higher or lower kv

I am planning a hub-middrive,the battery is the fixed factor it wil be a 30s (126volt) with 80amps.
So because the amps are limited i am better of with a low kv-motor for a fast acceleration-bike?
Or a high kv motor and gear it down? Still confused :shock: Controller will be sabvoton 96100.
 
Your motor will have an optimal "band" that it works in. When the motor is bogged down only maybe half of the energy is going towards forward momentum and the other half is generating heat; your motor will overheat at too low of rpms under high power.

You will want to either gear or kv your setup to reach an optimal power band. If you make your setup so it will go 150kph for instance it will get super hot on strenuous uphills going 30kph but actually run cooler the closer it gets to 150kph. Consider a two speed setup using moped parts that peaks the motor at 70kph, then a second speed that takes your setup to 150+kph.

You can use this online calculator to take some of the brain work out of it: http://www.recumbents.com/wisil/e-bent/rc_drive/rc_drive_calculator.asp

afterthought: Or if this is a e-dirtbike consider just keeping the top speed low in general to avoid overheat on steep inclines. I usually never exceed 100kph when I'm out playing in the dirt, so I would consider that a good target.
 
So because the amps are limited i am better of with a low kv-motor for a fast acceleration-bike?
Click to expand...
Actually the acceleration isn't effected that much by kv, just the motor temps. Its a mis-nomer to say a low kv motor accelerates faster. The torque to the wheel is the same, it's just that the motor doesn't like being run outside of it's peak performance power band.

It's wattage and torque that make a bike accelerate faster.
 
It wil be a off-road bike and will not do more then 70km/u. On the recumpent-site i always calculate for gearing and speed, just dont know for sure at what % for speed,with a hub motor?
 
bionicon said:
I am planning a hub-middrive,the battery is the fixed factor it wil be a 30s (126volt) with 80amps.
So because the amps are limited i am better of with a low kv-motor for a fast acceleration-bike?
Or a high kv motor and gear it down? Still confused :shock: Controller will be sabvoton 96100.
Click to expand...

Give us a motor you are thinking about, it will be an interesting exercise to match it to controller and battery output!
 
That was a bit of a thread.. I don't have the energy to read it just now. Where does the 90% claimed efficiency come from, do you know?

It's very uncommon for the manufacturer to understate performance specs, that's why i wonder :D
 
larsb said:
That was a bit of a thread.. I don't have the energy to read it just now. Where does the 90% claimed efficiency come from, do you know?

It's very uncommon for the manufacturer to understate performance specs, that's why i wonder :D
Click to expand...

no i don't just pick this hub for weight(9 kilos) and the power it deliver, I want to build a lighter version of my hub surron :eek:
 
According to Justin's data, Teslanv get the following phase current limits on the various windings:

MXUS XF40-45H "3000W" Direct Drive Hub Motor Series:

3T:
Max. Continuous Phase Current: 55A
Overheat in 10 Minutes: 85A
Overheat in 60 seconds: 242A
Click to expand...

Added from "teslanv"
Here is some more Hard data for the EE folks:
MXUS XF40-45H (3000W) DD Hub motors...

21X3T Winding
Phase resistance = 0.072 Ohms
RPM at 50.2V = 597, 11.89 Kv
1.78A/89.4 Watts No Load
Click to expand...

If this is correct then your choice is simple: 3T or less :D

your controller and battery will be able to drive it to the max torque and you can gear it down quite a bit.

126Vx11.89kV = 1498 rpm, geared to 70 in a 17" wheel, about 2.2x the original torque. Should be enough?
 
Wanted to check the erpm limit of the sabvoton. If this https://endless-sphere.com/forums/viewtopic.php?t=62240&start=50#p944624 is still valid then max erpm is 24000 and you can only drive your motor to about 1000rpm-->8.3kV-->4T

16X4T Winding
Phase resistance = 0.110 Ohms
RPM at 50.2V = 448.2, 8.93 Kv
1.08A/54.2 Watts No Load

And torque is 1.6x original
Invest in a 40kerpm controller? :wink:
 
Hi, actually i tried to do true mid-drive system with QS205 17,7kv and I can say I failed.

The system consist (as a "lab" bike). original it was idea to do with Revolt Rv160 PRO. But as it turned out it is crap in quality and so on. In making the frame I was kinda expecting that. So i made rear fork to accommodate for QS 205. I made reduction gearing for QS 205 of ~1,6. 53tooth rear sprocket and 32 for motor sprocket.

