kv that wont cause your controller to kill itself, while still giving you the top speed you desire for a given wheel size. then tune your controller perimeters accordingly, and you'll get the max power you can for a given rpm/controller/battery combo.
you could use a higher kv motor, with a smaller wheel, and get more power, but then you're wheels smaller (not good for offroad etc)
you could use a lower kv motor, but then your top speed will be lower.
you could use a higher kv motor with the same wheel, but your controller may not be able to run such a motor at such low rpms (low rpm means low back emf and thus lower duty cycle, and more heat/damange, though this is highly dependant on the controller).
However, in NONE of these situations will the motors torque per unit heat change. So if it takes 3kw to maintain your top speed, then every one of these cases will produce the same heat, with the minor exception of the first which will be lower. This is due purely to a larger reduction stage to the road (smaller wheel) which means the motor needs to produce less torque (same power at higher rpm = less torque), thus less heat.
it cant be escaped... motor continuous torque is the same for any kv (well not quite, but the difference is so small you'll never notice outside of a lab). So again, pick a kv based solely on what your controller can handle while still giving the speed you want with a given wheel size. if your controllers so weak that you cant hit your desired speed with a kv that it can handle, then get a better controller, or deal with the lower speed. if your controllers good enough that it can handle a motor that provides a speed higher than desired, then drop your battery votlage and save on BMS and wiring complexity, or leave it and be happy with the greater high torque near your top speed (lower back EMF means more phase current, thus more torque), or get a lower voltage controller. The motor doesn't care in any case.
