perpetualthoughts
1 µW
- Joined
- Sep 9, 2021
- Messages
- 2
The kv of a motor doesn't change the continuous power rating of a motor. Regardless of if it is a slower or faster wound, the output power will be the same.
When a manufacturer states the maximum continuous power of a motor, they are indicating the maximum power a motor can output at given rpm.
If however, you run this motor slower than the indicated max rpm, you will be able to attain or even surpass the maximum power output, but you will not be able to do so continuously without overheating the motor.
For example, when I calculated the power needed to propell a 120kg object up a 20% slope at 20km/h. P=grade*speed*weight*(gravity constant)
I found out that I needed a minimum of 1300watts of output power just to maintain that speed. With an estimated motor efficienty of 75%, a minimum of 1625 watts is needed. I pedal 100watts (i don't know). So the motor has to provide at least 1500watts just to maintain that speed, not counting acceleration nor friction losses.
Knowing this I bought a cheap 1000watt chinese motor, thinking that I could get to 1500watts with oil cooling and fins.
Now, what really happened was, I found the motor overheating at grades of only 7% and 110kg. Where 418 watts were theoretically needed to maintain such speeds. So, well below specs. Yet, it still over heated (coil temp 70C and rising fast). Max speed without load is 50km/h. 530rpm @ 48v in 20" wheel ~ 50km/h top speed. So I think it surpassed its current limitations and ran huge i²R losses.
The only way I see is to reduce the diameter of the wheel even further. Maybe mount a 12" tire directly on the 232mm hub motor ? Reducing the max speed to 30km/h, essentially reducing the amps required to go up the same grade, therefore reducing i²R losses and thermal runaway. I don't hope for 20kmh 20% anymore because the motor isn't powerful enough, but 10-12% continuous would be nice.
What do you guys think ? Is my way of thinking correct ? Or would a different kv motor rating change anything, how about rewiring ?
When a manufacturer states the maximum continuous power of a motor, they are indicating the maximum power a motor can output at given rpm.
If however, you run this motor slower than the indicated max rpm, you will be able to attain or even surpass the maximum power output, but you will not be able to do so continuously without overheating the motor.
For example, when I calculated the power needed to propell a 120kg object up a 20% slope at 20km/h. P=grade*speed*weight*(gravity constant)
I found out that I needed a minimum of 1300watts of output power just to maintain that speed. With an estimated motor efficienty of 75%, a minimum of 1625 watts is needed. I pedal 100watts (i don't know). So the motor has to provide at least 1500watts just to maintain that speed, not counting acceleration nor friction losses.
Knowing this I bought a cheap 1000watt chinese motor, thinking that I could get to 1500watts with oil cooling and fins.
Now, what really happened was, I found the motor overheating at grades of only 7% and 110kg. Where 418 watts were theoretically needed to maintain such speeds. So, well below specs. Yet, it still over heated (coil temp 70C and rising fast). Max speed without load is 50km/h. 530rpm @ 48v in 20" wheel ~ 50km/h top speed. So I think it surpassed its current limitations and ran huge i²R losses.
The only way I see is to reduce the diameter of the wheel even further. Maybe mount a 12" tire directly on the 232mm hub motor ? Reducing the max speed to 30km/h, essentially reducing the amps required to go up the same grade, therefore reducing i²R losses and thermal runaway. I don't hope for 20kmh 20% anymore because the motor isn't powerful enough, but 10-12% continuous would be nice.
What do you guys think ? Is my way of thinking correct ? Or would a different kv motor rating change anything, how about rewiring ?