So my system consist:
24S20Ah A123 battery
Vesc custom made controller. Battery 200+, 450A phase.
QS 205 17,7kv.
Yes it weight a lot ~75-80kg.


So when the wheel is lifted. I am not joking, the bike accelerates wheel in so much angry, that from momentum the bike can flip. I was super happy. But on ground it sucks big time. Vesc controller from beginning having even hard time to move bike. With all that setting from Stop. it was only able to pull like 500W max (data obtained from vesc monitoring data online in PC). So I switched to my old good Kelly 260A burst power from battery (do not know phase current). It starts ok. Even jumps but still it falls flat to even compare to my old full susension bicycle with
24S20Ah, Kelly, hubzila 9kv. It is like night and day. It doesn't have sharpness at all.
But it has ability to climp same steep hills no problem even felt not stressing with added weight compared to bicycle.
But sharpness is gone, acceleration feel almost twice as low as bicycle.
max speed for bicycle 70km/h.
with this bike 80km/h.
I would expected these results without reduction but now i feel like shit.
Here is video with wheel lifted with maybe 50% of power settings. In full power i have to hold handle bars for it not to fall.
https://www.youtube.com/watch?v=r9mxj4b9E7s
 
It might be as expected.

9/17,7=0,51
-->You'd need double the current to get same torque with the higher kV. With same controller you'd only get half of the torque from the new motor if it isn't already saturated.
downgearing 1.6 gets you back to 81% of the old setup.

Then Kellys are known for not holding their specs, especially at low rpm so the low end torque will be even lower than 81% which should explain the lack of sharpness. Time to buy a new controller :D

I bet you a pizza that your 260A kelly is not battery amps :mrgreen:

Like the bike :thumb:
 
Atleast I hoped to expect same result. Cause that hubzilla with 9kv was not able to stand a bit longer ride in bicycle. I have to cool it down with spray of water constantly. My BMS was reporting 210A from battery for hubzila (peak power). BMS is custom made with fluke calibration. so that Kelly I do respect.
With vesc controller I do reach 450A phase current cut-off. Shitty situation. Now atleast I will try crappy Revolt :)
 
bionicon said:
I wogndered what is best for middrive motor higher kv and bigger reduction or lower kv and less reduction? I have in mind a fast acceleration :D
Click to expand...

For any given amount of motor power, the lower kV with the larger reduction ratio will result in the strongest acceleration.

Edit:
Sorry about chiming in on a three year old thread. It got bumped to the top, and I failed to notice.
 
Large reduction off course gives torque. With a low kV it will be very slow but the high or low kV is not making a difference if you match the rest of the system to it given the same power. It's a common misconception.

See this for explanation:
https://endless-sphere.com/forums/viewtopic.php?f=30&t=102934#p1505648
 
larsb said:
Large reduction off course gives torque. With a low kV it will be very slow buth the high or low kV is not making a difference if you match the rest of the system to it given the same power. It's a common misconception.

See this for explanation:
https://endless-sphere.com/forums/viewtopic.php?f=30&t=102934#p1505648
Click to expand...

Nevertheless, a slow winding makes more torque out of the same number of watts. That's the term on the opposite side of the equation from RPM. It's not a misconception; it's math.

If you want to make equal torque with a fast wind, no problem. Just feed it more amps. But you will burn a lot more watts to do it, and most likely run at a lower efficiency.
 
Balmorhea said:
larsb said:
Large reduction off course gives torque. With a low kV it will be very slow buth the high or low kV is not making a difference if you match the rest of the system to it given the same power. It's a common misconception.

See this for explanation:
https://endless-sphere.com/forums/viewtopic.php?f=30&t=102934#p1505648
Click to expand...

Nevertheless, a slow winding makes more torque out of the same number of watts. That's the term on the opposite side of the equation from RPM. It's not a misconception; it's math.

If you want to make equal torque with a fast wind, no problem. Just feed it more amps. But you will burn a lot more watts to do it, and most likely run at a lower efficiency.
Click to expand...


Not quite. Assuming both motors have the same copper fill....

You're forgetting the fact that if you also proportionally reduce the voltage supplied to the higher kV motor at the same time as increasing the current, you get the same heat generated while producing the same torque from the same wattage at the same speed, both running at the same efficiency.
 
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